fuel pump will not prime

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. See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html for a description of the test connector. If the relay & inertia switch 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.
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 wire) then from the inertia switch to the relay coil and then from the relay coil to the computer (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) 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.

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

Download the diagram http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif and follow it and the tech description in my previous post. All the info you need to understand how the fuel pump control circuit works is there.

There must be an open circuit in the Orange/Lt Blue wire if there is no power at the fuel pump relay. Re-check your wiring at the fuse link. The Orange/Lt Blue wire joins the fuse link and goes to the fuel pump relay. The other side of the fuse link goes to a 16 gauge black wire and then to a 10 gauge yellow wire which connects to the battery side of the starter solenoid. According to the Chilton’s manual I have, there is at least one connector in the Orange/Lt Blue wire somewhere under the dash (?). I can’t confirm that since I have never found it.

Alternately, you could run a 14 gauge wire with a 15 amp fuse from the battery side of the starter solenoid to the Orange/Lt Blue wire on the fuel pump relay. This will keep the computer control of the fuel pump operation just as it did with the stock wiring.

Try resetting the Fuel Pump, In your hatch area, near the backing of the taillights there should be a black grommet on your drivers side panel in the rear, take off that cover, and look for a white box, there is a small switch or push button on top, push it to reset the system, then try turning it on.
-Nick

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