D-Jetronic articles series by Dr-DJet

Chapter 3 of D-Jetronic article series by Dr-DJet describes function, repair and tuning of the brain of D-Jetronic: The Engine Control Unit (ECU). All sensors are used here to calculate when and how long to open the fuel injectors. Opening times determine the amount of fuel injected.

 

The engine control unit (ECU) is the brain of the injection. It is an analogue computer, that does not rely on digital signalling but rather analogue threshholds with a pulse width modulator (PWM) being its heart. It is therefore sensitive to EMC disturbance and needs preselected components and a manual trimming to work reliably. After Bendix's experience with drifting und unreliable components Bosch even used MIL-spec components. And that has payed off, as this electronics is despite of its age of 40 years very reliable. If it fails, usual tricks like replacing all electroytic capacitors, will not work. The ECU consists of a main PCB and an additional auxiliary PCB screwed on to it and connected via cables. 2 or respectively 4 power transistors, a power diode and high power series resistors are mounted on heat sinks. These heat sinks can be mounted internally or externally. The ECU I show here is from an MB 8-cylinder engine. The intake manifold pressure (MAP) sensor is integral part of ECUs electronics. That is why there is no way of replacing broken D-Jetronic MAP sensors with today's cheap silicon MAP sensors. Treat you ECU like raw eggs as they get more expensive every year. Some hints:

 

3.1 ECU connector

Not much is standardized in D-Jetronic connectors, but all of them have a 25 pin AMP connector with a spacing of 5.0 mm (not 0.1 oder 0.2 inch), that connects them to engine wiring harness. Looking on that ECU connector, pin 25 is on the right side where wiring harness exits ECU, pin 1 on the left. Pin 25 and for 8-cylinders also pin 2 are diagnosis contacts that allow to monitor the drivers for injectors. They are not connected to motor wiring harnesses with the exception of MB 450, where pin 2 is also used for full-load contact. All other pins are not standardized, that's why you need a connector pinout for every car, partially even changing with year of make.

Earlier I had a pinout of Mercedes-Benz and Opel ECUs attached here. I have moved those tables to Appendix B: Pinout of ECUs by car manufacturer together with pinout of every D-Jetronic car.

3.2 Construction of ECU

The ECU is completely built with discrete components like transistors, diodes, capacitors and resistors and mostly a potentiometer. MAP sensor's coils are integral part of this electronics. That is why you sometimes face rules that a successor ECU can only be used with a successor MAP sensor. Today many of these components are not available any more and have to be replaced by modern components with suitable characteristic curve and size. The ECU is mounted in a zinc coated steel box that is only open towards engine harness connector. Lid is secured by metal latches which brake off easily and is sealed by a rubber ring. If ECU is placed in engine compartment its opening is sealed with a rubber cover over engine harness. If this rubber is broken, you have to replace it immediately. Especially if ECU is mounted in vertical position, water will collect in ECU and damage it. In MB W108/W109/W111 this tends to happen often. The advantage of this solution is easy access for measuring. Sometimes ECUs are also subject to water if a windshield sealing is broken and they are located behind glove box. As ECU and MAP sensor are placed differently in each car model, engine wiring harness is not only specific to the engine but also the car model.

The ECU has two boards, a main board and an auxiliary one. Both have a single layer coating on both sides and it seems that this layout was done manually. Rumours say that some ECUs were potted, but I have never seen one. Components on the PCB often have smaller tolerance than default or are of MIL-specification. Bosch is said to have pre-selected components and marked them via colour. First tests with manual assembly at Blaupunkt failed and Bosch built up its own line in Reutlingen. PCB boards are not always populated completely. Additionally assembly is not the same on each PCB, even it it carries same number and production time. Due to tolerance of components, tuning of each function block had to be done on a test stand by tuning resistors in order to reach reference values for each function block. Those resistors are not directly mounted on PCB, but rather on a kind of bridge to make them easily exchangeable. Totally an 8-cylinder ECU consists of 50 transistors, 70 diodes, 35 capacitors and 260 resistors. In later versions heat sink for power components in injector driver is attached to the box externally with a sealing. While early models for Volkswagen only have internal heat sink, MB's 8-cylinder have 2 external heat sinks for 4 transistors, a diode and 12 resistors. Surprisingly, this electronics is running over more than 40 years. But maybe you do not immediately notice a drift in a function block as deviations have to become big before you notice a bad idle-run or high fuel consumption. Compare to the unreliability of cruise controls or fuel pump relays in K-Jetronic!

