another fuel pump doing strange things

Discussion in 'Fox 5.0 Mustang Tech' started by jcgafford, Mar 14, 2012.


  1. jcgafford

    jcgafford Advanced Member

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    new problem. fuel pump is on the fritz. turn the key and nothing, tap the relay and it runs and won't shut off. replaced the relay with new one (twice) and same thing so three different relays with the same results. Went through the fuel pump checklist by jrichker and have hot to red wire key off, hot to all wires minus the green(ground) key on pump running(continuously) for the fp relay. jumped the pump off the test wires and it runs. inertia switch is working properly, pulled it an it killed the pump, pushed it in and it ran again. also checked it for power first and it was hot on both sides with the key on. what next? it drove into the garage a week ago and it ran great till i went out to work on it. now it is driving me crazy. any help would be great, thanks.
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  2. jcgafford

    jcgafford Advanced Member

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    checked the light blue and orange wire with a test light and it glows brightly, so that checks out....
    #2
  3. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    I need the year of your car to select the correct fuel pump troubleshooter. You will need a voltmeter or DVM to accurately check the circuit
    #3
  4. jcgafford

    jcgafford Advanced Member

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    1992 gt convertible... fuel pump runs everytime you turn the key now. yesterday it was hit or miss if the ignition would turn it on or not. it was 50/50 yesterday and today it is every single time. almost like it is wet wiring drying out, although it appears all dry and i did not water the engine!

    red wire is not hot with key off, all others besides light blue with orange stripe wire and red wire are hot with key off
    #4
  5. jcgafford

    jcgafford Advanced Member

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    light blue and orange wire has 5.3 v with key on pump running. all others have 12ish at same time
    #5
  6. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Fuel Pump Troubleshooting for 91-93 Mustangs

    Revised 21-Oct-2011 to Add location of fuel pump relay on 91 model year cars

    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.

    [​IMG]

    [​IMG]

    [​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'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.
    [​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 1.5 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.


    Your problem: light blue and orange wire has 5.3 v with key on pump running. all others have 12ish at same time
    The light blue/orange wire is used by the computer to provide ground to the relay coil. it should read less than 1.5 volts. See the control path, subsection relay in the troubleshooter above. That should help you figure out if it is the relay socket (corrosion in socket) or wiring (broken wire or bad connection) or the computer. Don't just swap the computer without doing the tests, you may spend $90-$120 and not fix the problem
    #6
  7. jcgafford

    jcgafford Advanced Member

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    thought that might be the problem. i will tear the socket apart tonight and check it, where does that wire lead to? the computer? So i can just run a wire from point A to point B and bipass that old one to test right? It does not go anywhere else but from point A to point B correct?
    #7
  8. powertrax91

    powertrax91 Founding Member

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    Ultimately the computer controls the fuel pump. If all of the other components test okay it's time to start looking at the computer. I had the same issue. jrichker's trouble shooting guide is fantastic, that's how is was able to fix my car. Get your hands on a computer.
    #8
  9. jcgafford

    jcgafford Advanced Member

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    yup, been through it all. the one wire with 5 ish volts is suspect. once i clear that as a problem i am on to the computer. friday night is the next time i can get into it again so we shall see.
    #9
  10. jcgafford

    jcgafford Advanced Member

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    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 1.5 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.


    is that with key on, wires jumpered and pump running like it always does? It does glow with the test light when checked with key on wires jumpered. but does have the high voltage too.
    #10
  11. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    All automotive circuits have to have a ground to operate. The computer provides the ground for the fuel pump relay coil (orange/blue wire). That wire should have less than 1.5 volts with the ignition switch On because the computer has switched the circuit to ground.

    When the ignition switch is on, the relay has 12 volts at the pink/black (power feed) and the red wire (relay coil power). The orange/blue wire is the ground circuit for the relay. With the computer grounds the orange/blue wire, the relay contacts close, and current flows from the pink/black wire to the dark green yellow wire connected to the inertia switch. If the inertia switch hasn't tripped open, it passes power to the fuel pump wiring.

    Re-read the troubleshooter description, and think about how it works. Then do the tests before making any more posts. I am here to help teach people to think logically, and this is your logic test... Get out your voltmeter and go at it.
    #11
    jcgafford likes this.
  12. jcgafford

    jcgafford Advanced Member

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    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.

    infinite , will not register on any ohm test setting .


    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 1.5 volts.


    almost 6 volts present, glows with test lamp.

    Relay:
    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.

    test light glows, both tests.

    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.

    fuel pump runs at full pressure.

    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.

    probed the light blue and orange wire at pin 22, test light was glowing.had test plug jumpered. turned key and no pulse/flash. fuel pump turned on and ran continuously. it is screaming bad computer to this novice, your take on it?
    #12
  13. jcgafford

    jcgafford Advanced Member

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    can i ask questions now jrichker? LOL
    #13
  14. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    I do not answer tech questions by PM or email.

    Did you measure the voltage at pin 22 on the computer when you turned the key on? You only have a 2 or 3 second window to see if the voltage drops below 1.5 volts before it shuts off. It shuts off to reduce the chance of flooding the engine with gas when starting. If you did this and saw greater than 1.5 volts, swap the computer.
    #14
  15. jcgafford

    jcgafford Advanced Member

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    Did you measure the voltage at pin 22 on the computer when you turned the key on? You only have a 2 or 3 second window to see if the voltage drops below 1.5 volts before it shuts off. It shuts off to reduce the chance of flooding the engine with gas when starting. If you did this and saw greater than 1.5 volts, swap the computer.


    0.00 volts key off. 9.9 volts constant with key on, probed in wire of pin22 or light blue and orange striped wire right next to a9p. 5.3 volts near the relay.

    looks bad to me. now a couple things that bother me and my take on it.

    1. the high voltage in that ground. i assume this is because the a9p is fried and it is bleeding over from somewhere inside.

    2. the fp relay coming on when flicked with a finger. i assume this has something to due with that voltage feeding up the ground wire to it and overloading it just barely .

    am i thinking right, or am i way out in left field? dont want to burn up a new one by missing something.
    #15
  16. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    The 9.9 volts on computer pin 22 tells me that there is a problem with the computer. However, in a effort to eliminate other possibilities, check the computer power ground.


    The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

    The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
    Picture courtesy timewarped1972
    [​IMG]

    Correct negative battery ground cable.
    [​IMG]

    3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
    In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
    In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
    You'll find it up next to the starter solenoid where the wire goes into the wiring harness.
    #16
  17. jcgafford

    jcgafford Advanced Member

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    already checked them all for broken wires, corrosion and being loose. pulled the computer quick disconnect with key on pump running to see if it was operational. it killed the pump instantly . i think it is a done deal as to ordering another computer , unless you have any other suggestions.
    #17
  18. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Looks like the computer is the last remaining suspect...
    #18
  19. jcgafford

    jcgafford Advanced Member

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    just wanted reassurance. Found the computer for 80 bucks at autozone. Dorman reman. Is that the best I can do or is there a better source you know of? Thanks for all your help.
    #19
  20. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    The quality of the reman and the length of the warranty are what you need to consider. Match the sticker on the end of the computer to the transmission type.
    Auto trans cars use an A9P
    5 speed cars use an A9L.. The A9P can be used if that is all you can get.
    Do not use an A9L in an auto trans car! the start circuit wiring is different and you will damage the computer.
    #20

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