Need Fuel Pump Circuit Help!!!! At Wits End

Are you able to trigger the relay manually then ohm between the power feed terminals?

there is a possibility that the new relay is defective; new doesn't always mean it's good.

I can't really think of anything else...

I would check the resistance of the EEC ground wire just to make sure - I know you said you grounded it yourself, though. The stock EEC ground wire has the quick-connect located to the right of the starter solonoid and has a wire running from that connection directly to the battery cable. You probably know that, but just thought I'd throw that out there.

Good Luck.

You say that when you jump power (bypassing the relay) that it all starts up and work correctly. Are you able to hear the relay *click* when you turn the key on? If so, you need you disconnect the power feed (not the trigger) wires from the relay and ohm out the two posts to ensure that contact is being made. If you're not hearing the relay click on then remove it completely and feed power and ground directly to the relay trigger posts to see if the relay is even functioning at all.

I like Dag's thinking about bench testing the relay. And I agree with James and him - It seems like you have all the elements needed, and a bad relay or an insufficient draw-path is the problem.

Good luck.

Fuel Pump Troubleshooting for 87-90 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. See http://www.mustangworks.com/article...c-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/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/t...witchWiring.gif

http://www.veryuseful.com/mustang/t...inks-ign-ac.gif

http://www.veryuseful.com/mustang/t...91eecPinout.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.

Glad to hear you got it. I was in full agreement with you before - your troubleshooting was the same route I would have gone as well.
Good to know that the parts store is not the place to get the EEC relay (I am sketchy on getting relays there period, though my bud did just get a FP relay which worked).

Thanks for sharing what the fix was.

I would assume that a relay is a relay - although I could be wrong; my bet is that quality was the factor between the AZ relay and the Ford relay.

Glad you got it fixed though; the fix wasn't really hard, just kinda expesive considering you had to buy two relays and all.

I've gotten a couple fuel pump relays from the parts store and they seem to work fine, but I agree quality isn't always there.

this may be a stupid reply but check the ground up front next to the battery..........went threw this a while back.

ohyouknow said:

this may be a stupid reply but check the ground up front next to the battery..........went threw this a while back.

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I think he got it fixed, but that is a good idea.

I'm not getting any hum from the fuel pump and no fuel pressure but am getting a clicking at the fuel pump relay. I jumped the realy and still nothing, so the pump is fried right?

If you have no voltage output at the N.O. terminal on the FP relay, and/or no voltage at the pump itself, then yeah, it sounds like the pump is gone.

It was the pump, new walbro in and started right up. Thanks.

BPH90LX said:

It was the pump, new walbro in and started right up. Thanks.

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Had a feeling about that. Nice work!

Sorry to bump an old thread, but I'm having a similar problem. My car when sitting over night will fire right up. If it start the car 4 or 5 times, then on the next try, it won't start. Could the pump be bad considering it starts occasionally? On the times when it doesn't start, I do here the pump relay under the seat clicking, but the pump isn't priming. Could the EEC power relay be intermittently working? Or is the pump just shot?

Turd , your best bet is to probe the relay terminals when it wont start. Clicking in the relay indicates the coil is energized and that it is a power-side issue (feed from the solenoid to the pump) or the relay itself.

THe first thing I would do is swap a known good relay in and see how it does (The WOT relay should be the same relay and can be used for this test).

Good luck.