Fuel Pump Wont Turn On

The FP relay has power going to it? There are a couple of power wires at the relay. Is the power going to it constant (i.e., battery power)?

Have you jumped the common and Normally open FP relay terminals to see if the pump runs?

If so, check for power and ground to the relay coil. The power to it is constant with the key on and the ground is modulated.

Jrichker's Cranks but No-Start checklist (in the useful tech sticky) and his fuel-pump-diagnostics post contain diagrams and a specific diagnostic checklist.

Good luck.

As suggested...

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 lower 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 seat.
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.


diagram of the wiring for 91-93 cars.

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.

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.

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.
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

coffmaad said:

Ok. I hope I'm not buggin anybody but I just want to figure this out cause its driving me nuts. I grabbed my test light tonight and placed it on each side of the inertia switch terminal and turned the key on. Both times it lit up for approx 2 secs and then went out. Is this a pretty good sign that the fuel pump is bad?

Click to expand...

No. What is your fuel pressure reading while this occurs and while cranking? Have you bench tested the FP with your own fused power and ground sources?

coffmaad said:

There is no fuel pressure at all. The pump is not turning on what so ever. I tried the eec fuel pump test point and placed my volt meter in the inertia switch on both sides and when I grounded it, it read 12 volts running through the inertia switch to the fuel pump and still nothing happend with the fuel pump. No pressure and didn't turn on while grounded.

Click to expand...

Ok, I kinda misread your earlier post (I assumed 12V through the inertia switch now meant the pump was priming for 2 seconds. My bad). Your logic seems sound.


I would now test for battery voltage at the pump's electrical connector (located above the tank) while the pump should be running. If you don't have it, ohm out the ground and check the 12 volt feed independently (dont use both at once as your power and ground sources).

If copacetic, I'd still use a fused jumper wire and ground from the battery to bench test the pump (while still in the car). If that test produces no pump noise and no pressure, test the connector at the pump itself. It can have a bad connection. If you have power and ground there, then I'd replace the pump......