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Thread: Fuel Pump Relay Testing

  1. #1
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    Fuel Pump Relay Testing

    Does anyone have a method of testing the Fuel Pump Relay from a 96 Silverado. It has the 5 pin type.

  2. #2
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    I'm assuming the truck won't run and you checked the pump already. You can try in this order to see where problem is.

    Check the following contacts when you key to run (not start) for a 12v signal which may last less than 2 seconds so you will have to key each time. Leave key off for at least 10 sec between.

    Check from A to ground, this is the hot being controled. If not 12v then look at fusible link to battery.

    Check from F to D(ground), this should be the signal for the relay to turn on. If not 12v then pull off the connector and check the wire to ground for 12v. If not 12V then your problem is towards PCM, may be the wire. Don't forget to reconnect plug before moving on.

    Check from E to ground, this is the "output" or pump side. Can be hot from PCM control if cranking or cold, or oil press switch if engine is warm and upping pressure. Relay is only need mostly to start eng, press sw will keep the fuel pump hot after.
    You can check if applying 12v to C will run your pump. First check to see if 12v is already there. If so the relay is staying open and may be stuck if the F to D was 12v. If no 12v at C, try applying it, and if pump runs relay is probably bad.
    There was a contact on the ALDL or DLC that was good for this but I forgot which one it is. I will try to find that.

    WARNING: This was mostly from memory, but I threw it out there because nobody else has jumped in yet. Good luck. I'm hoping you wanted the gas one not the diesel one.

  3. #3
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    This is good data.
    The truck will not start and I dont hear the pump making any noise iether.
    The fuses are all fine and I get 12V to the Relay socket.
    I dont understand all the ABCD stuff.
    My relay is labeled with 30, 86, 85, 87a and 87 (as u look at the relay with 2 pins on top).

  4. #4
    Senior Member scottie's Avatar
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    use an ohm meter and test 85 to 86.... you should have between 65-85 ohms of resistance. test 30 to 87... you should have 0.2 ohms and 30 to 87a should read "OFL" or "out of range"

    take a 9volt battery and put it to 85 & 86, the relay should "click" and 30 to 87 should be "OFL" and 30 to 87a should be 0.2 ohms.

    it sound like a lot of work, but it takes 2 mins
    im a straight shooter with upper management written all over me

  5. #5
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    Quote Originally Posted by scottie View Post
    use an ohm meter and test 85 to 86.... you should have between 65-85 ohms of resistance. test 30 to 87... you should have 0.2 ohms and 30 to 87a should read "OFL" or "out of range"

    take a 9volt battery and put it to 85 & 86, the relay should "click" and 30 to 87 should be "OFL" and 30 to 87a should be 0.2 ohms.

    it sound like a lot of work, but it takes 2 mins
    Scottie's advice is right. Sorry I tend to look from the wire side but you are looking at standard relay labels with the numbers.
    I think now maybe we're ahead of ourselves if you haven't heard the pump run.
    While you have that connector off, look for 12v to show up on 2 of the wires at key on (for only a moment on one).
    It maybe the relay but this is an easy check in case it's not.

    LOL it's only good data if it's right and you can use it.

  6. #6
    Senior Member NCroadracer's Avatar
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    First check your ECM B fuse, if it's good and there's power present check the relay terminal 87(B1) for power, if that's good, check for power on the relay terminal 85(B3) when you turn the ignition to run (should light for about 3 seconds), if that's good check with a test light from B+ to relay terminal 86(A1) and verify a good ground present.
    You can also jumper the terminals 87(B1) to 30(A3) for a direct feed to the pump and verify a functioning pump.

    There's also a red wire with a blank female connector on it just on the side of the fuse block, this is a pump prime terminal, when B+ is jumped to this terminal it will run the pump directly.

  7. #7
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    Quote Originally Posted by nnunez View Post
    96 1500 5.0L

    Has anyone done their fuel pressure regulator? I just want to gage how difficult it is to replace.
    Quote Originally Posted by nnunez View Post
    When I hit the highway when the truck is cold, I can feel a little hesitation. I've had a recent tune up and changed everything. I replaced the Cat as well........
    Quote Originally Posted by nnunez View Post
    ...It has a new fuel pump.
    History: Last month I took it to mechanic#1 (Engine light on) and he told me that the Fuel Pressure Regulator was bad and wanted $300 to replace it. So, I took it to mechanic#2 and he ended up replacing the Cat and the light went out.....the light is still out and it passed inspection. Mechanic #2 told me that the Press Reg was fine.
    I wonder if both mechanics are right.......?
    Is this the same situation just getting progressively worse ?
    The way some of your symptoms sound, especially after all these other things you're doing, are very similiar to a problem I've had.
    It was junk that was surgeing against the fuel filter making it intermittent and hard to chase down.
    I suggest that if you have not already done so, check your lines are clear and change the filter.
    Then double check your new pump will actually run.
    You can get pressure with poor flow. Driving the truck can change the way debris floats in the lines.
    Another truck I had someone had used lots of teflon tape on the lines and it just kept breaking off and floating in them
    clogging filters and other problems That was a diesel but no fuel system likes junk in it.
    One of the many reasons mechanics test drive a car after working on it. Just my 2 cents.

    There are some pros here (not me) trying to help you, good to listen to.
    Pretty soon you're gonna be an expert on your trucks fuel system
    You'll figure this out, there's not that many possibilities left.
    Please don't PM truck fix questions, bravely start a new thread for them. It is a forum afterall.

  8. #8
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    I dont think that this is part of the existing problem when its cold. I could be wrong though but I am optimistic that this problem is just the fuel pump.

    I was checking this yesterday and I get power (8. something volts) at the connection near the gas tank. I assume that the pump does not need 12v to run? Is there min voltage or amperage necessary?

    I will chech a few other things today and hopefully I can finally figure out (for sure) that its the fuel pump........

    I replaced the fuel pump about 6 months ago........

  9. #9
    Senior Member NCroadracer's Avatar
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    You should have 12V (bare minimum) at the pump. check to see if you've got 12V to the relay and what you're getting out of the relay.....

  10. #10
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    I was under the impression that the pump used 12v. I am getting 12v at the relay. I was going to use a battery to make contact with the battery and see if that helps.

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