You might find an electronics friend who will detect a broken component by measuring all components and will be able to replace that component. No matter which component: Transistors, diodes capacitors and resistors have tolerances of up to 20%. An analogue electronics works with thresholds and amplification factors that will change when replacing a component. Often original components are obsolete and you have to use replacements. By moving thresholds your ECU will still principally work, but it might do different corrections in a function block than you desire. That is how you risk your engine or can have an enormous fuel consumption. All function blocks have to be checked against reference values and re-tuned if necessary after a repair!

3.3 ECU functional overview

Remember that during 60s mechanical fuel injection pumps were linked directly to throttle opening via a mechanical linkage. Fuel pumps learned throttle opening and engine revolutions and dimensioned necessary fuel by reading a 3D-cam. To be complete in my statement, I mention that there was also a correction for altitude and engine temperature. But if something went wrong with the linkage between throttle and fuel pump, all mixture would be wrong.

D-Jetronic ECU was the first (commercially successful) injection to automatically adapt to engines load and revolutions to always give best performance, torque and least emissions and fuel consumption. Following parameters were measured for that:

1. Air volume sucked in, indirectly by measuring intake manifold vacuum
2. Air mass sucked in by compensation air volume with air temperature and altitude compensation (only late models) in MAP sensor
3. Engine revolutions by reading trigger points in distributor
4. Idle-run by reading idle-run switch in throttle switch
5. Full-load by reading either full-load contact in throttle witch (late models), having a full-load diaphragm in MAP sensor or an differential pressure sensor in intake manifold (very early models).
6. Acceleration when driver pushed down gas pedal
7. Warm-up run by engine temperature
8. Intake valve opening by reading trigger contacts for injector groups in distributor

For sure you noticed that there is neither a closed loop lambda control nor an emission control, just an adjustment of idle-run CO emissions is possible. That is why bypass air is the biggest enemy of every injection. The ECU calculates out of all possible engine states an injector opening time (about 1 to 10 milli-seconds) and opens injectors of respective injector group together. Injector groups follow ignition sequence. As fuel pressure is constant, opening times determine the amount of fuel injected before intake valve.

3.4 Testing ECU

Before you are suspicious about your ECU, you have to test everything else and also voltage between pin 11 (ground) and pin 16 and pin 24 should be at least 11.5 V. Unfortunately you cannot really test an ECU by installing it on an engine and for sure not with well known Bosch EFAW 228 D-Jetronic tester. You have to test it by simulating sensors and thus varying engine states externally. Then you test each function block by comparing resulting injection times with your references (incl. tolerances). I have now built up these references for every ECU.

Testing all function blocks is as important as verifying correct Bosch numbers on components. There is a dedicated ECU for each engine. During idle-run injector timing is often identical for various engines, but they will deviate more clearly for other engine speeds and loads. Don't get fooled by someone demonstrating to you how well your car runs with his ECU in idle-run or that he has tested it in a different car model! So called specialists have claimed that they can test everything ECU in their 8-cylinder engine. Read appendix B with ECU pinouts and you will notice that his is BS. And if you don't notice any difference let me show you the cost for a possibly higher fuel consumption of only 15%. When only riding 5000 km annually you will have additional costs of several hundred US dollars. A leaner mixture can cause a serious motor damage causing several thousand USD costs. It does not pay off to save on a proper ECU testing!. Let me also tell you that 8-cylinder ECUs have several doubled function blocks in parallel. These need to be synchronized. Your engine will run badly if both are within tolerances, but one at upper and one at lower limit.

