Good evening all,

I am at my wits end and would love some input or troubleshooting advice. I have a 2017 GS A8 that I recently put a new ECU in (physical damage to the old one, my fault). The car drives fine when it will start.

I have started the car sometimes 15 times in a row successfully then the next time, nothing (yes, I was patient to not deplete the battery). Sometimes the car will not start, I’ll press the start button and it’ll turn green and the DIC will act like the car turned on but the starter never spins. It does throw 2 codes however, p0615 p0617 (starter relay circuit and starter relay circuit high). Once this no start state occurs, I can typically let it sit for 30+ minutes, come back and it will fire right up.

I swapped the starter relay, no dice. My battery is from 04/24 and was rated at 87% charge from Autozone and I even have a chargeable jump box thing that had no effect. I have crawled under the car and tried to remote start (I am alone) it while tapping the bottom of the starter with a hammer, no dice. I am fairly certain that my starter relay is clicking when I try to start the car, I used my breaker bar as a stethoscope on it and hear her click. This would leave me to believe that power is being delivered to the starter but I am not positive of that as I do not know the exact flow of power to the starter and what might inhibit it. I will note that I do not hear any signs of life from the starter when I remote start it, no click or hum or anything.

My next steps were gonna be new battery and new starter. It is difficult to test the starter when the car won’t start because it is generally really hot by my cat and is difficult to get the plug off to see if 12V is coming into the starter. So, again, any help would be great. If it is just coincidence that my starter is going bad (would either the solenoid or motor going bad act like this?) just as I swapped the ECU, ok, but this behavior only started afterwards.

 

The 2 DTC's are "CIRCUIT" codes so that usually would not point to a starter or starter solenoid...do you have any electrical diagnostic experience to be able with a 12 volt test light to remove the starter relay and check each of the 4 terminals ??...can you follow a wiring diagram ??...looking at the diagram below if you were to "jump" the load side of the relay which are terminals 87 and 30 the starter should turn...you will need to figure out the terminal locations of the relay...also using a factory level scan tool can help you diagnose this as it will tell you if you get a "crank request" or other important data PIDS...below is how a professional diagnostician figures out why a C6 is not starting...similar wiring on a C7...3 part video...also having a factory level scan tool helps troubleshooting as you can see.

 

The OE wiring diagram below.

 

Most electrical diagram stuff I ever worked with was in college, comp sci major. So, my general electrical diagnostic experience on cars is slim to none. And the best diagnostic tools I have is HPTuners and a harbor freight scanner. I am no electrical engineer by any means. Good information though, I appreciate it.

 

One does not have to be a computer scientist to diagnose this...if you watch how the relay is checked in the video that is what you need to do...just need a 12 volt test light...you may have no power to the relay control circuit which goes to terminal 86...or the ground G100 is bad.

 

Cool, yea. I will take a look at that one of these nights, just tryna get this ole girl running like she once did. I have read that HPTuners can see a crank request and starter relay being enabled or not. I will do some poking once I can. In general, the car will start after having sat for a bit, but something in it trips eventually, not sure what, and it will just stop cranking and throw those 2 codes. Not sure if there would be a short somewhere, but they are always present

 

I have HP Tuners also but I use my factory level scan tools and the labscope for this kind of work...HP Tuners is just a "generic OBD2" scan tool...not much info.

 

Sure, I will concur that HPtuners is not a diagnostic tool, but it is the best that I have. I will do with it the best that I can. Thank you, again

 

Go back to the beginning of your repair process: Check your ECU wiring connector(s) (after disconnecting the negative terminal of the battery) to ensure that you have proper mating to the ECU module. I would disconnect the wiring connector(s), inspect for damage and then fully reseat the connectors(s) onto the ECU module, Reconnect the battery cable.

 

Let's see, where to start....

C5 gave you the best advice but I understand your reluctance to proceed forward.

Most people that look at a schematic have the 'deer in headlights' expression and thus are basically lost. (almost all dealership mechanics are not diagnosticians when it comes to the electrical side of a car, they are just 'parts' changers).

Expensive test equipment is nice but in your situation even a simple test light should suffice, if you know how to use it and understand what it is telling you. Knowledge is the key.

C5 gave you enough direction to track down the problem. If you are still at a loss then you can roll the dice at the dealership or find a shop that specializes in these kind of problems.

Many times over the years working in an electronics R&D lab technicians would come to me asking exactly what part to change to repair a board, never mind that there were over 1000 parts on the board! (ie: they were parts changers not someone who could diagnose a issue). Most times they were SOL.

My best advice is to get yourself a good test light, some of the higher end test lights can do a lot more than just check for voltage and can be very useful, and then study C5's schematic follow his advice and with your past knowledge you should be able to isolate the issue and / or come back here and state your results and ask additional questions.

Best of luck.

 

Just was out there and pulled the connectors off of the board and the behavior persisted, unfortunately

 

Update: Now car cannot talk to ECM at all. Reseated connectors, no dice

 

If "No Comm" with ECM you check its powers and grounds...if they are good you have a bad ECM...just reseating the connector is not how's it's done !!

 

C5 Diag - Thanks for the schematics. This will aid in my t-shooting of my no start situation. These I understand, I use them daily in my line of work.

 

Is the MIL (Check Engine Light) illuminated with ignition on ??...this is the proper way to check powers and grounds on the video below.

 

The CEL is not on, but the orange “service electric parking break” light is. HPTuners says that U0100 is a current, pending, and historic code when the car is on.

Got to do more diagnosing yesterday and today. It looks like all relays and fuses are fine. I did a power check going to the ECM connector and found this out… when the ignition is on, pin 70 on X1 has power, but pin 73 does not appear to have power. Does anyone know what is upstream of X1-73? It is the “main power relay” according to this website. I checked relay number 62 and it does click and has power on one side of the high circuit

 

My friend, you can't go by "looks like all relays and fuses are fine"...HOW did you verify this ??...fuse 4 (J Fuse) 30 amp is upstream of terminal 73 (X1) of the ECM...check the fuse !!...with a test light BOTH test points should illuminate with ignition and Ignition Relay energized...fuses aren't normally checked with a DMM...do NOT go by the Internet websites or AI to diagnose your car...use Factory Service Information !!...you can use Alldata DIY for help.

 

I have been using an multimeter, test light, and power probe to verify my fuses and relays, and pins on the X1 connector. I believe I checked fuse 4 last night, but I can look again

 

If the fuse is good you need to check for continuity between the relay and ECM...just see if both test points light your test light...this is an easy check !!...so you checked all 4 terminals of the ignition relay with a test light ??...a Power Probe can get you in trouble...also the MIL should be illuminated with ignition on.

 

Upon looking at fuse 4 and relay 62 (ECM), I discovered that the relay would not click if inserted into the fuse box. I believe that relay is fine, but the ground in the fuse box did not seem to want to play ball. This prevented electricity from getting to fuse 4, so we jumped the relay and fuse 4 was fine. Presumed electricity was making it to X1-73, but ECU was still undiscovered.

Uncertain why the relay would not click when plugged into box. ECU still dead in water, marked out a lot of things as the cause but no dice in solving this yet. Will continue to update as I learn things