Fuel Fuel Pump Not Working.

Discussion in 'Fox 5.0 Mustang Tech' started by JPD686, Aug 10, 2012.

  1. 1989 Mustang 5.0 LX hatch.

    Fuel pump does not work, have replaced the following, EEC Relay, Fuel pump Relay, Fuel pump.... still do not hear it prime and will not fire. Checking power under the seat i get power to the orange wire all the time, and never to the pink wire regardless of ignition status... I have bypassed the inertia switch so that is not the problem. I have also jumper the orange wire to the pink wire (from under the seat) and got the fuel pump to run.

    Any hints? Also, i have checked the fuse-able links(4-6 of them under the solenoid?) and they seem to be letting power travel through them while ignition is on.
  2. After studying the attached, If you can put power to the Pink wire and get the pump to run, it may be that you still have a bad relay (even if they are new). You have seemed to cover everything else.
  3. Fuel Pump Troubleshooting for 87-90 Mustangs

    Revised 10-Aug-2012 to update fuel pump run time on initial startup

    Clue – listen for the fuel pump to prime when you first turn the ignition switch on. It should run for 1-3 seconds and shut off. To trick the fuel pump into running, find the ECC test connector and jump the connector in the upper LH corner to ground.



    Turn the ignition switch on when you do this test.

    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 Mustangs built before 92. 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.

    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.


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

    All voltage reading are made with one voltmeter lead connected to the metal car body unless otherwise specified

    Check for 12 volts at the red wire on the inertia switch. No 12 volts at the inertia switch, the ignition switch is turned off or faulty or there is no power to the ECC (computer ) power relay. To be sure look for good 12 volts on the red wire on any fuel injector.
    Good 12 volts mean the ECC relay is working. No 12 volts and the ECC wiring is at fault.

    Look for 12 volts on the red/green wire on the ignition coil: no 12 volts and the ignition switch is faulty, or the fuse link in the ignition power wire has blown. No 12 volts here and the ECC relay won’t close and provide power to the inertia switch. Check the Red/black wire on the inertia switch, 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. Push the button on the side of it to reset it, and then recheck. Good 12 volts on one side and not on the other means the inertia switch has failed.

    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.

    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. In extreme ghetto cases, the pump relay may have been bypassed. 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 jrichker, Aug 10, 2012
    Last edited: Feb 12, 2014
  4. I have power at all the fuseable links, just double checked... no power to any injectors, no power to intertia switch(either red wire). I DO get power at the orange wire under the seat. I have replaced the ECC Relay as well as fuel pump and still the same thing. Im thinking something to do with computer? or the ignition switch..... (that would not be the lock cyl and keys, you are talking the peice the lock cyl and keys go into correct?
  5. and one other thing, it just happend out of the blue, came out of a conveinance store and it just would not fire, and im 100% sure the fuel pump is not running meaning no pressure at all.... my fuel pump was ALWAYS really loud... louder than any other mustang i owned, so i figured it was the pump that went on me....
  6. You talk about checking fuseable links and relays, but what about fuses?

    Edit: Nevermind, I looked at the schematic...
  7. The EEC relay provides power to the Fuel Pump relay coil. Look for not 12 volts at the red/green wire on the ignition coil. No 12 volts and you have a bad ignition switch or blown fuse link in the ignition circuit.

    Diagrams courtesy of Tmoss & Stang&2birds



    Some of the symptoms of ignition switch problems are things that don’t work or are intermittent like radio, turn signals, wipers or heater.

    There was a FREE recall on Ford ignition switches. They overheat and sometimes catch fire. That burns up the steering column and sometimes the car interior. Since this is very old information, you may not be able to get the switch replaced for free anymore. The auto parts stores sell the switches for $13-$15.

    Saleen0679 was nice enough to dig this up for us awhile back: Replace a 1979-1993 Ignition Switch Assembly

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

    Ignition switch wiring

    Fuel, alternator, A/C and ignition wiring

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

    Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

    Vacuum diagram 89-93 Mustangs

    HVAC vacuum diagram

    TFI module differences & pin out

    Fuse box layout

    87-92 power window wiring
    http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

    93 power window wiring
  8. fuse links are all ok (ones under the solenoid? they all let power on both sides of them. the red/green wire on the coil has no power either.... so im guessing faulty ignition switch???
  9. Sounds like the ignition switch is the problem.
  10. **** me... i replaced the ignition switch, still a no go...... i grabbed the computer out of a 92 5.0 that i have sitting around that i know runs, "a9l" was stamped on both. Still not working... i have about 5 different relays that im swapping in and out so i dont think relays are the problem..... where should i begin my **** up wire search??
  11. Go back and study the two diagrams I posted showing the computer power wiring and ignition switch.

