92 MUSTANG NO POWER TO FUEL PUMP

Discussion in 'Fox 5.0 Mustang Tech' started by LARRY J, Sep 19, 2009.


  1. LARRY J

    LARRY J New Member

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    I HAVE A 92 MUSTANG. IT SAT UP FOR ABOUT 2 MONTHS. I TRIED TO START IT AND THE FUEL PUMP WOULD NOT RUN. I CHANGED THE FUEL PUMP AND STILL NOT WORKING.I HAVE NO POWER TO THE PUMP OR THE INERTIA SWITCH. I JUMPED THE FUEL PUMP LEAD AND THE FUEL PUMP CONTINUOSLY RUNS.I HAV E CHANGED THE FUEL PUMP RELAY AND STILL NO LUCK. ALSO AT THE SAME TIME I HAVE NO FIRE TO THE PLUGS. ANY HELP WOULD B E GREATLY APPRECIATED.
    #1
  2. 90mustangGT

    90mustangGT I felt sorry for girls because Founding Member

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    check the coil, and go back to the distributer pickup and see if you are getting signal. The computer does not turn the fuel pump on if there is no crank signal besides the initial prime when you first turn the key.
    #2
  3. HISSIN50

    HISSIN50 "How long does it take to get help in here? SN Certified Technician

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    What did you jump? Are you talking about the ground for the relay coil?
    #3
  4. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Fuel Pump Troubleshooting for 91-93 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 lower RH corner to
    ground.
    [​IMG]
    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 Relay:
    On 91 cars, it is located under the driver seat.
    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.

    Theory of operation:
    Read this section through several times. If you understand the theory of operation,
    this will be much easier to troubleshoot. Refer to the diagram below frequently.

    Diagram of the fuel pump wiring for 91-93 cars.
    [​IMG]


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

    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.


    The control path consists of 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 relay coil and then from the relay coil to the
    computer (light blue\orange 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 (pink/black wire) goes to the fuel pump relay
    contacts. When the contacts close because the relay energizes, the power flows
    through the pink/black wire to the contacts and through the dark green\yellow
    wire to the inertia switch. The other side of the inertia switch with the
    brown\pink wire joins the pink/black wire that connects to the fuel pump. The fuel
    pump has a black wire that supplies the ground to complete the circuit.


    Power path:
    Power feed: Look for 12 volts at the pink/black wire (power source for fuel pump relay).
    No voltage or low voltage, bad fuse link, bad wiring, or connections. Remember that on 92
    or later models the fuel pump relay is located under the Mass Air meter. Watch out for the
    WOT A/C control relay on these cars, as it is located in the same place and can easily be
    mistaken for the fuel pump relay.

    Relay: Turn on the key and jumper the ECC test connector as previously described. Look
    for 12 volts at the dark green\yellow 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.

    Inertia switch:
    The location for the inertia switch is under the plastic for the driver's side taillight.
    There should be a round plastic pop out cover over it, remove it to access the switch button.
    With the test connection jumpered and ignition switch in The Run position as described above, check
    the brown/pink 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 dark
    green\yellow (inertia switch input) and brown/pink wire (inertia switch output). Power on the
    dark green\yellow wire and not on the brown/pink wire means the inertia switch is open.
    Press on the red plunger to reset it to the closed position. Sometimes the inertia switch will
    be intermittent or will not pass full power. Be sure that there is 12 volts on both sides of the
    switch with the pump running and that the voltage drop measured across the switch is less
    than .75 volts.

    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.

    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.


    [​IMG]

    Control path:
    Relay: The red wire for the fuel pump relay coil gets its power feed from the ECC relay.
    No 12 volts here, and the ECC relay has failed or there is bad wiring or bad connections
    coming from it. The ECC relay is located on top of the computer, which is under the passenger’s
    side kick panel. It is not easy to get to, you must have small hands or pull the passenger side
    dash speaker out to access it.

    Relay: The light blue/orange wire provides a ground path for the relay power. With the test
    connector jumpered according to the previous instructions, there should be less than .75 volts.
    Use a test lamp with one side connected to battery power and the other side to the light blue/orange
    wire on the fuel pump relay. 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. Remove the test jumper from the ECC test connector.
    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.

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
    for help on 88-95 wiring Mustang FAQ - Engine Information

    Fuel pump runs continuously:
    The fuel pump relay contacts are stuck together or the light blue/orange 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 light blue/orange 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
    #4

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