How likely is it for fuel pump relay to go out?

Discussion in 'Fox 5.0 Mustang Tech' started by SuperStang83, Sep 5, 2008.

  1. This morning my car wouldn't start, and the fuel pump wasn't priming at all. So when I got home for lunch I unplugged and plugged the relay, then checked for power in the fuel pump wires. I was messing around with the power seat and I heard the pump priming again after I had put the key in so I tried to start it and it started right up. So I think that since I unplugged and plugged the relay that it may have triggered something. I just replaced the pump 5 months ago, so more than likely that is still good. How often do the relays go out, and is this most likely my problem? Thank you very much.

  2. Fuel Pump Troubleshooting for 86-90 Mustangs

    Clue – listen for the fuel pump to prime when you first turn the ignition switch on.
    It should run for 5-20 seconds and shut off. To trick the fuel pump into running,
    find the ECC test connector and jump the connector in the upper RH corner to



    If the fuse links are OK, you will have power to the pump. Check fuel pressure –
    remove the cap from the Schrader valve behind the alternator and depress the
    core. Fuel should squirt out, catch it in a rag. A tire pressure gauge can also be
    used if you have one - look for 37-40 PSI. Beware of fire hazard when you do this.

    No fuel pressure, possible failed items in order of their probability:
    A.) Tripped inertia switch – press reset button on the inertia switch. The hatch
    cars hide it under the plastic trim covering the driver's side taillight. Use the
    voltmeter or test light to make sure you have power to both sides of the switch

    B.) Fuel pump power relay – located under the driver’s seat in most stangs built
    before 92. On 92 and later model cars it is located below the Mass Air Flow meter.
    C.) Clogged fuel filter
    D.) Failed fuel pump
    E.) Blown fuse link in wiring harness.
    F.) Fuel pressure regulator failed. Remove vacuum line from regulator and inspect
    for fuel escaping while pump is running.

    The electrical circuit for the fuel pump has two paths, a control path and a power

    The control path consists of the inertia switch, the computer, and the fuel pump
    relay coil. It turns the fuel pump relay on or off under computer control. The
    switched power (red wire) from the ECC relay goes to the inertia switch
    (red/black wire) then from the inertia switch to the relay coil and then from the
    relay coil to the computer (tan/ Lt green wire). The computer provides the ground
    path to complete the circuit. This ground causes the relay coil to energize and
    close the contacts for the power path. Keep in mind that you can have voltage
    to all the right places, but the computer must provide a ground. If there is no
    ground, the relay will not close the power contacts.

    The power path picks up from a fuse link near the starter relay. Fuse links are like
    fuses, except they are pieces of wire and are made right into the wiring harness.
    The feed wire from the fuse link (orange/ light blue wire) goes to the fuel pump
    relay contacts. When the contacts close because the relay energizes, the power
    flows through the contacts to the fuel pump (light pink/black wire). Notice that
    pin 19 on the computer is the monitor to make sure the pump has power.
    The fuel pump has a black wire that supplies the ground to complete the circuit.

    Remember that the computer does not source any power to actuators, relays
    or injectors, but provides the ground necessary to complete the circuit. That
    means one side of the circuit will always be hot, and the other side will go to
    ground or below 1 volt as the computer switches on that circuit.

    Diagram courtesy of AutoZone


    Now that you have the theory of how it works, it’s time to go digging.

    Look for 12 volts at the Orange/Lt. Blue wire (power source for fuel pump relay).
    No voltage or low voltage, bad fuse link, bad wiring, bad ignition switch or ignition
    switch wiring or connections. There is a mystery connector somewhere under the
    driver’s side kick panel, between the fuel pump relay and the fuse link.

    Turn on the key and jumper the fuel pump test connector to ground as previously
    described. Look for 12 volts at the Light Pink/Black wire (relay controlled power
    for the fuel pump). No voltage there means that the relay has failed, or there is a
    broken wire in the relay control circuit.

    Check the Red/black wire, it should have 12 volts. No 12 volts there, either the
    inertia switch is open or has no power to it. Check both sides of the inertia
    switch: there should be power on the Red wire and Red/Black wire. Power on the
    Red wire and not on the Red/Black wire means the inertia switch is open.

    Pump wiring: Anytime the ignition switch is in the Run position and the test
    point is jumpered to ground, there should be at least 12 volts present on the
    black/pink wire. With power off, check the pump ground: you should see less
    than 1 ohm between the black wire and chassis ground.


    The yellow wire is the fuel tank sender to the fuel quantity gage. The two black wires are grounds.
    One ground is for the fuel tank sender and the other is the fuel pump. The ground for the fuel pump
    may be larger gauge wire that the fuel tank sender ground wire.

    Make sure that the power is off the circuit before making any resistance checks. If the circuit
    is powered up, your resistance measurements will be inaccurate.

    You should see less than 1 Ohm between the black wire(s) and ground. To get some idea of what
    a good reading is, short the two meter leads together and observe the reading. It should only be
    slightly higher when you measure the black wire to ground resistance.