3.5 Repair and tuning of an ECU

As a warning: Mostly ECUs are considered broken because a garage does not find the real problem in motor wear, valves, ignition, fuel supply and D-Jetronic. 2 thirds of all ECU I have tested were correctly working, only those that people bought on eBay had higher failure rates. I also had cases where someone tried to repair himself and caused a short-cut with major damage. If you are not sure whether your ECU is working, first try with a friend's ECU whether the problems on your D-Jetronic are still there. Or send it to someone who can test ist properly. That will save you a lot of anger. I do not know anyone besides Bosch itself or Bosch Classic Services who can properly repair and test such an ECU.

If an ECU shows deviations from references or failures during a run on a test stand, it has to be repaired and possibly re-tuned. Just replacing a broken semiconductor or resistor often demands re-tuning it. Partially Bosch has also changed tuning of ECUs during year of make to increase richness in warm-up run or idle-run. And they did not always use a new Bosch number. Most extreme is the case of BMW. 0280001007 has a colourless, a blue and a red version that can only combined with a MAP sensor of same colour. Also VW and Citroen have such setups. I prefer MB where they always used a new number in such a case. Replacing of components can be really time-consuming, especially on 8-cylinder ECUs where one has to weld off all power resistors, power transistors and the power diode before the basic PCB board can be removed. Many transistors do not just operate as a switch but as an amplifier. Most functions in ECU are based on realignment of thresholds, that are adjusted by 41 resistors. This is also the way how it was originally done by Bosch. That is time consuming but necessary to have proper engine mixture in all states with optimal fuel consumption and emissions. Once I even had a case where the original tuning by Bosch in 1975 was misaligned. Fortunately that is very seldom the case. That is why the charming electronic friend replacing a component is not really the cheapest solution. It is a long lasting task to record and analyze a wiring diagram But it takes even much longer to understand tuning. Do me a favor and always ask if your workshop can also test against each function block reference and do a re-tune if necessary! To my knowledge that is only Bosch itself and some Bosch Classic services. You will not find such service providers on eBay. 10% higher fuel consumption quickly eats up cost advantage of a cheap repair.

A little warning of false repairs

You will find people offering you a refresh of your ECU or improving it by swapping it to y higher number ECU. They tell you they can refresh it by resoldering all solder points. Cold soldder points are however very seldom on these ECUs. A complete resoldering leads to forcing a lot of heat into these semiconductors. That lets them age quicker and I reommend you not to do so! I have even seen ECUs where this has caused short-cuts with strange effects.

Some also recommend to swap your ECU against one with a higher no. This should improve part-load via better electronics by swapping a 0 280 002 004 against a 0 280 002 007 ECU for 1'850€. This is wrong as both ECUs share the same electronics. Only housing is slightly different with 3 additional bolts and 2 lugs. 0 280 002 007 was built for Mercedes-Benz W116 350 engine, 0 280 002 004 for same engine in R/C107, W108, W109, W111 (and also fits W116) so that this ECU could be fixed easily to chassis. By swapping you loose this fixing possibility and gain nothing in engine run. If you feel that you have 1'850€ too much, donate them for operation of this web site!

Unneccessary to say that testing and repairing an ECU is a matter of trust. You gain nothing if your service supplier declares every ECU broken and you still don't know what's wrong with your D-Jetronic.

3.6 Injector driver

4 and 6-cylinder ECUs have 2 drivers, triggered by contacts in distributor. 8-cylinder drivers have 4 drivers with 4 contacts in distributor. Drivers directly control the injectors via high performance resistors of 6 Ω. These resistors only forward the direct injection pulse to the injectors. It is important to know that these drivers have a direct power supply through pin 24, while the ECU itself is supplied through pin 16. That means that the ECU may still work but not inject if power to pin 24 is missing.

Jaguar's 12 cylinder XJ12 and XJ-S are very special. Lucas finished that system only in 1975 for Jaguar. As trigger contacts would not have fit in their own ignition distributor, Lucas developed a board with 2 reed relais or later with hall sensors instead. In principle that is the same as the old trigger contacts, only more modern and without wear (but not withour problems). In order to control 12 injectors, Lucas installed an external driver, called injection amplifier. This receives signals from ECU and does inject in 2 groups of 6 injectors each. A further specialty of this system is emission gas reduction valves that are controlled by another electronics and a full-load contact in throttle switch that only existed in the US.

