Author: Paul B. Anders
Date: 7/16/01
Version: 1.5
Introduction
The D-Jetronic Electronic Control Unit (ECU) controls most aspects of the fuel injection system. Also known as a "brain" or a "computer", it's a combination of digital circuits, analog sensor interfaces, and control circuits. Given that the first D-Jetronic equipped cars date from  the mid-to-late 1960's, the technology of the ECU is primitive in comparison to today - all discrete components and simple printed circuit board technology. Regardless, the engineers at Bosch did a good job, and the ECU accurately maintains proper mixture control for many different engine operation modes.
The ECU would continue to remain a mystery to owners if it weren't for the efforts of Frank Kerfoot, a former Bell Labs engineer. Frank was an SCCA racer in the 1970's who also had a 914. He took it upon himself to reverse-engineer a schematic of the ECU for analysis and tuning purposes. Frank used a 039 906 021 A ECU, which was used on the 1975-1976 2.0L cars. This ECU is similar to all D-Jetronic ECU's.
This document is an attempt to analyze the ECU using Frank's drawings and information I have obtained by disassembling several ECU's. Frank has graciously agreed to help answer questions I've posed to him and I've incorporated his answers into the analysis below. While I'm an electrical engineer, I'm pathetic at circuit analysis, so I've also had to draw upon my 914 and EE friends to help me muddle through. If I don't understand how a circuit in the ECU works, I'll say so below and hopefully will get information on it for future versions of this document
ECU Operation Overview
The ECU controls the timing and duration of voltage pulses to the injectors, and the operation of the fuel pump relay (FPR). The operation of the cold start valve (CSV) is independently controlled by the thermo- or thermo-time switch (TS), not the ECU. The ECU determines the timing and duration of the injection pulses by processing signals from the ignition start switch (Start), trigger contacts (TC) , manifold pressure sensor (MPS), temperature sensors (TS1 - air temp, and TS2 - cylinder head temp), and throttle position switch (TPS w/ idle switch IS). The ECU controls the operation of the FPR by processing signals from Start and the TC.
The timing of injection pulses is determined by signals from the TC. In 4-cylinder implementations of D-Jetronic, the injectors are paired and pulsed together - this is called "group injection". Determination of the injector pulse duration (Ti) is fairly complex. Without going into fuel injection theory in detail, there is a basic injection pulse duration (Tb) for a specific engine load. In the D-Jetronic system, engine load is determined by measuring the intake manifold pressure with the MPS. Ti is also affected by specific operating conditions - cold start, engine speed, idle, part-load, full-load, cold/warm/hot engine, acceleration, and air temperature. These conditions are sensed by inputs to the ECU from Start, TS1 and TS2, TC, TPS, and the MPS. The ECU adds additional pulses (during acceleration) and modifies the value of Tb for these conditions, resulting in the final injection pulse of Ti. The synchronized pulse of duration Ti is gated to the appropriate paired injector drivers, which are designed to provide the proper drive characteristics to control the injectors. For over-run conditions (closed throttle and coasting), a special circuit in some versions of the  ECU shuts off the injection pulses when the engine speed is above a threshold level.

The ECU supplies ground to the FPR to run the fuel pump under three conditions. First, the fuel pump is run for 1.5 seconds after the key is turned to the "on" position and the ECU is powered-up. This pressurizes the fuel system in preparation for starting, but prevents the fuel pump from continuing to pump if the key is left in this position. This reduces fire hazards from leaking, and also wear and possible damage to the fuel pump. A check valve in the fuel pump prevents the system pressure from being relieved through the return circuit after the pump has pressurized the system, preventing fuel vapor formation and improving hot starting. When the key is turned to the "start" position, a signal to the ECU from the ignition switch turns the fuel pump back on while cranking. Lastly, if the engine starts, and the engine speed is greater than 100 rpm, the ECU will keep the fuel pump running after the key is released to the "on" position. If the engine does not start after cranking, the fuel pump stops when the key is returned to the "on" position. If the engine stops running (stalls) while the key is in the "on" position, the fuel pump also stops. This reduces fire hazards in the case of an accident where the fuel system might be damaged.
Main Board - This board has all the circuits on sheets 1 and 2 of the schematics below. D502 is the large diode on the heat sink on the left. The two power transistors for the injector driver stage are also on the heat sink on the left. The power resistors are in the heat sink pack on the bottom (and are bonded to the ECU case for thermal transfer).
Daughter Board - This board has all of the circuits on sheet 3 of the schematics below. The 3-wire cable in the upper right is for the idle mixture adjustment potentiometer

