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SacSantorin
07-10-2018, 06:51 PM
Hi guys

I could use some advice as how to proceed with an issue. I'm pretty strong with working on the car mechanically, but electronics are foreign and scare me. I'm having a no start issue after a bad jump where the donor vehicle got the leads crossed. I currently have no spark or fuel, and can't talk to the ECU with vagcom or my handheld scanner. The check engine light is not coming on with the key on engine off position which is telling me there is an issue with the ECU. I'm also not getting any power to the ECU & fuel pump fuses in the driver side fuse box (fuses 28, 29, 32, 34) when testing with a multimeter with key on engine off position. I have power to other fuses in the box.

My first thought was either the ECU relay or the Fuel Pump relay, but after throwing parts at that it seems to be something else. Any ideas on another relay or fuse that could be causing this? I've done some searching but haven't found any similar issues with not having power to the fuses. It's a 2000 ATW B5 A4 1.8TQ.

Thanks in a advanced!

scaphan
07-11-2018, 12:13 AM
So you put the + to the - and the - to the +?

Did you check all the fuses? If they are all good then you’ve probably fried the ecu.

Do you have access to another ecu to rule that out?


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walky_talky20
07-11-2018, 04:25 AM
Sounds like you are on the right track. You have only a few possibilities:

A - ECU is not getting constant power / ground
B - ECU is not seeing the key turn on (ignition switch)
C - ECU is getting everything just fine, but it is smoked (faulty ECU)

There are just a few things to do to check for these:

**Step 0: Pull an accurate Engine wiring diagram so you aren't flying blind here.**

A - Unplug the ECU and check that all power and grounds are present
B - Unplug the ECU and check that ignition switch power signal is present
C - If A and B check out fine, it's C. Swap in a used ECU for $50 and give it a shot.

mobildetroit
07-12-2018, 09:58 AM
For the most part you are shorting out your battery when you cross jumpers like that.
First thing to do is disconnect the battery from the car, charge it, then do a load test to make sure the battery is still good.
Maybe voltage is not high enough to turn the ECU on because you melted a few plates inside your battery.
If it's good then check all your fuses.
If you are lucky you just have some blown fuses.

If you are not lucky you may have a dead ECU and/or alternator (may no longer charge if regulator was blown).
If you are really really not lucky you fried the computer in your dash cluster.
If you are really really really not luckly you fried wires in your wiring harness (Audi likes to use small gauge wires...)....and you won't be able to get a replacement from Audi....because you could buy a new used Audi for the cost of the new harness.

SacSantorin
07-14-2018, 09:07 PM
Thank you everyone for your insight and advice. This community has always been incredibly supportive and has a wealth of knowledge on these cars and one of the main reasons I will always own an Audi.

At this point, I'm thinking that the ECU might be fried. I'm currently trying to source a used cheap ECU to swap in and test it, but since I bought a new car today (not because of this problem, I've been shopping for the last few months) so this is no longer my daily and I can take my time chasing this problem. I'll be sure to update this thread with anything I find for people with similar issues in the future.


Thank you all!

walky_talky20
07-16-2018, 01:21 PM
Do you have an engine diagram so you can verify power and grounds to check at the ECU location? If you need one, just drop me a PM with your email address.

walky_talky20
07-23-2018, 04:36 AM
Sent diagrams. [wrench]

Fasterd
07-24-2018, 07:41 PM
Hate to be the bearer of bad news but if a main fuse isn’t cooked then your ecu is.

SacSantorin
07-25-2018, 04:50 PM
That's what I'm thinking. I've got a new ECU coming on Friday to test.

Fasterd
07-25-2018, 07:34 PM
pulling for you bruv [race]

SacSantorin
07-26-2018, 07:05 PM
Well, I swapped in the new ECU today and it made no difference. I'm done playing parts darts with it so hopefully Saturday I can dig into the cause of this. I'm thinking there may be a short somewhere in the wiring harness that is causing power not to make it to the ECU.

Fasterd
07-27-2018, 02:54 AM
Fuses all checked for sure? And relays?

