Electrical Does Anybody Have The Fuel Pump Wiring Schematics For A 91?

Discussion in 'Fox 5.0 Mustang Tech' started by Boostedpimp, May 28, 2013.

  1. Long story short my little brother shorted out a fuse link while relocating his wiring into the fender. Battery was disconnected but when he connected it something was touching anyways we replaced the bad fuselink and I ruled out the inertia switch, pump and wiring to both. Fuel pump relay is good and ecm is good too so the problem seems to exist somewhere from the engine bay to the fuel pump relay under the seat. Jumping the relay obviously doesn't work neither.

    Does anybody have a wiring diagram or schematic that I can use?


  2. 64326d1287785504-fusible-link-burned-up-need-help-87-93-5.0-mustang-fuse-links.gif

    Fuse links come with a current rating just like fuses. A clue as to what current they are designed for is to look at the size wire they protect.

    Choose the fuse according to the wire size.

    Wire size current 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.

    Keep in mind that the wire size in the chart is for the circuit itself, not the size of the fuse link. The packages of fuse link repair material you can get at the auto parts stores also will have a current rating on them.

    See http://fordfuelinjection.com/?p=7 for some excellent tips of soldering wires for best electrical performance.

    Fuel Pump Troubleshooting for 91-93 Mustangs

    Revised 08-Dec-2012 to add check for 12 volts at ignition coil to prove the ignition swich is good.

    Clue – listen for the fuel pump to prime when you first turn the ignition switch on.
    It should run for 2-5 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 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's seat.
    On 92 and 93 cars it is located under the MAF. Be careful not to confuse it with the A/C WOT cutoff relay which is in the same area. See the diagram to help identify the fuel pump relay wiring colors.
    Be sure to closely check the condition of the relay, wiring & socket for corrosion and damage.
    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.

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

    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.


    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.

    Another possibility is that the ignition switch is faulty: look for 12 volts on the red/green wire on the ignition coil. No 12 volts with the ignition switch in the Run position and the ignition switch is faulty or the fuse link in the ignition wiring is blown.

    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.
    Boostedpimp likes this.
  3. As always... you are the man!

    Much appreciate the help sir
  4. Jrichker, or whoever else might be able to shed some light on this

    Question.. that first diagram you posted with the wiring to the solenoid. Does that apply to a 91 hatch? I ask because looking at mine the colors don't match up at all.

    Also found a yellow wire grounded.... This sounds wrong to me but looking at your wiring diagrams I can't seem to find where this wire should go or what it's for. It was in a loom with a Red/green strip, three Black/yellow strip wires and a Orange/yellow strip wire. Any idea on these?

    This grounded yellow wire i found is different from the main yellow from the starter solenoid that splits into four other wires with fuse links... so im thinking maybe this grounded yellow is probably something to do with the ignition switch?

    Thanks for any help in advance

    Here's a few pics of the wiring
    And what wire im talking about
  5. From what I've seen here buddy I purchased a diagram for my 91 hatch and it seems all the grounds are black if you would like I could send you the link for a ford factory wiring diagram for our year car its really nice to have and shows all the connectors and their locations
  6. Damn appreciate the link but you don't see any yellow wire as described and show getting grounded?
  7. I can check for you tomorrow you said it is located near the starter solenoid? Which way does it go head lights or tail lights direction?
  8. Thanks

    It comes from behind the driver side strut tower. Not sure where it should go because the guy before did a wire tuck and moved everything into the inside fender.
  9. Are you sure it isn't a wire added by someone else? Almost all the OEM wiring had a solid wire color and a tracer stripe for each wire.
  10. Yeah the yellow wire in question is a factory wire. It comes from loom of wires that starts at the rear of the driver side strut tower area and runs towards the front of the car but goes as far as the starter solenoid area before it's grounded inside the fender by the previous owner.

    Previous owner can't but thought it was a ground... he put heads on the car and while he was doing that decided to disconnect everything on the driver side and run the wiring through the strut tower into the inner fender so all the wiring was out of site. He lenghtened a few wires which were actually soldered and wrapped nicely but as he turned the key to fire it up a wire cooked inside that inner fender which turns out to be this yellow wire. So with that issue the fuel pump does not power on/prime. Fuel relay checked out, computer and the relay for it checked out as well as the inertia switch.
  11. This is all the info I have on a solid color yellow wire in that area.

  12. Damn well I appreciate the help.

    The yellow wire in question is factory and I guess I'm just gonna have to traces it into the car and try to find where it goes but I'm thinking it probably belongs on the starter solenoid.

    Also I dont have a blue wire on my starter solenoid like pictured above.
  13. OK that yellow wire definitely goes onto the solenoid
  14. Mike that's what im thinking but are you certain? were you able to tell from your diagram?

    Reason why I ask is because the yellow wire that splits off to four or five fuse linked wires is actually attached to the solenoid already so I want to make sure were not getting that one confused with the mystery yellow wire here.

    Here's a pic of the starter solenoid and whats currently attached to it. The yellow one you see is not the wire in question.. that wire is behind there on the inside of the fender (due to the attempted wire tuck)


    Thanks for all the help guys really appreciate it.

  15. No I'm mistaken my thoughts is strip the wire back in the loom to see if she changes colors and also post a pic of what the connector looks like too.
  16. Yeah ok figured it wouldn't be that easy ha. I'll follow the sob and try to find it inside the car because I know it runs by the brake booster.
  17. Yeah unveil it and just trace her on back
  18. ok got sometime to get over to the car and undo some sleeving... looks like this sob 12g yellow wire goes from behind the driver's side strut tower across the firewall and into the pas side interior and it's part of the factory main harness. I can't find this wire on any of the diagrams online... On the inside it's hard to tell which direction this wire goes but seen a 12g yellow going to the ignition switch, above the fusebox and maybe the ecc relay. If it's one of those mentioned wouldn't it require power and should I hook it up to the starter solenoid?

    I hooked the battery back up and verified 12v at the alt, starter solenoid and at the fuse box.

    There is no interior lights, no door chimes, no fuel pump prime, no dash lights, no dumby lights nothing.

    here's some pictures



  19. There is a yellow wire that goes to the EEC relay and pin 1 on the PCM, but it should be connected to 12v through a blue 20g fusible link.