ECU Schematics
	Sheet 1: Timing Logic (TL), Pressure-Sensing Loop (PL), Over-Run Shutoff (OS), Injection Logic (IL), Switching Logic (SL), and Output Drivers (D1 & D2)	
	Sheet 2: Acceleration Enrichment (AE), Fuel Pump Control (FPC), and Cylinder Temperature Compensation (CTC)	
	Sheet 3: Engine Speed Sensor (ES), Engine Speed Compensation (SC), Pulse Width Multiplier (PWM), Idle Mixture Adjustment (IM), and Idle-Cold Mixture Compensation (ICM)	

ECU Block Diagram
A block diagram of the ECU can be found on page 0.1 - 1/8 of the Porsche 914 Factory Workshop Manual vol. 2, "Fuel System". While instructive, it does not accurately reflect the organization and components of the ECU. The diagram below is in the same spirit as the factory diagram, but was derived from review of the ECU schematics:

	Engine speed sensor / Engine speed correction (ES & EC) - The ES stage determines engine speed using pulse data from the PL. The EC stage corrects the mixture for engine speed (volumetric efficiency), using the output of the ES.	
	Fuel pump control (FPC) - Controls the FPR using inputs from Start and the ES.	
	Pressure-sensing loop (PL) - Creates the basic injection pulse using inputs from the TS1, MPS, IM, and SC. The TL supplies the injection timing signal for injection pulse initiation and synchronization.	
	Idle mixture adjustment (IM) - Adjusts the mixture at idle using inputs from the IS and IA.	
	Idle-cold mixture compensation (ICM) - Adjusts the mixture for cold start, and provides partial idle mixture control using inputs from Start and IS.	
	Over-run shutoff (OS) - Shuts off injection on overrun, using inputs from the PL and IS. Not present on all ECU's. 	
	Pulse width multiplier (PWM) - Delays and stretches the basic injection pulse duration using inputs from the ES, FC, AE, and CTC.	
	Cylinder temperature compensation (CTC) - Adjusts the mixture for cylinder head temperature using inputs from TS2 and ICM.	
	Injection logic (IL) - Combines inputs from the FC, PWM, and AE to create the final injection pulse.	
	Acceleration enrichment (AE) - Provides immediate and delayed acceleration enrichment using input from the TPS.	
	Switching logic (SL) - Routes the final injection pulse to the correct injector group using inputs from the IL and TL.	
	Timing logic (TL) - Creates group injection gate pulses and injection timing signal using input from the TC.	
	Output driver stages (D1, D2) - Converts the final injection pulse for each injection group to the proper level to drive the injectors I1-4.	
ECU Circuit Block Analysis
Timing Logic (TL) - Sheet 1 
The TL controls which injector group is active, and signals the beginning of each injection cycle to the PL. The two switches in the TC are connected to the inputs of a flip-flop, creating square wave outputs from T251 and T252 that are 180 degrees out-of-phase. Each square wave output goes to an edge detector and the input NOR gate of the injector drivers (IL). 
The edge detector is a differentiator circuit with a normal output of +12 V. When a leading edge of the input square wave is detected, the differentiator generates a short negative-going pulse down from the nominal +12 V level. Pulses from both edge detectors are combined and are sent to the pressure-sensing loop circuit (PL). Each pulse is the starting signal for an injection pulse. 
Pressure-Sensing Loop (PL) - Sheet 1 
The PL creates a synchronized basic injection pulse, with the duration primarily controlled by the MPS. The heart of this circuit is the transistor pair T201 and T202. The basic injection pulse appears at the collector output of T202.  
T201 is normally on, turning T202 off. With T202 off, its collector output is +12V. The negative-going pulses from the TL edge detectors turn T201 off, turning T202 on, which drives its collector output to zero volts. This causes a current to flow through L1 and the diode-resistor network (R401, R402 {if present}, R403, R405, D401, and TC1), inducing a negative voltage across L2. The time constant of the current decay is proportional to L1 * Re, where Re is the effective resistance value of the diode-resistor network. Since the inductances of L1 and L2 are dependent on the MPS armature position, the induced negative voltage on L2 is dependent on the manifold pressure (see my MPS document for a description of how the MPS works).  
The negative voltage across L2 holds T201 off (keeps it turned off) through D202. As the current through L2 decays, the induced negative voltage decays, eventually to the point (starting at +0.7V) where T201 turns back on, turning T202 off, and completing the basic injection pulse. Bias voltage supplied from the SC and IM affects the basic injection pulse duration by shifting the voltage decay curve across L2. The more negative the bias voltage from the SC and IM, the longer the hold-off period and the longer the duration of the basic injection pulse. 

