Fuel pump...can I run a dedicated power line

Sure, but why add a switch, you already have an ignition switch, just use that

Do it right. Bypasing the computer control of the fuel pump is asking for trouble.

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.

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.

Diagram courtesy of Tmoss & Stang&2birds

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.

seijirou said:

Sure, but why add a switch, you already have an ignition switch, just use that

Click to expand...

Not a good idea. The computer shuts the pump off after about 5 seconds when you turn the key to Run but don't start the engine.
This keeps the pump from running continously anytime the ignition switch is in the Run position. This does several things: it saves
the battery, and reduces wear on the fuel pump. More importantly, if you had a leaking injector and left the pump running continously,
you would flood the engine. The other thing that would happen is that it would wash all the oil from the cyinder walls of the cylinder
with the leaking injector.

As you can see there are good reasons why things work the way they do. Bypasing computer control of the fuel pump on an EFI car
is a bad idea.

jrichker said:

Not a good idea. The computer shuts the pump off after about 5 seconds when you turn the key to Run but don't start the engine.
This keeps the pump from running continously anytime the ignition switch is in the Run position. This does several things: it saves
the battery, and reduces wear on the fuel pump. More importantly, if you had a leaking injector and left the pump running continously,
you would flood the engine. The other thing that would happen is that it would wash all the oil from the cyinder walls of the cylinder
with the leaking injector.

As you can see there are good reasons why things work the way they do. Bypasing computer control of the fuel pump on an EFI car
is a bad idea.

Click to expand...

I completely agree, I would (and have spent many hours doing so once) restore the stock function myself rather than running a new circuit. My post was basically in reguard to 'will it work'. Good looking out the the words of wisdom

Please read the description again and look at the diagram.

The red/black wire on the relay should have 12 volts. The tan/green side will have 12 volts when the computer turns it off. When the computer turns the relay on, the tan/lt green wire will go to less than 1 volt.

Jumpering the test connector tan/lt green wire to ground will turn the fuel pump relay on when the ignition switch is in the Run position. Use this feature to aid you in troubleshooting the wiring. The computer grounds the tan/lt green wire to turn on the relay. Grounding the tan/lt green wire at the test connector for test purposes won't hurt anything and does not disturb the computer.

If you're getting 11.7 at the inertia switch your relay is working.

The only thing left downstream is the connector after the inertia switch, the pump itself, and ofcourse the ground comes last.

Check out those 3 things, it's one of those.