fuel pump not working after restomod

The computer does not power the fuel pump. The computer sends a ground pulse to the FP relay's coil, which when combined with key-on 12 volts from the EEC relay at the other relay-coil terminal, connect battery power to the pump.

I'd access your FP relay and test the terminals with your meter so you can see what element is missing. This gives you a direction to go.

Good luck.

You can check the relay under the driver side seat.
Kevin

I have 3 different test paths for fuel pump power problems. I will be glad to post the proper one to help you fix your car. But you need to update your sig so that we all know what year car and wiring harness you are using. The wiring changed at least 3 times between 79 and 95.

Fuel Pump Troubleshooting for 86-90 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 power relay – located under the driver’s seat in most stangs built
before 92. 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. All the applicable fuse links are in the wiring
harness up next to the starter solenoid.
F.) Fuel pressure regulator failed. Remove vacuum line from regulator and inspect
for fuel escaping while pump is running. If you find fuel escaping, the regulator has failed.

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 (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). 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.

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/

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

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif



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

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.

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.

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.

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.

The computer does not source power to any relay, actuator or injector. It supplies ground for all of these items.
This is known as current sink technology. It protects the computer from damage if a short circuit to ground occurs
in the wiring between the computer and the device it is controlling. If a short to ground does occur, there is almost
no current flowing from the computer to the short.

The computer provides a ground to complete the circuit for the relay coil. One side of the relay coil has power
anytime the ignition switch is in the run position unless the inertia switch is tripped.

To add to JRichker's sentiments, there have been instances of the ground strap failing in the computer. If this occurs, the ground pulse to the control side of the relay is missing (remember, that ground pulse is modulated - it's not a constant ground. If it were, the pump would not prime-out but instead stay on whenever the key was on). In other words, it would act like the circuit does when you manually ground this wire at the self-test connector.

Good luck.

Spook302 said:

So I got my other stang here today (89 coupe,another project of mine) and I popped the hood and started checking wires all over the place to compare my install and I noticed a quick disconnect for ground that I dont have the cars side on my car. I must have lost it when I pulled everything to paint the engine bay. Its a petty thick groung wire. I can only imagine that this was my problem all along. This was the only scew up I have found in the entire project.

Click to expand...

Follow the wire back a little ways and you will find 2 black/green wires. They are the computer's main power ground. Without them, the computer will not operate. The singe ground beside the computer is a ground for the shield on the TFI wiring.