Note that since the resistance of TS1 is a component of Re, the time constant value is a function of the intake air temperature. From reading the resistors on a scrap ECU, I found the following resistance values: R401 = 1.5 K ohms, R403 = 510 ohms, and R405 = 1.5 K ohms. Ignoring the diode drop of D401, and using the nominal value of TS1 at 68 deg. F of 300 ohms, I calculate that Re = 395 ohms. If TS1 is open (disconnected), then Re = 436 ohms. If TS1 is shorted, Re = 381 ohms. Higher values of Re increase the time constant and richen the mixture. A common mechanic's trick is to disconnect the TS1 sensor on an older motor, increasing the injection pulse time by about 10% for a richer mixture. Older motors tend to have vacuum leaks and lower intake manifold vacuum (due to wear) than a new motor, so this trick often helps performance.  

Over-run Shutoff (OS) - Sheet 1 
The OS circuit prevents injection during over-run to reduce emissions. When the throttle is fully closed, the idle switch is activated, grounding terminal "B" in the over-run shutoff circuit. This ground is one of the inputs to the base of T807, along with the output of a low-pass filter circuit that uses the basic injection pulse from the PL as an input. When the engine speed is above a threshold level (>2000 rpm?) and the throttle is fully closed, T807 is turned off, forcing T301 on, which inhibits injection. When the engine speed drops below the threshold, or if the throttle is opened, T807 is turned on, and the state of T301 is now determined by the basic injection pulse. 
Note that this circuit is not present on all 914 ECU's. I am currently researching which ECU's had the OS. 

Injection Logic (IL) - Sheet 1 
The IL combines multiple injection signals to create the final injection pulse. The T507 NOR gate enables injection when it receives a high signal from either inverter T301 (when the basic injection pulse from the pressure sensing loop goes low), the AE pulse output, or from the inverted output of the PWM. 
Switching Logic (SL) - Sheet 1 
The SL routes the final injection pulse to the appropriate injector group. The SL uses two NOR gates that each take the final injection pulse from the IL as one input. The other input of each NOR gate is from the two TL flip-flop outputs, which are square waves that are 180 degrees out-of-phase. When both the flip-flop output and the output of the IL are low, D1 or D2 are activated and the injectors are opened for the duration of the injection pulse. Because the inputs to the NOR gates are 180 degrees out-of-phase, only one injector group can fire for each injection pulse from T507.  
Output Drivers (D1 & D2) - Sheet 1 
D1 & D2 provide the appropriate current and voltage to drive the inductive load of the fuel injectors. Each driver uses two power transistors and load resistors to provide the proper voltage output to the injectors. Accounting for the 0.7 V drop across D502, and assuming a system voltage of 13.6 V when the car is running, 12.9 V is present at the emitters of the final driver transistors.  When the driver transistor turns on, there is an additional 0.7 V drop across the junction, so the effective supply voltage to the injectors is 12.2 V. 
To understand the actual voltage waveform that appears across the injector, it is necessary to model the injector as a series combination of inductance (L) and resistance (R). I have not measured the value of L, but the "book" value of R is 2.4 ohms. When the driver transistor turns on and current begins to flow through the injector, a magnetic field begins to build in the coil and starts to pull the injector open. Due to the energy required to open the injector, the inductive reactance (XL - the effective resistance of the inductor to a time-varying signal, measured in ohms) is initially very large. Because XL is initially the largest resistance in the series path of components, almost all of the voltage is dropped across the injector - that's why right after the injector turns on you see +12.2 V across it. 

The rate the magnetic field builds is limited by the product of L * R. As the field builds, XL decreases and less voltage is dropped across the injector - that's why you see the voltage dropping away from +12.2 V. Eventually, the magnetic field is fully developed and stabilizes, and the voltage drop across the inductor is solely due I * R, where I is the steady-state current. If you use the "book" values of R = 2.4 ohms and an assumed battery voltage of +12 V, you would get a sustained voltage drop across the injector of 3 V. However, measuring the injectors in my car at the ECU socket gives values of R between 2.8 and 3.8 ohms.  When the car is running, the actual system voltage is more like 13.6 V. Accounting for the higher injector resistance over the "book" values and using the system voltage of 13. 6 V, the actual sustained voltage is more like 4 to 5 V. Note that the calculated drive voltage corresponds exactly with data given for the flow rates of the injectors, which states that the tests were performed at 3 V (data courtesy of Roland Kunz).

At the end of the injection period, the driver transistor turns off, which rapidly changes the current drive to the injector to zero. The rapid collapse of the magnetic field in the coil now has an opposite effect - it induces a high negative voltage across the inductor. The mathematical expression of this voltage is VL = L * dI/dt , where dI/dt is the time derivative of current through the inductor. The net effect is that you get a large negative spike of voltage after the injector turns off. I have measured a peak of -32 V, quickly decaying to zero volts. The inductive voltage spike (and resultant current) is dissipated by the series combination of a 6.8 MF capacitor and load resistor in parallel with the injector. 
Note that this analysis ignores the high impedance path from the final driver transistor emitters through R510 (820 ohms) and D501, which is negligible in comparison with the low-impedance path through the injectors.  
Below is a sketch of an injector waveform I measured on my car: 