    Look for 12 volts on the 2 yellow wires on the ignition switch connector. No 12 volts and the fuse link that feeds them is blown. The fuse link is in the wiring harness up near the starter solenoid.

    Since you replaced the ignition switch electrical part, we can skip that as being a problem source. Here's a picture of the ignition switch for reference:

    Good 12 volts on the 2 yellow wires, turn the ignition to Run and look for 12 volts on the red/green wire on the ignition coil. No 12 volts and the fuse link inside the wiring harness for the red/green wire has blown.

    Remove the passenger side kick panel and dash speaker. The computer power or EEC relay is mounted up above the computer. Remove the two screws that hold the computer in the plastic strap mount and pull it down. The computer power or EEC relay is mounted on a small plastic Christmas tree shaped plug that pushes into a hole. You will have to pry it loose out of the mounting hole without breaking anything. It is stuck in there very good, so it will be hard to get out. Removing the glove compartment box may provide more room. Pull the relay down so that you have good access to the wiring.

    Check for good 12 volts on the black/orange wire that feeds power to the computer power or EEC relay. No good 12 volts here, and the dark blue fuse link coming off the starter solenoid has blown.

    Turn the ignition to Run: good 12 volts on the red/green wire for the ignition coil, look for 12 volts on the computer power or EEC relay red/green wire. No 12 volts and there is a broken wire or bad connection between the ignition switch and the computer power or EEC relay.

    Turn the ignition to Run: good 12 volts on the red/green wire for the computer power or EEC relay, look for good 12 volts on the orange/black wire on the computer power or EEC relay. No good 12 volts and the relay isn't working. This is either a bad relay, corroded relay socket or missing relay ground.

    Look for 12 volts on pins 37/57 and on the fuel pump relay socket. No 12 volts and you have a broken wire or bad connection in the junction between the orange/black wire and the red wire.

    End of test path.
  12. no power to coil and i get power on the two yellow wires and the red/green wire coming out of the ignition switch. How do i replace the fuseable link inside the harness and where in the harness is it... thanks for the quick replys by the way its really helpful
  13. i have been screwing with this all day to no prevail.... all the fuseable links are good, i have taken apart the harness and check that it lets current through them and im still not getting power at the coil, injectors, or inertia switch. I get power to the black wire for the eec/ecc relay and power at the orange wire at the FP relay.... under the solenoid i have quadruple check and all the links there let power through. The pump went off once, but then i tried to start and it never went off again... i have re-grounded the terminals to the battery and the wiring harness to chassis/battery as well.
  14. FOUND IT! god damn fusible link that i did not know where it was.... the one for the ignition switch to coil fusible link was ****.... its a small blue plastic thing that i didnt think was fusible.... fixed... finally. thank you all who helped.... after this i will be able to diagnose any electrical problem on my mustang lol.
  15. I am glad you got it fixed. For future reference, can you give a better description of the blue fuseable link and it's location? It could save someone else a lot of time and trouble.
    Jarvis360 likes this.
  16. The location of this link was by the brake booster going into the main harness.... i may have been one that was replaced because it did not look anything like the other fuseable links. It was a small blue plastic peice, i tested continuity and it wasnt letting anything through it.... so i cut it out, grabbed a regular fuse socket and put a 20A fuse in there and wired it back up... Turned key and boom, pump was on, fires right up, runs better then ever now that the circuit it 100%
  17. Thanks for you input. I will add this little tidbit to my tech note.
  18. I know this is an old thread but it seems most relevant.

    if I have a bad ignition switch, could I still have power at the coil? And still have a dead fuel pump?
  19. Not likely.

    Go back and look at my first post in this thread. It contains a complete fuel pump troubleshooter. Use it and you can find your fuel pump problem.

    Otherwise, see The cranks OK but will not start checklist for Fuel injected 5.0 Mustangs. http://www.stangnet.com/mustang-for...t-for-fuel-injected-mustangs-fox-sn95-5-0l.7/
    #20 jrichker, Feb 12, 2014
    Last edited: Feb 12, 2014