dvs_dave

Hey guys. Dropping in from my usual hangout in the D3 section.

Anyway, a buddy of mine has an 02 S8 with an intermittent no crank and I’m looking for pointers as to what could be causing this. Battery is good, and there are zero VCDS error codes. When it does crank, it fires right up, and the starter sounds normal.

When it’s not cranking, there is no telltale failing starter click, or anything. Just nothing, no warnings no codes, no clicks. Ignition all comes on as normal, just it won’t crank. Car is garaged so not a water ingress issue either.

It feels like something simple as when it does crank, it starts and behaves completely normally, and it runs well. Is there a starter relay, or some other electrical/electronic component that’s known to cause this?

Geomeo

Probably the starter solenoid. You can check the voltage on the back of the starter in a couple of places if you have a voltage meter. If the small wire is getting voltage then it’s likely the starter solenoid is bad.

dvs_dave

That was my initial thought, but the symptom of it going from cranking perfectly normally one minute, and the next, literally nothing, no clicks, clunks, or any other symptoms typically associated with a failing starter makes me question that diagnosis. Where does the voltage to the starter solenoid come from? Is there anywhere that can be tested further upstream without having to get up under car....like an easily accessible relay to test, or short the pins to test the starter, etc.

Geomeo

As you’re likely aware the starter is earthed to the engine. Which unless you have a bad grounding strap from body to block typically does not fail. If you google search starter solenoid the very first YouTube video will show how a starter should be tested. The small wire on the back gets its power from the relay. The large gets its power from the battery. If you have enough room you could whack it with a mallet, which would free the solenoid temporarily. If it’s a manual you can also rock the car back and forward In gear. That might free it.

dvs_dave

Yes, that’s all good stuff. However for various reasons getting physical access to the starter isn’t presently a straightforward option. And it’s an auto so bump starting isn’t possible either.

Do you know where the starter relay is located on the D2? That’ll potentially be a much easier spot to perform initial starter electrical diagnosis and function from.

jfrahm

I don't suppose the gearshift cable needs to be adjusted or the F125 switch replaced? If it won't start try moving the selector to N and moving it a but high or low of N to see if there is a position it likes and will crank.
If it's somewhere cold where the selector might freeze up I could see the cable getting stretched or pulled out of adjustment.

dvs_dave

We did consider that, but the gear selector graphic in the DIS displays and moves correctly, and the “put foot on brake to start” message comes up too. So it appears to think that the gear selector is in an appropriate position to allow a start. However I’m not familiar with the day to day operation of the d2, so not sure what it says in the DIS if F125 is faulty and you try and start it with the gear selector in a start blocked position.

medinton

Hi, I have the same issue with my '94 A8 4.2. Maybe this can help you as well. For me this issue only happens if the engine is up to temperature and I sit in traffic or a drive thru for a longer period of time. I either wait about 30-45 minutes for something to cool down and then it fires up straight away, or I disconnect the battery and touch the + and - wires together for 10-15 seconds. Then I only reconnect the + to avoid sparks in the trunk. I turn the key into cranking position for 30 seconds. I reconnect the - cable and the car usually starts up. If it doesn't, I repeat this 1-2x.

I checked the fuses and found one that melted the plastic around it, but the fuse never broke (yay cheap crap fuses). This fixed my battery drain issue at least.
I tested the relays I could find, but none of them acted up.

I tried pushing and pulling on the ignition to see if the ignition switch could cause this.
Starter always works, no weak cranking or clicking.

I would love to start throwing money at the car, but not sure where to start.

dvs_dave

I found the wiring diagrams and the J207 starting interlock relay, located right side of the passenger footwell is where the starter motor gets its signal from. The F125 transmission switch is connected to this relay’s coil. So the start relay isn't able to engage and fire the starter if the F125 transmission switch isn’t connecting the start relay coil to ground by being in the P or N position.

So its possible to test, diagnose, and bypass the interlocks for pretty much the entire starting system via this relay; by measuring voltages/resistance/continuity at each terminal and jumpering terminals.

I’ve attached the wiring diagram for info so I hope it’s useful.

hillpc

If the F125 switch or cable is just slightly off, the indicator might move correctly, so I second jfram's suggestion about looking for a spot it likes. Keep the key turned to start and move and jiggle the gear selector around and between positions.

I had a car on which the solenoid wire's little attaching nut had backed off a turn or three, and the starter intermittently failed to crank. It would start every time if I had someone pull on the wire from under the hood, causing it to make contact, while someone held the key to start. That was easy because the wire was easily accessible under the hood. I finally fixed it by crawling under the car to tighten the loose connection.