Acceleration Enrichment (AE) - Sheet 2
The AE circuit provides immediate and delayed enrichment when the throttle is opened for acceleration. Throttle opening is signaled by the TPS through alternating ground signals from two inter-digitated traces that are each connected to the inputs of a flip-flop (see sheet 2 of the schematic for a drawing of the switch details). A drag switch in the TPS prevents these signals from being sent when the throttle is closing. The outputs of the flip-flop are sent to two edge detectors, whose outputs are combined and sent to pulse shaping and narrowing circuits. The pulse shaper provides immediate injection pulses to the IL. The width of these pulses is independent of engine speed and load, and from Kerfoot's schematic, they are about 1.5 ms in duration. Additionally, a delayed (approx. 1 to 2 seconds) acceleration enrichment effect created by sending the inverted, shaped, and narrowed pulses to the mixture enrichment circuit. The negative-going pulses pump down the voltage on capacitor C901, controlling the output of the T901 current source. The current source output is used by the PWM for the mixture adjustment.  

Fuel Pump Control (FPC) - Sheet 2 
The FPC operates the FPR during starting and normal running. The FPR is controlled by T552, which when on, provides ground to the FPR. When the key is turned to the "on" position, +12V is supplied to the ECU, and the FPR is activated for 1.5 seconds. At power-on, T101 is on and T102 is off because C101 has not charged to 1.4 V yet. While T102 is off C102 is charging and T103 is On. Eventually C101 charges and turns T102 on which pulses T103 to off via the discharge of C102. The pulse charges C551, turning the FPR on, then C551 discharges (taking about 1.5 sec),  turning the FPR off. Turning the key to the "start" position sends the Start signal to the base of T552, activating the FPR. If the car starts, and the engine speed is greater than 100 rpm, pulses from the ES pump up capacitor C551, turning T551 and T552 on, activating the FPR . If the engine speed drops below 100 rpm, there is insufficient pumping to keep the voltage on C551 high enough to keep T551 on, shutting off the FPR.
Cylinder Temperature Compensation (CTC) - Sheet 2
The CTC adjusts the mixture for the engine temperature. The operation of the circuit is controlled by T451, which is operated in the linear region. T451 is part of a resistor-diode voltage divider circuit. The resistance across T451 in this circuit is controlled by the base drive. Ignoring start and idle conditions (CTC effects under these conditions are covered in ICM section), the drive is defined by the voltage divider of TS2 and  R464 (assuming near zero base current). When the engine is cold, TS2 is about 2300 ohms, and the base voltage is about 7.7 V, driving T451 towards saturation, decreasing the voltage drop across the transistor. I did not do the circuit analysis of the resistor-diode voltage divider circuit, but Frank Kerfoot notes that the effect of this decreased voltage drop across T451 is to drive the output of the CTC to a higher voltage, increasing the multiplication effect in the PWM. Similarly, when the engine is hot, TS2 is only about 50 ohms, resulting in a base voltage of only 0.4 V, turning T451 nearly off, and increasing the voltage drop across the transistor. This drives the output of the CTC lower, decreasing the multiplication effect in the PWM.  
What happens to the CTC output if TS2 is open or shorted? If TS2 is open, the base of T451 is effectively at +12V, driving T451 into saturation. This causes the output of the CTC to go to a maximum, which results in a very rich mixture, preventing the car from running after starting. If TS2 is shorted, the base of T451 is effectively grounded, and T451 is fully off. Under cold engine conditions, this results in a very lean mixture, lean enough to prevent the engine from running. For a hot engine, the effect is not as significant, and the engine may run, but will be lean.  

Engine Speed Sensor (ES) - Sheet 3 
The ES creates an engine speed signal used by the FPC and the SC. The input to the ES is the basic injection pulse from the PL. T101 and associated circuitry act as an edge detector for the trailing edge of the injection pulse. The output of the edge detector is shaped by T102 into a 0.5 ms pulse (this point is also coupled to the PWM, apparently for voltage clamping) and to T103, where the output is stretched and inverted to form the engine speed signal. This signal is used by the FPC to detect if the engine is running faster than 100 rpm.

