Fuel Pump on 90 GT won't prime after driving

jeffuhde

New Member
Jul 8, 2006
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Anyone seen this one before?

Have 90 GT that won't start after I've run it about 15-30 min (time seems to vary). Example is if I take it out to run errands - at about the second or third stop, the car won't start again. Fuel pump and car was running fine up until the point where I parked it and shut it off. I troubleshot it down to the fuel pump system I think because I'm not hearing the pump prime once I turn the ignition on at the point it won't start. If I leave it sit 3-4 hours, pump primes and it'll start right back up. I'm thinking it may be the relay.

Anyone have any thoughts on this?
 
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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.
attachment.php

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.
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.


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

Diagram courtesy of Tmoss & Stang&2birds

fuel-alt-links-ign-ac.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.

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.

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.
 
No, amount of gas in the tank doesn't seem to matter. I probably have 7/8 right now as it is.

I'll try the fuel pump relay first and see where that gets me. Autozone has 'em for only $9.99, so it's a cheap/easy try at fixing it. After that I'll trouble shoot with jrichker's troubleshooting if need be.

Appreciate the insight!

Thanks!
 
when my pump went, it wasn't just a 'done' kinda deal. i could drive for a bit after it cooled down but then it would heat up, and then my car would die. i could start it up after a while, but when it heated up again, sputter and die. dont know if that helps
 
I CAN ALMOST BET $100 bucks it's the fuel pump relay! Mine was doing the same thing and i got a new one after looking threw hours of jrichker's scamatics (witch he has posted i see :D ) replaced it with a $15.00 or so one and fired right off no problems ever since. On yours it's located under the driver seat! DO this first then keep searching! peace


john:p
 
Well, I hope John didn' t bet the $100. Installed the new relay last night then took it out for a spin for 30 minutes. Pulled back into the garage and same ole same ole, no prime. Guess it's the pump. :bang:

Can you guys recommend a brand of pump? I'm not looking for anything special, don't race or drag, just looking for something good and reliable. Thanks again.
 
Well, finally got around to trying a new fuel pump and that was it. Car has been running great now for 3-4 weeks. Nice to be able to "count" on it starting everytime no matter what.... Thanks again guys.
 
Well this sucks after reading all this again and doing the tests, i'm shure the ECM on mine is bad! I'm going today to get a new one from a buddy for $50 hopefully that will fix it?! It says the ECM are good and durable on these UNLESS it's suffered trama. Well stupid be arced out the battery preatty dam good when changing the alt one day and the problem started after that. I tested the jumper wire from the ECM test port and the fuel pump works then, also replaced the fuel pump relay first cause it's cheap and i thought it was that before doing these tests. Wish me luck and hopefully in about 3 hours from now it will be purring again! peace



john:p
 
Make sure you are getting spark when the pump quits working. No spark means the PIP sensor
inside the distributor has died. The computer turns the fuel pump on and then off for a start cycle.
When the computer see a continous stream of pulses from the PIP sensor above a certain speed,
it turns the fuel pump on and holds it on. That's why it is important to check for a spark when you
have a starting problem.