Fuel Pump Not Engaging

Fuel Pump Troubleshooting for 86 Mustangs

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. This on and off again cycle helps to prevent flooding the engine when cranking. 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 power relay – located under the driver’s seat in most stangs .
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 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 (yellow 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.

Diagrams courtesy of Tmoss & Stang&2birds



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

Look for 12 volts at the yellow 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.

Check the Red/black 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 Red wire and Red/Black wire. Power on the Red wire and not on the Red/Black wire means the inertia switch is open.

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 will have to drop the tank to inspect the pump power and ground connector and the pump wiring chassis ground.

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. You should see less than 1 Ohm between the black wire(s) and ground. The chassis ground is up near the spare tire shell. 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. 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)

http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif

Did you check the control-side wiring? If either input is missing, a functional relay will not energize.

michaelg10 said:

Yes, checked all the wiring at the relay and the intertia switch.

Once I verified all the wires except the Pink were correct, with key on jumped the yellow spade to the pink spade in the back of the FP relay socket (bypassing the relay). The fuel pump whirred on, confirming a faulty relay.

Just followed the process instead of throwing parts at it and it worked! Thanks again!

Click to expand...

That's a concept I try to convey (sometimes with limited success) to others. I am glad that it proved successful for you.

I am having a strange issue with my 93 LX.

I started it up a couple of weeks ago and it ran for about 45 seconds and died and has not started since.

I will go into detail what I have done so far and if anybody has some other things to try let me know.

I bought new FP relay form the parts store the day it stopped working, that did not fix the problem. Thought well maybe got a bad relay so I returned it and got another relay from another parts store, that did not fix the problem. Got pissed closed the hood.

This past Sat I bought a new ECC relay that did not fix the problem. So, I noticed that when I put both of the new relays in (FP and ECC) the pump did not turn of. If I put the old FP relay in and kept the new ECC relay in pump don't come on. If I put new FP relay in and old ECC relay in pump wont come on. If I put old FP relay in and old ECC relay in the pump primes up but, does not shut off and car will not start.

Check spark from coil and I have spark, had the Ignition Control Module tested and it past.

I don't understand the relay issue, did I get 3 bad relays from 2 different stores? Do I need to get an OEM relay? From the check list posted from jrichker where should I start checking?

Any help would be great.

Thanks
Wade

stang4wade said:

I am having a strange issue with my 93 LX.

I started it up a couple of weeks ago and it ran for about 45 seconds and died and has not started since.

I will go into detail what I have done so far and if anybody has some other things to try let me know.

I bought new FP relay form the parts store the day it stopped working, that did not fix the problem. Thought well maybe got a bad relay so I returned it and got another relay from another parts store, that did not fix the problem. Got pissed closed the hood.

This past Sat I bought a new ECC relay that did not fix the problem. So, I noticed that when I put both of the new relays in (FP and ECC) the pump did not turn of. If I put the old FP relay in and kept the new ECC relay in pump don't come on. If I put new FP relay in and old ECC relay in pump wont come on. If I put old FP relay in and old ECC relay in the pump primes up but, does not shut off and car will not start.

Check spark from coil and I have spark, had the Ignition Control Module tested and it past.

I don't understand the relay issue, did I get 3 bad relays from 2 different stores? Do I need to get an OEM relay? From the check list posted from jrichker where should I start checking?

Any help would be great.

Thanks
Wade

Click to expand...

You should probably start your own thread.

What values did you find at the relay sockets at any given time?

The problem with trying to infer anything from the switching of relays is that if the issue is intermittent (sounds like yours is), the inferences drawn are invalid.

Well since I did not really know what to look for I just tested voltage by probing the relay connectors and had 12-volts at the FP, FP and ECC relays and 12-volt at the FP only with the old relays installed but the pump won’t shut off.

I am going to pick out what I think I will need to test from the posted check list and go from there. It sucks that it started and then died at that’s it, no rhyme or reason just like I turned the key off.