SacSantorin
07-27-2018, 07:21 PM
I replaced the ECM and Fuel Pump relays with no effect. Checked fuses 28, 29, 32, and 34 and were all good. Fuse behind the radio blew but I replaced it and the radio now works.

SacSantorin
07-28-2018, 02:20 PM
Sounds like you are on the right track. You have only a few possibilities:

A - ECU is not getting constant power / ground
B - ECU is not seeing the key turn on (ignition switch)
C - ECU is getting everything just fine, but it is smoked (faulty ECU)

There are just a few things to do to check for these:

**Step 0: Pull an accurate Engine wiring diagram so you aren't flying blind here.**

A - Unplug the ECU and check that all power and grounds are present
B - Unplug the ECU and check that ignition switch power signal is present
C - If A and B check out fine, it's C. Swap in a used ECU for $50 and give it a shot.

So I checked power and grounds for the ECU today. I used the below diagram as a reference and it looks like I am not getting power to pin 3 & 21 in the ECU connector. I a actually getting a ground signal to pin 3. I have power at pin 62. Does anyone have a pin out for what these pins correspond with? Could this be an ignition switch issue?

77861

SacSantorin
07-31-2018, 11:14 PM
Bump. Hoping someone has some insight here.

scaphan
08-01-2018, 12:32 AM
No power to pin 3 and 21 is “good” news I guess. It means that your ecu could still be alive but there’s a power supply problem.

Do you have a wiring diagram? You can then trace where the power comes from. All the fuses are working right?


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walky_talky20
08-01-2018, 04:38 AM
Good work.

Pin 21 is not really an "applied" power to the ECU. It is actually a ground signal output of the ECU to engage the Motronic ECM Power Supply Relay (sits next to the ECU in the plenum box). So the wire at pin 21 does go to the battery, but it goes through that relay coil to get there. If the relay coil is open, then that will cause a no-start.

Pin 3 should be "Hot in Run or Start" positions of the key. It comes through Fuse #7 (10 amp). So check Fuse 7 to make sure it is intact. And check for power on both sides of Fuse #7 with key in Run and Start positions.

I would check into the Pin 3 problem first, then go to Pin 21 only once Pin 3 is all good. Due to the Relay coil attached to Pin 21 (and possibly a flyback diode on the relay coil), you may get unexpected measurements on that pin, depending on how you test.

SacSantorin
08-02-2018, 03:04 PM
I checked fuse 7 this morning before leaving for work. It looked fine but when i tested with a multimeter it was bad. Ill grab a new fuse on the way home today and hopefully that does the trick. Thanks for the continued support guys.

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SacSantorin
08-02-2018, 08:29 PM
Thank you to everyone for your support and advice with this problem. Happy to say, she started up after replacing the fuse!!!!
78770

Fasterd
08-03-2018, 04:05 AM
<<< this guy said fuse.

walky_talky20
08-03-2018, 07:57 PM
Several people did say to "check fuses". The problem is that peoples definition of "checking fuses" can be different, and some methods (like visual inspection) can be inaccurate. That was one of the issues in this case. Key takeaway: check fuses on both sides with a test light or multimeter to be sure.

SacSantorin
08-05-2018, 08:56 PM
Several people did say to "check fuses". The problem is that peoples definition of "checking fuses" can be different, and some methods (like visual inspection) can be inaccurate. That was one of the issues in this case. Key takeaway: check fuses on both sides with a test light or multimeter to be sure.

Absolutely, several people did say to check fuses and I really hate that this turned me into "that guy". Like I mentioned in my initial post, I really don't understand electrical, and even after this I still don't have a clue. The problem here was that I didn't understand that fuses other then ones labeled as "ECU" could be effected. I only checked the ECU & Fuel Pump fuses, and when asked whether I checked all of the fuses, thought they meant 28, 29, 32 & 34. I really appreciate all your advice and help with this problem, and apologize for misinterpreting what you all were telling me.

Fasterd
08-06-2018, 10:15 AM
I was having fun, I’m not that tight butthole as to need credit