Please help fuel pump not turning on!

Discussion in 'Fox 5.0 Mustang Tech' started by stang_380, May 15, 2006.

  1. Ive been through many of the no start check list threads already but i just cant pinpoint why my fuel pump wont turn on. Ive already replaced the fuel pump relay and still its not priming.
    The wiring diagrams for my 90 5.0 show four wires to the fuel pump relay and my car has 4 wires but one of them is green not org/lt blue. The green wire to the fuel pump relay does however have power to it. Im also not getting spark. the coil has no power to it. The inertia switch also has no power to either side. I thought maybe it could be the fuse links, or possibly something with the EEC power relay but i dont know where to find either of these, where are they?

    Please help me figure this out.

    Thanks \

  2. When the car gets back this evening I'll take pics of the fusible links if no one else has gotten answer for you by then. Can't remember where mine was but I think it was on the driverside near the firewall. Mine did pretty much the same thing when my fusible link burned up on me..
  3. The wiring colors you spoke about are for a 91-93 Mustang.
    Here's the fuel pump test path for those model years.

    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 Upper RH corner to ground.

    . If the relay & inertia switch 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 path.

    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.

    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.

    [​IMG] diagram
    of the wiring for 91-93 cars.

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

    Power circuits:
    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: 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.

    Control circuits:

    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 . Remove the plastic
    cover over the computer wiring, but leave the computer wiring connector plugged into the computer.
    With the ignition switch in the run position, connect a test lamp to the battery and back probe pin 22,
    the light blue/orange wire with it. The lamp should glow brightly. No glow and the computer
    has died a sad death. :( If you used a voltmeter instead of a test lamp, you should see battery
    voltage, whatever that may be…

    Fuel pump runs continuously: The light blue/orange wire has shorted to ground. 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.
  4. I had the same problem, my fuse link went out and the wiring to the relay was melted into the carpet under the seat, so I ran a bigger wire and an inline fuse directly from the battery to the relay. Also added a kill switch at the same time
  5. It sounds like your fuse link wire has gone bad, or you have a bad ground to it. If the fuse link wire is not grounded correctly, you will not get power to the fuel pump. This happened to me last week and that is all it was, check that wire out.

  6. Bladez_50 i could really use those pics if you could manage it. I really want take care of this problem but i dont know where the fuse links are or what they look like.
    I just want to say thanks to those that have given me their expertise so far.

  7. ok i think i found one of the fuse links but how do i know if it is bad? Is it visibly melted? I found the fuse link for the red/LT grn wire it has 2 wires going in each end. Is this the fuse link that would be causeing my problems or should i keep looking for another one?

  8. Measure the voltage drop across the link with the fuel pump or other device with the load applied or device running. You should see less than 1 volt.

    Here's a help tip to use the DVM to measure voltage drops if you aren't familar with how to do it.

    A voltmeter is handy if you are familiar with how to use it to find bad connections. Measure the voltage drop across a connection: more than .5 volts across a connection indicates a problem.
    See for help
  9. i had the same problem recently. after lots of frustrating hunting and pecking i found the culprit - one of the wires to thge fuel pump relay was corroded in two inside the electrical connector. maybe that's the problem you are having. should be relatively wuick and easy to check. good luck man.
  10. thanks for the help everyone the problem has been fixed. The red/lt grn wire was the problem the fuse link was toast but i couldnt find it. It is somewhere between the drivers fender and the dash but i could only open up so much of the wireing harness so i added a jumper wire of the same gauge and now everything works. Will not having a fuse link cause a problem in the future? should i put one in?

    Thanks everyone

  11. Just put an inline fuse and you are good to go.

    Fuse to wire size table:

    18 gauge wire = 5-8 amps
    16 gauge wire = 10-12 amps
    14 gauge wire = 15-17 amps
    12 gauge wire = 20-25 amps
    10 gauge wire = 30-40 amps
    8 gauge wire = 50-60 amps.
  12. fuel pump -

    I am just responding here to see if I get some help. I have look in other forums here but not found this much response.
    I have a 91 LX 2.3 - and after leaving the parking lights on I came back to a car that turns over well but no fuel. I am told I have the same problem but I don't know where to look. I have never tried to put pictures on here but if you guys will help me I a sure will put them up and soon.