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  1. #1
    Senior Member Two Rings
    Join Date
    Oct 08 2007
    AZ Member #
    21638
    Location
    Minnesota

    ECU won't ground the fuel pump relay on key-on

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    Pretty stumped with this one. I can't get the ECU to ground the fuel pump relay when i move the key to on. Everything on the chassis harness seems to work fine, I can connect to all the modules with VCDS, ABS, radio, etc. and all the lights work. I found that if i manually ground the relay via pin 8 (red/blue wire) with a jumper wire in the black plenum connector then everything seems to work as normal. I've tried 3 relays and 2 different ecu's so far and a new ignition switch just to remove one more variable. I double checked all the ground connections on the engine harness and all show good continuity. I verified continuity on the ground pins 1 and 2 in the ecu connectors, and also verified continuity between pin 65 on the ecu connector and the black plug. I did take the relay holder out and checked continuity between all the pins on that as well and they all checked out fine. Not sure where to go next, I may try to track down another ECU but it seems odd that it would be the ECU since I can connect to it fine to both with vcds if manually ground the fuel pump relay. Its a manual ATQ v6 for anyone that can help.
    2012 S4 DSG EPL Stg 2 DP
    B5 A4 Lemons racer in progress

  2. #2
    Veteran Member Four Rings
    Join Date
    Sep 24 2010
    AZ Member #
    64817
    Location
    Costa Mesa, SoCal

    Have you checked that the trigger wire going to the fuel pump relay is energizing? (is that pin 65?)

    It's possible the ECU is at fault. If it uses an output transistor to trigger the relay, then that could have gone bad.

    Lastly, I'm not sure about this, but one of the Output Tests in VCDS may be for the fuel pump. It's worth a shot as another diagnostic step.
    2001.5 S4 Avant: JHM shifter, 3" exhaust, Stage 1, APR bipipe, lowered, receiving lots of love ($$$)

    2001.5 A4 1.8TQM: so many upgrades. Parting out.

    2001 Corvette (C5) Z06: just getting started

  3. #3
    Established Member Two Rings
    Join Date
    Mar 15 2018
    AZ Member #
    415568
    Location
    Romania

    Quote Originally Posted by MetalMan View Post
    Have you checked that the trigger wire going to the fuel pump relay is energizing? (is that pin 65?)

    It's possible the ECU is at fault. If it uses an output transistor to trigger the relay, then that could have gone bad.

    Lastly, I'm not sure about this, but one of the Output Tests in VCDS may be for the fuel pump. It's worth a shot as another diagnostic step.
    yes the 1st output test is fuel pump in vcds
    2000 B5 A4 Avant - S Line - Sport package

    N/A ARG bottom end, AEB Head | Planning t3/t4 turbo conversion soonTM

  4. #4
    Senior Member Two Rings
    Join Date
    Oct 08 2007
    AZ Member #
    21638
    Location
    Minnesota

    Power for the relay comes from the relay distribution block, and pin 65 is the ground coming from the ECU. The fuel pump itself does run when i ground the relay so its not the pump. Seems to me at this point its more ECU related than anything unless I missing some random wiring problem between the ignition switch and the ecu.
    2012 S4 DSG EPL Stg 2 DP
    B5 A4 Lemons racer in progress

  5. #5
    Veteran Member Four Rings walky_talky20's Avatar
    Join Date
    Jun 30 2008
    AZ Member #
    30427
    Location
    Erie, Pennsylvania

    So, just to be clear: The Fuel Pump Relay is not expected to be on solid whenever the key is on. It *may* prime for a couple seconds when you first turn the key on. But it won't necessarily do that every time, or at all. It depends on some variables (mainly temperature, I think). It should turn the relay on whenever the engine is physically rotating (ie: starter is cranking, or bump start by popping the clutch). This means ECU is on and Engine speed sensor is showing some RPM.

