Classic Audi Technical Mech/Tech Electrical problems » Request: 3A Engine ECU Connector Pinout

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Old 22-02-21, 12:27 AM   #1
thunderpumpkin
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Default Request: 3A Engine ECU Connector Pinout

Hi all, new member here and apologies in advance for the long read...I've tried to summarise my diagnosis so far! I drive an Audi B3 Coupe 2.0 with the 3A engine 1990 UK model (it's been in the family since the 90s) and I'm hoping that someone may have a pinout for the 35-pin 3A Motronic connector.

I dived into trying solve some idling issues following a leaking DPR which was replaced. Also replaced the following with matched part numbers and genuine Bosch:

1. Coolant temperature sensor (old one was over 15k Ohms at room temperature)
2. Oxygen sensor (old as the hills)

Further testing revealed the oxygen sensor is not receiving power to the sensor heater, the idle microswitch has high resistance with a closed throttle and the ISV does not receive a voltage which is probably explained by the failed microswitch not telling the ECU to idle. Vacuum leaks should not be a problem as I've smoke tested and found the injector holder o-rings needed replacing, one holder is cracked but a replacement is on its way.

After replacing the DPR, coolant temperature sensor and oxygen sensor the car would start idle at around 1,000rpm and then settle to just above 800rpm. While all seems okay, the DPR current fluctuates from 6-15mA not 0-5mA and there is no power going to the oxygen sensor heater which would explain the problem. I am not able to get a sensible voltage from the oxygen sensor either, the voltage is around 5mV which would suggest a very lean condition. I've also managed to pull error codes from the 2x2 diagnostic connect under the steering wheel, 4431 which suggests an ISV problem or a break in the cable.

Also since trying to go through the cabling, I've gone from having a steady idle to the the car not being able to idle at all. I had richened the replacement (second-hand) DPR screw since it was not able to idle when warm. Yes, perhaps I should have not tinkered but I also have a couple of Mercs with KE-Jetronic and I've gotten curious about the system so hence the tinkering

Since I have a number of electrical issues with the engine harness, I'm hoping someone may have (or point me in the direction) the pinout of the connector for the 3A Motronic ECU so I can at least rule out a problem with the harness as no power is going to the oxygen sensor heater and the ISV.

Thanks!
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Old 22-02-21, 02:04 PM   #2
mikes2
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Welcome,

I may well have wiring diagrams for this car rather than the ecu pin outs. I'll have a look later this evening.

Something you may want to check - N71 ISV normally goes through a fuse to the fuel pump relay. Check you have this continuity and check your fuses. The ISV code is almost never the ISV itself - there's no control signal so either the fuse, the fuel pump relay or the wiring has a fault.

Also, whats a DPR ?

For example, this is for a 16v ACE but the circuits will be the same

http://s2-audi.co.uk/workshop/schema...6v/16v_ace.htm

Lambda probe heating and N71 ISV are both linked to the fuel pump relay through fuses
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Old 23-02-21, 05:18 PM   #3
thunderpumpkin
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Quote:
Originally Posted by mikes2 View Post
Welcome,

I may well have wiring diagrams for this car rather than the ecu pin outs. I'll have a look later this evening.

Something you may want to check - N71 ISV normally goes through a fuse to the fuel pump relay. Check you have this continuity and check your fuses. The ISV code is almost never the ISV itself - there's no control signal so either the fuse, the fuel pump relay or the wiring has a fault.

Also, whats a DPR ?

For example, this is for a 16v ACE but the circuits will be the same

http://s2-audi.co.uk/workshop/schema...6v/16v_ace.htm

Lambda probe heating and N71 ISV are both linked to the fuel pump relay through fuses
Hi, thanks for the welcome and much appreciated if you have a chance to have a look! Another forum member kindly linked me a ECU pinout diagram for KE-Motronic which should solve my problem ECU pinout problem. But if you do have wiring diagrams for my Coupe, that will save me many hours of trial and error!

Yes, your suggestion about the ISV was something that crossed my mind as I have cleaned the ISV and tested it for function. I'll trace the circuits for the oxygen sensor heater and ISV and check the relay is doing the job is supposed to do! I replaced all the fuses a couple of years ago but notice over the weekend the right of the two "Motor" fuses had fused but the engine was still able to start.

The DPR is the differential pressure regulator with a VAG part number of 034133482 (for engines AAD,3A,6A & ACE). It's black Bosch unit on the side of the fuel metering head. My basic understanding of it is that it makes small adjustments to the richness/leanness of the fuel being fed to the cylinders. I have since discovered there may be two versions of the DPR and my problem may be from using the wrong version. I sourced a grey version to replace my black DPR, I've read that the black version is designed to work by fluctuating from 0-5mA to achieve the ideal mixture. Whereas the grey version works by fluctuating around 10mA. Each time my Coupe stalls, it occurs as the current to the new (old) grey DPR goes to 0mA.

Last edited by thunderpumpkin; 23-02-21 at 06:47 PM.
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Old 23-02-21, 11:06 PM   #4
mikes2
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It's your lucky day. Link to follow
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Old 23-02-21, 11:56 PM   #5
mikes2
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This should help

http://s2-audi.co.uk/workshop/schematics_3a/3a.htm

If you need assistance de-ciphering, feel free to ask
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Old 24-02-21, 12:45 AM   #6
thunderpumpkin
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Quote:
Originally Posted by mikes2 View Post
This should help

http://s2-audi.co.uk/workshop/schematics_3a/3a.htm

If you need assistance de-ciphering, feel free to ask
Cheers! That's amazing, thank you! I'll happily send you some beer money too for getting those uploaded! I'll have a read over of everything and let you know if something doesn't make sense

Just a quick question, I've just noticed that my grey fuel pump relay marked 215 with a part number of 443951253L has a fuse inserted in the fuse holder. Does that fuse need to remain in there permanently...I thought it was only when pulling blink codes? It looks like it's been in there for years!
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Old 24-02-21, 01:50 PM   #7
mikes2
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No worries - happy to assist.

The slot in your relay should be empty unless you want to enable diagnostics.
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