    The Tan/Lt Green wire provides a ground path for the relay power. With the test
    connector jumpered to ground, there should be less than .75 volts. Use a test
    lamp with one side connected to battery power and the other side to the
    Tan/Lt Green wire. The test light should glow brightly. No glow and you have a
    broken wire or bad connection between the test connector and the relay. To test
    the wiring from the computer, remove the passenger side kick panel and
    disconnect the computer connector. It has a 10 MM bolt that holds it in place.
    With the test lamp connected to power, jumper pin 22 to ground and the test
    lamp should glow. No glow and the wiring between the computer and the fuel
    pump relay is bad.

    Computer: If you got this far and everything else checked out good, the computer is suspect.
    Remove the test jumper from the ECC test connector located under the hood.
    Probe computer pin 22 with a safety pin and ground it to chassis. Make sure the computer
    and everything else is connected. Turn the ignition switch to the Run position and observe
    the fuel pressure. The pump should run at full pressure.
    If it doesn't, the wiring between pin 22 on the computer and the fuel pump relay is bad.
    If it does run at full pressure, the computer may have failed.

    Keep in mind that the computer only runs the fuel pump for about 2-3 seconds when you turn
    the key to the Run position. This can sometimes fool you into thinking the computer has died.
    Connect one lead of the test light to power and the other lead to computer pin 22 with a safety pin.
    With the ignition switch Off, jumper the computer into self test mode like you are going to dump
    the codes. Turn the ignition switch to the Run position. The light will flicker when the computer
    does the self test routine. A flickering light is a good computer. No flickering light is a bad computer.
    Remove the test jumper from the ECC test connector located under the hood.

    Fuel pump runs continuously: The fuel pump relay contacts are stuck together or the Tan/Lt Green wire
    has shorted to ground. Remove the fuel pump relay from its socket. Then disconnect the computer and use
    an ohmmeter to check out the resistance between the Tan/Lt Green wire and ground. You should see
    more than 10 K Ohms (10,000 ohms) or an infinite open circuit. Be sure that the test connector isn’t
    jumpered to ground.
    If the wiring checks out good, then the computer is the likely culprit.

    Prior to replacing the computer, check the computer power ground. The computer has its own
    dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to
    it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a
    black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. You'll find it up
    next to the starter solenoid where the wire goes into the wiring harness

    If all of the checks have worked OK to this point, then the computer is bad. The
    computers are very reliable and not prone to failure unless there has been
    significant electrical trauma to the car. Things like lightning strikes and putting
    the battery in backwards or connecting jumper cables backwards are about the
    only thing that kills the computer.

    See the following website for some help from Tmoss (diagram designer) &
    Stang&2Birds (website host)
  3. Thanks, a lot man. I got it running, but I still haven't found the problem. I will keep using your techniques until I find the problem. Thanks again man!

  4. Jrichker's post is outstanding. :nice:

    Jacob, I would also simply check the wiring harness itself [under the seat]. If moving the seat was related to it working again, the harness might be snagged or have a borderline connection. In such circumstances, I would have the engine running and wiggle test the harness to see if I could make the pump shut off.
  5. Good thinkin' JT, I'll give that a shot as well. Thanks a lot guys!
  6. Okay, so I have power going into the relay, but no power coming out. So I replaced the relay and there is still no power coming out of it, and the car won't start again. So where does this leave me?

    I took out the seat JT and everything looked good under there.

  7. I diconnected the wires at the tank and checked for power there, but there was no power. But then I went to start the car and it started. So I have no iodea what the problem is.
  8. Where is the ground for the fuel pump? I'm thinking that it could possibly be the ground. Thank you for the diagrams!

  9. The ground is the black wire in the connector and bolts to the body under the tank.
    You will need to drop the tank to inspect the area where the ground wire bolts to the car body.

  10. Thank you man, I am gonna drop the tank tonight.
  11. The ground bolt actually goes up into the hatch area and can be checked from inside the car. It is on the left side under the rear plastics.
  12. That may vary with the model year of the car.
  13. Man, I'm lost and I've got to drive this thing 2,400 miles next Thursday!
  14. I just want to give a huge shout out to JT (HISSIN50). He came over to my place today and figured out what was going wrong with the fuel pump issue I was having, He lives about 45 minutes away and had no complaints coming over just to help someone out. I just want to say thanks a lot man, you are truly a stand up guy with a lot of integrity! I'll make sure to keep in contact bro.

  15. what was wrong?? Im having fuel pump problems myself.
  16. Jacob,
    It was my pleasure. I didn't do much - you were really close to finding the exact issue before I showed up. Intermittant wiring problems can make anyone nutty.

    Thank you to you and your family for your hospitality, and for your service to our country! :flag:

    Have a safe and uneventful trip home. We'll definitely keep in touch brah. :nice:

    For those inquiring, it was really an individual issue with the car - an intermittant wiring connection due to an aged and impaired plastic connector.