Engine Speed Correction (SC) - Sheet 3
The ES output is fed to the SC circuit. The function of this circuit is to correct the mixture for the volumetric efficiency curve of the engine. "Luau_BoB" saw my pitiful first draft on this section and took it upon himself to provide the excellent analysis I have paraphrased here, with modified versions of his diagrams to match the nomenclature of this document.
Volumetric efficiency (Ve) is the ratio of the actual volume of air taken in by a cylinder divided by the cylinder volume. The maximum Ve is mostly determined by the design of the intake system (2 vs. 4 valve, valve sizes, intake runner design, throttle body design, air filter, etc.), and the engine speed at which the maximum Ve occurs is mostly determined by valve timing and by intake runner design. Typical max Ve for an engine with a overhead 2-valve configuration is about 75 to 80%. A generalized Ve curve for an engine is shown below:

Here are two good basic references on Ve:
http://www.tpub.com/engine1/en1-105.htm 
http://www.ponycarburetors.com/calculating_cfm.htm 
A good reference that shows how to measure the Ve of an engine is:
http://eric.virginia.com/install_university/installu_articles/volumetric_efficiency/ve_computation_9.012000.htm
To maintain a constant air/fuel ratio, the fuel quantity must take into account the Ve at a specific engine speed. The SC circuit is used to realize this effect. 

The following diagrams are used to describe the SC operation:

In this simplified model, the SC is replaced by a generalized waveform generator (Wf), and the PL is represented by T201, T202, and the MPS. As the Wf bias voltage changes, the threshold for turn-on of T201 changes, which in turn, changes the basic injection pulse duration. See the diagram above in the PL analysis to see how this works. 

The effect of the Wf bias voltage on the basic injection pulse generated by T202 is shown below:

In the example above, the actual Wf output is modeled by a hypothetical linear voltage decay, V(t) = -k * t + b , where k is the slope and b is the offset voltage. Note that the value of V(P) determines the turn-on voltage for T201, which determines the basic injection pulse duration. In the case of this hypothetical Wf, then as engine speed increases, P decreases, and V(P) increases. As shown in the diagram above, as V(P) increases, T becomes shorter.

A model of the actual waveform generator in the SC is shown below:

The output of each of the Wfm generators are "OR'ed" together, which results in an output signal that is the maximum voltage of all of the waveforms at a given time. Why four Wfm generators? Presumably, that's how many that the Bosch engineers determined were necessary to create the desired characteristic. Note from the schematic on sheet 3, that the Wfm 1, 2, and 3 generators each have selected components (resistors) that were picked, probably to match the specific Ve curve of the engine application the ECU was designed for. The output of the Wfm's are buffered by T107. The buffered output signal is combined with the output of the IM and sent to the PL, where it sets the threshold for the turn-off of T201. Note that for non-idle conditions, the output of the SC controls the T201 threshold, whereas at idle conditions, the output of the IM dominates.

The final waveform at the base of T107 in the diagram above is from Kerfoot's schematic. Randy Montellato has recently measured this characteristic on his ECU. See below:

The characteristic is very similar to Kerfoot's measurement of the base of T107, noted on sheet 3.The particular value of bias from this characteristic that determines the turn-on threshold of T201 in the PL is equal to the voltage at period P, the reciprocal of the engine speed (see the diagram above). To understand the effect on the injection pulse duration at a specific engine speed, first note that lower values of bias voltages affect the PL circuit to extend the injection period (see the PL section for a description of this behavior). On sheet 3, the opposite behavior is noted, so this observation is in disagreement with Kerfoot's notes. Next, since the bottom axis is time, engine speed is proportional to the reciprocal of time. Therefore, you need to invert and flip the characteristic as shown below:

Randy has also actually characterized the effect of the SC on the injection pulse width by monitoring the pulse width on a bench setup of the D-Jetronic system as a function of engine speed at constant intake manifold pressure. A chart of his results is shown below:

The results shown compare favorably with the inverted and flipped Wfm output, especially under full-load (0 in. Hg) conditions. Note that it appears that the long injection durations at low RPM implied by the Wfm bias characteristic apparently are at engine speeds well below the idle point, where the bias value is controlled by the IM instead of the SC. 
Pulse Width Multiplier (PWM) - Sheet 3 
The PWM stretches the basic injection pulse width to correct the mixture for the effects of engine temperature (CTC) and linked effects, starting enrichment (ICM), and the delayed accelerator pump action (AE). The basic injection pulse from the PL is fed to the base of T302. The collector output of T302 charges the 0.68 mfd capacitor, which is also charged by the current source from the AE that provides the delayed accelerator enrichment effect. The collector output of T303 sets the voltage on the other side of the capacitor, also affecting the charging rate of the capacitor. The drive on T303 is set by the output of the CTC, and the emitter level of T303 is set by the resistor-diode network, and also by a start signal effect from T651 (see the ICM description below). Diode D302 is then used to shape the sawtooth signal output into more of a square wave. This shaped signal is then sent to the IL to multiply the effect of the basic injection pulse from the PL.   

