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fuel pump relay diagnostics - coils check fine, no signal to relay coil 700 1990

posted by  760 turbo wagon on  Mon Oct 18 10:38 CST 2010
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last visit: Thu Oct 21 03:02 CST 2010
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I should have checked the post more carefully, i just joined, before sending this question - I see others have the same problem. I will follow their suggestions first.

Having problems with a fuel pump relay for my 760 turbo wagon, 1990. I jumpered the inner relay contact to the fuel pumps and verified that both the in tank and the under floor board pumps are functioning fine. When I jump the pump relay by removing the relay cover and press the contact closed while turning the ignition, the car starts fine. The outer relay operates on and off with the key turned on and off, but the inner fuel pump relay does not energize. Both relay coils check out on an ohm meter to be around 75 ohms. They both operate if I apply voltage to their coils with a power supply. I thought the diode on the relay board might be bad but when I apply voltage to the relay contacts, where the relay would plug into the car, both relays operate. It looks like maybe something is keeping the voltage to the fuel pump relay from coming on - isn't there an engine sensor to tell when the engine is rotating? How do I know if that is working? Other ideas appreciated. I will order another fuel pump relay but it looks like it might be something keeping the signal from getting to the relay. Thank you for the help.

Mike








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    fuel pump relay diagnostics - coils check fine, no signal to relay coil 700 1990

    posted by  spasske on  Tue Oct 19 02:29 CST 2010
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    last visit: Thu Oct 23 10:11 CST 2014
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    The problem with the fuel pump relay is normally not a dirty contact or a bad coil. These relays experience cracks in the solder which conduct poorly at times. Usually they fail sporadically.

    Pressing on the relay may make the crack conduct as you test it. The repair for a relay is to reflow the solder with a soldering iron. Or just prevetively replace it with a new relay. You will need one sooner or later so it is a smart invetment to replace it now.

    You may have other issues but it is hard to rule out the fuel pump and good plannig to replace it in any case.
    --
    Paul NW Indiana '89 744 Turbo 145K/ '90 745 turbo 127K








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    fuel pump relay diagnostics - coils check fine, no signal to relay coil 700 1990

    posted by  B.B. on  Mon Oct 18 22:10 CST 2010
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    last visit: Mon Oct 6 13:15 CST 2014
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    Hi Mike,

    Check the misnamed "Radio Supression relay" "RSR"
    It's in the engine area.

    It should be called the "Fuel shutdown relay", It's a safety item

    Bill







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    fuel pump relay diagnostics - coils check fine, no signal to relay coil 700 1990

    uid 27's pic
    posted by  Art Benstein on  Mon Oct 18 10:53 CST 2010
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    last visit: Fri Oct 17 20:10 CST 2014
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    Hi Mike,

    Sounds like your ECU is failing to supply the "ground" that would pull in the pump relay. This is a common symptom of failure with -5XX ECUs bearing a pink label. Remedy is to replace with white label version of same ECU, if available. Someone with 740 turbo experience will have the details.
    --
    Art Benstein near Baltimore

    Then a lady at the airport backed into the spinning propeller of a plane—disaster!








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      fuel pump relay diagnostics - coils check fine, no signal to relay coil 700 1990

      posted by  760 turbo wagon on  Mon Oct 18 11:04 CST 2010
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      last visit: Thu Oct 21 03:02 CST 2010
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      Hi Art,

      Thank you for the reply. Is there another sensor, like the engine rotation sensor, or that would send a signal to the ECU that may have failed? The engine runs fine if I manually hold in the fuel pump relay, the outer relay in the green enclosure seems to work correctly and energizes when the key is turned on. Is there a safe way to jumper the relay so it functions with the key / engine run condition if it is a portion of the ecu that failed? Take care. Thank you for the information.
      Mike








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        fuel pump relay diagnostics - coils check fine, no signal to relay coil 700 1990

        uid 27's pic
        posted by  Art Benstein on  Tue Oct 19 02:24 CST 2010
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        last visit: Fri Oct 17 20:10 CST 2014
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        Is there another sensor, like the engine rotation sensor, or that would send a signal to the ECU that may have failed?

        Your being able to drive the car with the fuel pump relay clapper held down is a mostly complete diagnostic pointing to the common ECU failure. If the CPS, or the RSR as Bill notes, had failed, you would not be able to do this.

        Is there a safe way to jumper the relay so it functions with the key / engine run condition if it is a portion of the ecu that failed?

        Yes, it is possible to wire in an old-style fuel relay from a k-jet which uses ignition pulses to keep its clapper down. This relay would be connected to provide the "ground" the ECU no longer does. I'm thinking it has been written up in the FAQ but am too lazy to look for you. The mod is OK for someone who will never take their car to a pro for service, never give it to their kids, and never sell it.
        --
        Art Benstein near Baltimore

        The roundest knight at King Arthur's round table was Sir Cumference.







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