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Thread: J538 Fuel Pump Control Module, cable FB? twin Walbro 450?

  1. #1
    Registered User RS8's Avatar
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    J538 Fuel Pump Control Module, cable FB? twin Walbro 450?

    Hi.
    I'm planing to use the RS6 J538 Fuel Pump Control Module in my S8-97 to control two Walbo 450L fuelpumps.
    I will control J538 with my standalone ECU = MaxxECu Pro

    First question:
    Is J538 a good functioning unit? or is it often a problem with these?

    Second question:
    What does the abbreviation FB mean on the cable that is in the red circle in my picture?
    Click image for larger version. 

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    Last edited by RS8; February 24th, 2019 at 13:49.
    RS6/S8 engine, 6spd tdi 01E, EFR7163 turbos, custom built exhaust manifolds, S8 camshafts, MaxxEcu Pro, wagners ic's, bigger fuel rails, AN8 fuel line, 2xwalbro 450lph fuel pump
    and EV14 1500cc fuel injectors in my S8/RS8

  2. #2
    Registered User hahnmgh63's Avatar
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    Difficult to say without seeing the wiring colors. At the relay (in the diagram-Bentley) I see one to Ground br, one to power gn/ge(fuse box), two going to the transfer fuel pump ro/gn, br/gr, two going to the main fuel pump br/sw, ro/sw, two going to the ECU gn/gr, gn/ge. Wiring page no. 28/18
    ro=red, gn=green, br=brown, gr=grey, sw=black, ge=yellow. These should be at the connector in the RS6 wiring at the Controller. According to the wiring diagram these all change to different colors through plug connections before they make it to their devices which I can list. You probably already know but the controller only delivers 10volts to the pumps under most conditions and delivers the full 14volts (Alternator voltage) when full throttle or close to it. They did this as at lower fuel consumption the return lines and Fuel pressure regulator cannot handle that much fuel being returned to the tank so they cut the volume by reducing the voltage. Hope this helps but with a full wiring diagram next to the connections it might be a little tedious but fairly easy.
    2003 White RS6 2013 Midnight Blue S5
    2013 Daytona RS5 2x944 Turbo's 1974 911 w/'91 3.6ltr motor
    Roy, WA

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    Click image for larger version. 

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    We'll see how this looks.
    2003 White RS6 2013 Midnight Blue S5
    2013 Daytona RS5 2x944 Turbo's 1974 911 w/'91 3.6ltr motor
    Roy, WA

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    Registered User RS8's Avatar
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    Thanks, but I have this wiring page and it is the cable gn/gr on contact T6aw/2 that is the one marked with "FB" on J538 module in my picture,
    but in the wiring diagram it only shows that it goes to the ECU, but it doesn't says what the cable is for?
    what does the ECU do with this cable marked FB, is there any information about this in the Bentley manual?


    And yes the idea I have is to lower the speed on my Walbo 450 pumps when I don't need full speed so I can get lower heat on the fuel, so 10volt I think will be good or does anyone have problems with Walbo 450 on RS6?
    RS6/S8 engine, 6spd tdi 01E, EFR7163 turbos, custom built exhaust manifolds, S8 camshafts, MaxxEcu Pro, wagners ic's, bigger fuel rails, AN8 fuel line, 2xwalbro 450lph fuel pump
    and EV14 1500cc fuel injectors in my S8/RS8

  5. #5
    Registered User hahnmgh63's Avatar
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    It's too bad the wiring diagram doesn't show a schematic of the inside of the J538 like it does some relays so you would know what it does. One or both of the ECU wires are supposed to be a signal telling the relay to go from 10V to 14V for the fuel pumps. I can't imagine an easy way to find out. You could put a jumper on both wires and go through a start/drive procedure and see what kind of signal/voltage the put out and when but it wouldn't be 100% helpful, maybe. If you saw a signal out of one of the wires as soon as you gave full throttle then you would know that is the one that kicks the fuel pump power straight to Alternator power. Sorry, I'm not much help without a better schematic, looks like you've come as far as I can already.
    2003 White RS6 2013 Midnight Blue S5
    2013 Daytona RS5 2x944 Turbo's 1974 911 w/'91 3.6ltr motor
    Roy, WA

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    Registered User RS8's Avatar
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    Well it is the other cable marked S that is the signal to make it change between 10V and 14V, and it works with only this connected so I know how to make it work in my car.
    But it would be good to find out what the other cable is for, maybe it is only a communication cable for fault codes? Or something else that must be used or I will get problems?

    And I also wonders if any RS6 cars use Walbro fuelpumps together with J538 ?
    RS6/S8 engine, 6spd tdi 01E, EFR7163 turbos, custom built exhaust manifolds, S8 camshafts, MaxxEcu Pro, wagners ic's, bigger fuel rails, AN8 fuel line, 2xwalbro 450lph fuel pump
    and EV14 1500cc fuel injectors in my S8/RS8

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    Quote Originally Posted by RS8 View Post
    Well it is the other cable marked S that is the signal to make it change between 10V and 14V, and it works with only this connected so I know how to make it work in my car.
    But it would be good to find out what the other cable is for, maybe it is only a communication cable for fault codes? Or something else that must be used or I will get problems?

    And I also wonders if any RS6 cars use Walbro fuelpumps together with J538 ?
    Hey RS8, did you end up putting a Walbro in the tank with the stock controller? How did the pump do with 10V?

  8. #8
    Registered User RS8's Avatar
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    Yes it worked, BUT discovered that there were different volts on the 2 outputs? since I use 2 walbro pumps next to each other then one pump now pumped more than the other !
    so I then replaced this RS6 control module with a MaxxEcu PWM module which now controls my pumps,
    but if you only control 1 pump, it should work.

    https://rejsa.nu/forum/viewtopic.php...asc&start=8454
    RS6/S8 engine, 6spd tdi 01E, EFR7163 turbos, custom built exhaust manifolds, S8 camshafts, MaxxEcu Pro, wagners ic's, bigger fuel rails, AN8 fuel line, 2xwalbro 450lph fuel pump
    and EV14 1500cc fuel injectors in my S8/RS8

  9. #9
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    It won't work without ECU mods. Pump controller will report issues with one of the pumps & ECU will go limp mode.
    Also, Walbro 450 is too powerful for stock FPR even at 10 volts. You'll end up with too high pressure in idle & low load - your long term trims will drift.
    So you're going to need a bigger regulator if you go this route.

  10. #10
    Registered User RS8's Avatar
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    You're probably right, I myself have no original left,
    AN8 fuel lines, Nuke fuelrail, Turbosmart FPR and standalone ecu = MaxxEcu Pro with home built engine wiring harness.

    But for me 2pcs of Walbro 450 was to much for this big Turbosmart FPR so that is why I needed so slow down my fuelpumps, So now I controll them with a MaxxEcu PWM unit made for this.
    RS6/S8 engine, 6spd tdi 01E, EFR7163 turbos, custom built exhaust manifolds, S8 camshafts, MaxxEcu Pro, wagners ic's, bigger fuel rails, AN8 fuel line, 2xwalbro 450lph fuel pump
    and EV14 1500cc fuel injectors in my S8/RS8

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