Idle-Cold Mixture Compensation (ICM) - Sheet 3 
The ICM provides cold-start enrichment clamping, idle mixture enrichment, and starting mixture enrichment to the PWM.  
	Cold-start enrichment clamping: The Start signal turns on T651, which clamps the cathode of D652 to the ratio of R653 and R656 (~6.7 V). The anode of D652 is connected to the input of T451 in the CTC. Without the clamping voltage, the cold start (70 deg. F engine temp or less) value of this input would be 8.2 V or greater, which would result in a very rich starting mixture. Therefore, the clamping voltage limits the effect of the CTC circuit on cold start mixture. 	
	Idle mixture enrichment: Under normal running conditions, the throttle is open and T452 is on, shunting R471 in the CTC. When the throttle is closed and IS is on, T452 is off and the additional 300 ohms of R471 appears in series with TS2. This increases the input voltage to T451, increases the output of the CTC, and enriches the mixture.	
	Starting mixture enrichment: The Start signal turns on T651, which increases the voltage at the emitter of T303 in the PWM, increasing the drive and lengthening the pulse width output of the PWM. The longer pulse enriches  the mixture as long as the Start signal is present.	
Idle Mixture Adjustment (IM) - Sheet 3 
The IM sets the mixture (adjustable with IA) during idle operation. The output of the IM is combined with the output of the SC to control the voltage threshold in the PL for turn-off of T201. When the throttle is open, the IM appears as an open circuit and the SC controls the PL threshold. When the throttle is closed and the IS is on, engine speed is low and the IM controls the threshold.  T701 acts as a current source through R705 and the 5 K adjustment potentiometer (IA). The voltage divider implemented with the potentiometer sets the PL threshold level.  

ECU Waveforms 
TL1 and TL2 are the outputs of the flip-flop in the TL circuit. The edge detector generates a short negative-going pulse (down from +12V) on each rising edge from either TL1 or TL2. This signal goes to the PL, which generates the basic injection pulse, which is modified by the IL for effects from the PWM and the AE (additional AE pulses are not shown here). The IL signal and the TL1 and TL2 signals are inputs to the NOR gates for D1 and D2. When both inputs are low, the injector group for D1 or D2 will be energized. Note that the injector groups fire every other crank revolution. 

Common Questions and Answers

	Q: How do the various ECU's differ?
A: Some of the later 1.7L and 2.0L ECU's do not have the OS circuit. The various ECU's have somewhat different resistor values in various circuits, and some ECU's lack some of the ICM functions. Early 1.7L ECU's did not have an idle adjustment control. I am currently trying to acquire a full set of 914 D-Jetronic ECU's to document the differences.

	Q: Can I replace the MPS with a modern semiconductor MAP sensor?
A: It would be difficult. The MPS inductances are active components in the PL circuit and it would be very difficult to duplicate the characteristics by using a semiconductor MAP sensor. It may be possible to create an analog or digital circuit that could duplicate the exact transfer function of the MPS, the difficulty of this feat is unknown. Additionally, the MPS handles full-load sensing and mixture enrichment. If a MAP sensor were implemented instead, it would require additional circuitry to provide the correct signal at full-load for the ECU to properly richen the mixture. See my MPS document for details on its operation.
 	
	Q: Why does an open or shorted TS2 sensor keep the engine from running?
A: Other than the MPS, the TS2 sensor input is the second most significant input to the ECU in setting the mixture. An open TS2 sensor drives T451 into saturation after the Start signal is removed, causing a very rich mixture that immediately stalls the engine. A shorted TS2 sensor causes a very lean mixture that is insufficiently rich for starting, especially when the engine is cold. See the CTC section above for more details. 

	Q: What is the effect of disconnecting the TS1 sensor?
A: The TS1 sensor produces a fairly weak effect on the mixture, to compensate for the varying density of air with temperature. Removing the sensor will slightly richen the mixture. This is a common mechanics "trick" for an old engine that is running lean due to wear or vacuum leaks. See the PL analysis above for a detailed explanation.

	Q: I want to convert my car to closed-loop operation by adding an oxygen sensor - can this be done?
A: Possibly, but with some caveats. Closed-loop operation of FI systems was necessitated by the use of 3-way catalytic converters. Catalytic converters require stringent mixture control (e.g. lambda in the range from 0.98 to 1.02) for proper operation and low emissions of HC, CO, and NOx. Most 914's don't have catalytic converters and those that do only have single-bed converters with an air pump, which don't require stringent mixture control. A well-tuned D-Jetronic system maintains sufficiently accurate mixture control and closed-loop operation is unnecessary. Additionally, setting up a 914 to run with lambda at 1.00 would be a mistake, as the engine doesn't run well unless the mixture is closer to a lambda of 0.93 (about 13.7:1 mixture). Of course, to do this, you will probably need a broadband O2 sensor (big $$$), as the standard Nerst-cell O2 sensors only provide a step response at a lambda factor of 1.00.