    A good test for the relay control circuit is VCDS Output Test Mode. During output test mode, it will command the FP Relay to be on solid for the duration of all the tests. This is because a lot of the solenoids and injectors and such are actually powered from the Fuel Pump Relay as well.

    If the car runs perfectly otherwise, you could always permanently ground the FP relay coil and power the other side of the coil from the key ("15" circuit, Hot in Run or Start). Then it would no longer be computer controlled and just run with the key. This removes a safety feature: the fuel pump does not stop pumping when the engine stalls. Even a fiery accident. If you wanted to retain that feature, you could employ a tachometric relay. That sort of relay was used to provide that exact feature in older Audi's (and lesser cars) where the ECU did not directly control the pump.
    ^Don't listen to this guy, he's not even a mechanic.
    2001 Laser Red A4 1.8TQM, 5-Speed Swapped, 4.11 Final Drive, APR 93, R8 coil packs, Labree HFC & DP, 2.5" Exhaust, ST Coilovers, 034 RSB, A8 Brakes Front & Rear, 18" RS4 Reps
    2006 Passion Red Volvo V50 T5 AWD 6MT
    "You need to know what you are doing...Its like a rainbow jungle in there." - Redline380

  6. #6
    Senior Member Two Rings
    Join Date
    Oct 08 2007
    AZ Member #
    21638
    Location
    Minnesota

    The FP relay won't trigger on its own right now even when I crank the engine. I think its an internal issue with the distribution block, I found I've got no power to pin 3 of the ECU connector at any time which if I'm reading the wiring diagrams right should be powered all the time (comes off circuit 30 in the diagram), although I found a pinout diagram from an S4 which says its a switched power from circuit 15. The diagram is a bit confusing though because I pulled apart a spare harness I have and pin 3 from the ECU connector is a black/yellow and on the diagram it says it should be Red. I traced the black/yellow back to the load reduction relay. If I get motivated enough I may create a pinout diagram for the 2.8 because it doesn't seem to match up with the diagrams I've found. I ordered a new distribution block and a new load reduction relay since thats the only relay I haven't changed yet so we'll see when that gets here. One of the pitfalls of taking on a project car that hasn't run in 8 years and was mostly disassembled by a shop that went out of business.
    2012 S4 DSG EPL Stg 2 DP
    B5 A4 Lemons racer in progress

  7. #7
    Established Member Two Rings
    Join Date
    May 29 2017
    AZ Member #
    400260
    Location
    Fort Collins, CO

    Has the dash every been fully removed from the car, like to do a heater core or anything like that? Could be loose ground wire maybe down where all the ground wires connect?

  8. #8
    Senior Member Two Rings
    Join Date
    Oct 08 2007
    AZ Member #
    21638
    Location
    Minnesota

    Doesn't appear to have been, the grounds in the kick panel area were all tight and had good continuity.
    2012 S4 DSG EPL Stg 2 DP
    B5 A4 Lemons racer in progress

  9. #9
    Veteran Member Four Rings walky_talky20's Avatar
    Join Date
    Jun 30 2008
    AZ Member #
    30427
    Location
    Erie, Pennsylvania

    Diagram for a 2000 A4 2.8:



    Last edited by walky_talky20; 12-04-2018 at 02:21 PM.
    ^Don't listen to this guy, he's not even a mechanic.
    2001 Laser Red A4 1.8TQM, 5-Speed Swapped, 4.11 Final Drive, APR 93, R8 coil packs, Labree HFC & DP, 2.5" Exhaust, ST Coilovers, 034 RSB, A8 Brakes Front & Rear, 18" RS4 Reps
    2006 Passion Red Volvo V50 T5 AWD 6MT
    "You need to know what you are doing...Its like a rainbow jungle in there." - Redline380

  10. #10
    Senior Member Two Rings
    Join Date
    Oct 08 2007
    AZ Member #
    21638
    Location
    Minnesota

    That is fantastic. Thanks!
    2012 S4 DSG EPL Stg 2 DP
    B5 A4 Lemons racer in progress



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