3.7 Diagnosis contacts

Every ECU has at least one diagnosis contact not connected to engine wiring harness. Normally that is pin 25, 8-cylinders also have a second one connected to pin 2. Here you can monitor the input to the driver before over-run shutoff and injector group selection is applied. You can watch this signal on pin 25 with an oscilloscope. The 8-cylinder also uses a second signal, on MB 450 pin 2 has a double usage as input and output. Input in that case is full-load contact.

3.8 Box variants of Mercedes-Benz

Mercedes-Benz has added an additional difference to its already high number of electronics (450 engine has 4 different ECU electronics, especially for the US): Housing for models R/C107, W108, W109 and W11 have additional 3 bolts and latches to fix the ECU in the car. W114 and W116 did not know such bolts and latches.

As I write in Attachement A, electronics in this case are absolutely identical, but Bosch has given them different numbers to both housing variants. So you can use them still, all you have to do is fix them with cable binders or similar. W114 and W116 do not know such additional fixings.

Additionally ECUs and MAP sensors of Mercedes-Benz have a color coding.- An "E" stands for Europe variant, "A" for the American variant with lower engine compression. Please do not mix this with even lower compression cars "NV" for countries like gulf states. Those cars were again equipped with standard components. Additionally there is a color coding similar to Bosch's injector coloring. Green is for 250 M114, white 280 M110, yellow is 350 M116 and blue is 450 M117. In summary that means small colored labels showing E or A on ECU and SDF, that often also fall off.

3.9 List of all ECUs with usage and compatibility

I started a table for myself with all ECUs in order to identify them, their usage and compatibility with MAP sensors. Then I decided to share it with all of you. Registered users can even read, whether ECU has over-run shutoff (1 in column Schub), an idle-run potentiometer (1 in column poti) and full-load enrichment via differential pressure switch ("D") or full-load contact ("K"). They will also see possible replacements. Especially in VW/Porsche this is a real puzzle. They have created many replacement ECUs and MAP sensors. But you have to be careful what to combine and have to change thermo switches if necessary. I do understand that VW workshops were unable to cope with that, especially as they mainly had carburettor cars. I have also added how to set the selector switch on VW tester 1218 or AES 000050. So it is not an issue of the year of make of the VW car but of the ECU number. VW always has 2 Bosch no.s, one for the version with Bosch no and one for only VW no. One can make its life so easily miserable! In somce cases you will find an ECU date of make (ECU). That is the start date when this version was implemented by Bosch. Hopefully you will still find that 3 digit no. on your ECU. As long as you do not have changing colour labels on your ECU in parallel, these changes are normally not important. But with VW, Citroen and BMW you have to be very careful.

Acceleration enrichment is always present except in first 2 VW ECUs.

What is still to be done?
References for 2 ECUs are still missing: First VW ECU 0 280 000 001 and US BMW ECU 0 280 001 013. If you own one of these, please be so kind to let me measure references on them. And I have data for a 0 280 000 024 ECU where I do not know its usage. Who knows in which car it was used?

 

Hint for searching:
I have entered all ECU no.s without spaces. Search is smart. Entering 0 280 000 001 will actually search for 0, 280, 000 or 001 and will filter all rows with any of these strings. You could also use "Lancia" as a search string. But you can also sort the table in each column.

How to read date of make FD:
This date is a 3 digit symbol. It is often punched into ECU and MAP sensor, but sometime it is just stamped and after 40 years you cannot read it any more. Its meaning is as follows:

 

3.10 References

Porsche 914 owners have an excellent website by Paul B. "Brad" Anders. Earlier I learned a lot from this website. Unfortunately ECUs for 6 and 8-cylinder are mostly different from Porsche ones. Even the 8-cylinder ECUs of 350 and 450 differ a lot, both in layout and functionality. That is why recording and analyzing a wiring diagram is always a long-lasting task to be done for each new ECU. I always recommend to have Bosch's service manual for your engine. It is worth the money and describes every pinout, every cabling and a lot more.

Your Dr-DJet

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