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Thread: Q7 4.2 TDI Turbo Control Unit

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  1. Q7 4.2 TDI Turbo Control Unit 
    #1
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    Hi,

    I have a 2008 Q7 4.2 TDI S-Line.

    A few months ago it displayed an error: Turbo Control Unit faulty. The check engine light came on, the glow plug indicator flashed, and it went into limp mode. I took it to the dealer, who plugged in their diagnostic tool, and they said it's the left side turbo. But, after clearing the error it was 100% for about four months.

    Then, it happened again. I took it back again, but this time the error would not clear. The dealer's advice was to completely replace the left side turbo at huge expense.

    I figured it's worth a try to repair it first, so a local mechanic swapped out the control unit, which the had imported from Netherlands. They had it programmed and tested at the dealer, so it should be OK. However, the same error keeps being displayed.

    They tried everything they could think of, including cleaning all wire connectors and having the turbo pressure tested. They even took the new control unit out and replaced it with a second one. No luck. For some reason the error remains.

    It is now back at the dealer who wants to start replacing parts without knowing what the core problem is. I am afraid costs might run await with such a trail-and-error approach, and seeing that it is an old car it wouldn't make much sense to spend more than the car is worth.

    Please help! I am out of ideas.
     
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  2. Re: Q7 4.2 TDI Turbo Control Unit 
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    Has anyone checked the wiring loom to the turbo? Also, a mechanical issue with the turbo could cause a control unit to show as faulty if associated readings are not what the ecu is expecting to see.
    Alan.

    A6 Avant (C7) 3.0 TFSI Quattro S-Tronic
    APR St1 Custom Exhaust - now gone
    VW T6 150 DSG Camper £££
     
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  3. Re: Q7 4.2 TDI Turbo Control Unit 
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    At first pass all the pins on the wiring seem to register at least some voltages/signals, but we're not sure it these are correct values. Also, I am not convinced that even though there is some signal, there is not maybe an intermittend short or break in contact, perhaps during driving vibration. Someone else suggested to replace the wiring regardless, just to be sure. Not sure how difficult/expensive that would be...

    We are quite confident that it is not mechanical. The turbos were both taken out and tested on a bench. The vanes actuated correctly and pressures measured fine.

    Do you think it could be the ECU itself that's faulty, or would this register weirder issues?
     
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  4. Re: Q7 4.2 TDI Turbo Control Unit 
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    Hi, where can I find wiring diagrams for this car? I bought the full workshop manual with 8600+pages of technical details for $40, but it only has the diagrams for the 3.0 TDI, not the 4.2.
     
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  5. Re: Q7 4.2 TDI Turbo Control Unit 
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    Try looking at the A8 manual as they use the same 4.2tdi.
     
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  6. Re: Q7 4.2 TDI Turbo Control Unit 
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    Thanks. I'll suggest that to the dealer
     
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