A further complication is that independent detection of the full-load condition is necessary, as all modern FI systems operate at full-load in an open-loop mode, supplying a rich mixture (12.7:1) for maximum power. Since the D-Jetronic full-load system is integrated into the MPS and is not externally accessible, an independent means of detecting full-load and disabling the closed-loop system is required. This could potentially be accomplished by accessing the unused wide-open-throttle contact on the TPS. Note that this requires very accurate setup of the TPS and accelerator mechanism to work, and was discarded by Bosch as a means of detecting full-load in the D-Jetronic system early in its implementation.
  
Regardless, it should be possible to implement closed-loop control by developing an interface circuit that would integrate an oxygen sensor into the TS2 connection to the ECU, with full-load sensing from the TPS to disable closed-loop operation at full-load. I am not aware of anyone who has done this or exactly how it could be accomplished.
 	
	Q: How can I change my mixture only at idle?
A: Idle mixture can be adjusted by using the idle mixture adjustment knob on the ECU. Note that some early 1.7L ECU's did not have an adjustment knob. Clockwise clicks richen the mixture, counter-clockwise clicks lean the mixture. The factory made a mark on the plastic surround of the knob to indicate the "factory" setting, the correct mixture for proper operation is usually within +/- 3 clicks of this position. I suggest that you use an air/fuel meter to adjust the idle mixture to the factory setting of CO. If the proper setting of CO cannot be achieved within the range of the idle mixture adjustment knob, then it is likely that there is a problem with the FI system - mismatched FI components (see my D-Jetronics Parts Document for assistance), failed FI components, or a maladjusted engine (valve clearance and/or ignition timing and dwell, other ignition faults, vacuum leaks, etc.). From in-car road measurements (using a Heathkit Air/Fuel meter) on my 2.0L with 1974 FI components, the idle mixture is about 13.3:1 (lambda of 0.90) - this is a guideline value only, I have not measured the full-load mixture on other 2.0 or 1.7L cars in good tune. 

	Q: Does the idle mixture adjustment knob on the ECU only affect the idle mixture?
A: Yes, with one exception. It has no effect unless the throttle is closed and the idle switch in the TPS is on, indicating the idle condition. For ECU's with the OS circuit, note that when the engine speed drops below the threshold level of the OS operation, the over-run mixture is partially controlled by the idle mixture knob position. If you have an ECU with the OS circuit, and you have light backfiring during over-run, adjusting the idle mixture knob to a slightly richer mixture (1 or 2 clicks clockwise) may help.
 	
	Q: How can I change my mixture only at part-load?
A: The MPS can be adjusted to set the part-load mixture. See my MPS Document for details. "Tricks" such as adding ballast resistance to the TS2 sensor and disconnecting the TS1 sensor generally affects the mixture across all conditions, not just part-load. From in-car road measurements (using a Heathkit Air/Fuel meter) on my 2.0L with 1974 FI components, the part-load mixture is about 13.6:1 (lambda of 0.93) - this is a guideline value only, I have not measured the part-load mixture on other 2.0 or 1.7L cars in good tune. 
 	
	Q: How can I change my mixture only at full-load?
A: The MPS can be adjusted to set the full-load mixture. See my MPS Document for details. Again, "tricks" such as adding ballast resistance to the TS2 sensor and disconnecting the TS1 sensor generally affects the mixture across all conditions, not just full-load. Use of a rising-rate fuel pressure regulator can also be used to richen full-load mixture, but may result in injector spray problems if the maximum fuel pressure exceeds 35 psig. From in-car road measurements (using a Heathkit Air/Fuel meter) on my 2.0L with 1974 FI components, the full-load mixture is about 12.7:1 (lambda of 0.86) - this is a guideline value only, I have not measured the full-load mixture on other 2.0 or 1.7L cars in good tune. 

	Q: How can I tell if my ECU is working properly?
A: A good  way that is accessible for many owners is to use a VW1218 tester (available for rental from Pelican Parts). This tester can verify the operation of the following ECU functions: 
	
Engine speed correction	

Basic compensation rich	

Basic compensation lean	

Engine speed correction, full load enrichment	

Acceleration/full load enrichment	

Start & warm-up enrichment	

Fuel pump control	

The VW1218 also does comprehensive testing of the FI sensors, including the MPS. Note that the Bosch EFAW tester also available for rental from Pelican Parts only provides rudimentary ECU testing. 
In lieu of using the VW1218, a suspect ECU can be swapped with a new/rebuilt ECU. Note that there is also an aftermarket D-Jetronic tester that was available about 20 years ago called the "MPC Analyzer". A few shops still have this tester, which is an advanced tool for evaluating the ECU operation. It can also be used to adjust the MPS to factory settings. John Larson on Rennlist has one of these testers at his shop in Central California, please contact him for further information. Jim Thorusen on Rennlist also has developed his own D-Jetronic tester that can be useful in testing the ECU. This tester is available for purchase, please contact him for further information.
 	
