ground path for the fuel pump

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 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.

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

I have a little to add. I will start on a circuit by finding the ground wires. They can be located by color if you use the links jrichker gave you. Then I jump some temporary grounds to them with alligator clips and pieces of wire I have in my box. Sometimes the circuit will start to work again. You cannot leave the primary fuel circuit permanetly grounded for safety reasons. But it will tell you if its a ground issue or not. Many times now I will just use my meter to check for ohms to ground.

When I first got my 90 I had all sorts of problems with my fuel system. First I replaced the tank... then the pump relay, then the ECC fuse ( POS ),then the filter, I bypased the inertia switch, I pulled the carpet & went over the wiring, thought mabey it was a low voltage problem... so in went a new alternator, and finaly a new 190 BBK pump. All of this over 2 months time & it still continued to have intermitent problems, I finaly traced the problem to a faulty conector on the top of the fuel pump. A trip to the local salvage yard, a harness for 20 bucks, & I have not had a problem since.