Greetings everyone,

I've been grappling with a persistent issue in my 2006 A6 C6 2.0TDI for the past 7-8 months, and despite my efforts, I haven't been able to resolve it. I've scoured forums and pored over electrical drawings, but I'm still getting nowhere, so I'm reaching out to seek your valuable insights.

To provide some context, I've successfully tackled every problem with my car except for the starter issue. Initially, the car would shake violently during every startup, specifically when the starter engages (not during idling afterward). Despite changing glow plugs, fuel filters, and fuel lines, the problem persisted.

I sent the starter for repair once, where they replaced the brushes holder, but that didn't solve the issue. Subsequently, I took matters into my own hands, disassembled and cleaned the starter, and replaced the solenoid and bendix (the starter gear engaging with the flywheel). After this DIY repair, the car started flawlessly for about 10-15 days, only for the problems to resurface.

Further research revealed that my car has two starter relays. Testing them, they clicked when applying 12v, but the resistance reading was either non-existent or too high for my multimeter to register. I purchased two new relays, which showed a resistance of around 80-90 Ohms. Now, the car starts without issues on the first attempt of the day, but after the engine heats up, turning it off and restarting results in violent shaking and strange noises from the starter. All goes back to normal after letting it cool down for 30-60 minutes.

During the starter repair, I could only find parts from the "AS" manufacturer, and a mechanic suggested that replacing them with Bosch parts (bendix, solenoid, and relays) might resolve the problem.

My primary question is about the function of the two starter relays. If they control the feature where you don't need to hold the key for the car to start (just click it once), can I bypass them without causing damage? I wouldn't mind manually holding the key to start the car if that resolves the issue.
Additionally, has anyone encountered a similar problem and found a solution? I would greatly appreciate any advice or insights you can offer.

 

Odd, the contacts for the relays should read 0-0.5 ohms, as in perfect continuity. Sounds like you are metering the coil, not the contacts.

Also meter the volts on the battery as you crank the car over. Maybe the battery is weak?

 

No battery is not weak. After replacing bendix and solenoid it was starting all okay. I will try to attach a video of starting it warm.

 

But when the car is warm, there's more compression, so the starter has to work harder to turn it over. Even more so in a diesel.

EDIT:
Just watched the vid... sounds like a diesel starting up. My wife has a Benz GL320 CDi (diesel) It pretty much sounds like that. Unless in the vid I cant hear what you're talking about. (glowplugs ??)

 

Everything shakes, you can see it in the video that camera is shaking on startup :-) Sound of the engine is normal but starter is making weird sound.
Under is a picture that I aquired from VCDS Log file. It is normal, going linear to about 1100rpm, then dropping to around 900rpm.

Secound picture is DIY drawing because I cannot find the VCDS Log file but I remember how it was. It goes to about 300rpm, then drops, then goes little bit more, |
maybe 600rpm then drops, then little bit more, 900 maybe then drops. But it fires up at the end.

 

I'm new on this forum. I don't know how to edit a post, sorry.
I changed the glowplugs, it starts in the morning when it's cold no problem. But then when it heats up, then I have this problem. Maybe compression as you said. I will try other battery, maybe that helps.

 

At the bottom right of your post you'll see some buttons, "EDIT" "QUOTE" etc. Hit the EDIT link and then you can edit your posts.

 

I have put new battery in the car. Still having same issues when it is starting warm (90°c).

 

If you can, try going under the car and applying voltage directly to the starter solenoid using jumper cables if required but otherwise you can just shove something metal between the two large posts. If that test seems to work then you may have an issue with too much voltage drop to the coil on the start solenoid. If you have replaced the relays you mentioned above then they're probably OK as long as you confirmed that you were actually measuring the coil and not the contacts as Sallad suggested. I would check for proper voltage at the "starter wire" on the starter solenoid and see if it is dropping too low when trying to start the car. I had a problem similar to this on a diesel air compressor that I have inherited which was not starting and threatening to melt down the starter in the process. It turns out, all I needed to do was to replace the wire from the ignition switch to the starter solenoid with a heavier gauge wire and all my problems disappeared! It was never the starters fault even though it seemed it might be after first replacing it. Another experiment you could try would be to use a jumper cable to go from the battery terminal on the starter solenoid directly to the start terminal. If not a jumper cable, which is way longer than required, then a length of 8-12 gauge wire directly to the start terminal.