	Q: Which is the right ECU for my car? I have a different ECU from the one that came in my car, will it work?
A: See my D-Jetronics Parts Document for information. Surprisingly, ECU's from different years and displacement engines often can be substituted with little or no problems - and sometimes they cause very rich or very lean mixtures. If you feel like experimenting, give it a try, but keep an eye out for mixture problems over the full range of operation. 
 	
	Q: I want to turbocharge my D-Jetronic car - can the ECU support this?
A: Amazingly, yes! There is at least one D-Jetronic turbocharged 914 that I'm aware of (Jeff Shyu on Rennlist), and it seems to be working, despite some pretty interesting stuff that sounds like to me will eventually blow it up (e.g. 60 psig fuel pressure from a rising-rate fuel pressure regulator through stock injectors, with large exhaust flames during full-load operation, vacuum switch for activating the CSV to provide enrichment at full-load, etc.). The ECU does not seem to be a limiting factor here, but personally, I'd go for an aftermarket FI system or carbs if I were turbocharging a 914. My MPS Document has some additional information in the tuning and modifications section. 

	Q: My fuel pump isn't working, I think the ECU may be bad. How do I debug the circuit without renting one of those fancy testers?
A: Here is a link to a page I've created with a set of flowcharts to troubleshoot the fuel pump circuit.
  	
	Q: I'd like to use a high-lift, long-duration cam to improve my engine's performance. People have told me that this won't work with the D-Jetronic system. Why?
A: First off, I can't say for sure that it won't work, but there are a couple of reasons why it might be a problem. First, when you start to get a significant amount of valve overlap (intake and exhaust simultaneously open), there are significant intake manifold vacuum pulsations. The MPS responds very quickly to manifold vacuum - how fast? Nobody really knows, it needs to be characterized. If the MPS does respond to the fluctuations, it's likely that the MPS response to the engine load will be incorrect. A more significant problem relates to the speed correction circuit of the ECU. Significantly changing the basic cam parameters (timing, duration, lift) will change the shape of the volumetric efficiency (Ve) curve. Since the SC is hard-wired to correct the mixture for the stock Ve curve, a "hot" cam will likely result in a non-linear effect on the mixture - possibly low and possibly high at varied engine speeds. Note that none of this has been actually verified by measurements on a real engine, and that there are reports of people having success in using "mild" changes to the stock cam grind while retaining the stock D-Jetronic setup.

	Q: OK, if I can't use a hot cam, what kind of modifications can I do to improve the engine's performance and still retain the stock D-Jetronic setup?
A: Basically, anything that you can do that doesn't change shape of the Ve curve, doesn't change the linear response of the MPS, or doesn't exceed the fuel delivery capability of the injectors (i.e,  exceed the injector duty cycle and maximum fuel pressure (36 psig)) should be OK. These modifications (also see the turbo discussion above) include; increased displacement (limiter: fuel supply), increased compression ratio (limiter: knock threshold), free-flow exhaust, bored-out throttle body, larger valves, and other head modifications. All of these types of modifications (within limits) should be within the limits of the ECU to handle, and the increased fuel demands can be handled by adjusting the MPS and/or by increasing the system fuel pressure (to a max of 36 psig - beyond that pressure the spray pattern degrades into a stream). See my MPS document for a description of how the MPS can be adjusted. 	
Acknowledgements
Special thanks to Jim Thoursen, Dave Darling, Roland Kunz, and others on Rennlist who have assisted me with this analysis. Big thanks to Frank Kerfoot for the excellent work he did on the schematic and his assistance with the analysis. Big thanks to "Luau_BoB", who took it upon himself to analyze the SC circuit, and improve the description of how the SC and IM bias voltage affects the basic injection pulse duration. Thanks also to Randy Montellato for his measurements of the effects of engine speed on injection duration. Thanks also to Jeff Bowlsby for his comments and assistance with some of the measurements.
Changes by Version
	0.0 - Pre-release	
	1.0 - Initial release. Still missing descriptions of the ES, SC, and PWM circuits. 	
	1.1 - Added initial description of the ES, SC, and PWM circuits. Corrected  block diagram to remove ES to PWM control signal - turns out that the connection is only for voltage clamping of the output of the ES, not as an input to the PWM.	
	1.2 - General editing, added acknowledgements section. Also added calculation of the injector drive voltage from D1 and D2.	
	1.3 - Included analysis by "Luau_BoB" of the SC circuit and how the SC + IM bias voltage affects the basic injection pulse duration. Big thanks to him for this contribution!!! Also included measurements taken by Randy Montellato on the effect of engine speed on injection duration, independent of intake manifold pressure. Added additional Q&A questions on performance modifications. 	
	1.4 - Corrected an error in the PL circuit model, added description from "Luau_BoB" on the start-up operation of the FPC	
	1.5 - Corrected and improved the D1 & D2 section to accurately describe the circuit behavior to explain the observed injector voltage waveform.