Fuel Pump Problems


New Member
Jan 3, 2003
Tampa, FL
My friend has a 1990 LX Mustang...and the valve cover was leaking a decent trickle so I told him I'd replace the gasket for him. Well the swap went smoothly...we go to start it up and the fuel pump is dead as a doornail. It won't even prime and then shut off.

I immediately checked everything under the hood, every connector hooked back up properly and I didn't take off that much. The only two grounds I could find was the one that hooked up to the bolt underneath the power steering pump and the one behind the rear of the valve cover that attached to the firewall. Both were tight and undisturbed...just for ****s and giggles, I took off the one that bolted to the P/S pump bracket and hooked it back up, still nothing.

I searched here, replaced the fuel pump relay underneath the drivers seat and nothing. I then checked all of the fuses and my checker lit up everytime and just for assurance, I removed everyone of them and physically inspected them and they were ok. The only 3 relays on the fuse block (The car doesn't have a fuse cover to show which fuse does what, so I have no idea)...one was the flasher (round) and the other smaller one controlled the power windows. The third relay type metal one (sorry, don't know the technical name) is unkown...its on the left hand side that is the only thing on the fuse box that could possibly be bad and I don't know what it conrols.

Now, I think inertia switch. So I get to that, its not tripped. I reset it anyway, try it and nothing. Now I think ok is there power getting past the Fuel Pump relay at lest to the inertia switch. I turn the key to the on posistion and my volt meter says 11.8x volts consistant.

Crap...the freaking pump must be dead. Being an F-body owner, I'm thinking GREAT this **** is gonna take ALL day. To my pleasant surprise it was very easy comparitively. Get the tank dropped, new fuel pump in and STILL NOTHING. :damnit: :damnit: :damnit:

At this point I lean over the engine in frustration running through my mind over and over again what I did and trying to trace myself...and the alternator is very warm...but the car has been off since 10am this morning and its now 6pm. I take the alternator out and eat dinner and come back, and sure enough it has cooled down...so somehow, the alternator is hot.

In summary now, the car cranks, all the accessories work. There is 12 volts getting to the inertia switch, but its not going down the the Fuel pump at all. Put my meter on the pump connectors and nothing. The alternator gets very warm for some reason but if it was internally shorted, why would the car still crank and get the power back to the switch at all? I have checked everything three times, everything is hooked up properly..grounds ok..fuses all working..

My only two guesses at this point is the switch being faulty or the ECM not sending a signal to the fuel pump. :shrug:
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Check the computer relay.... it's behind the passenger kick panel. My 88 LX did the exact samething a few weeks ago. The relay enables when the switch is turned on and then the computer will then turn on the pump relay for 3 seconds. I suggest checking there.
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)




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.
In the passenger side floor board there is a kick panel on the right, it's plastic. Remove the kick panel. Behind the kick panel you will see the computer it's rectangular 6 to 7 inches high and maybe 4 or 5 inches wide. It should be aluminum in color. There are a couple of relays in this area around the computer. One should be brown in color. 2 x 1 x 1 in size hooked to a wiring harness. Mine had rust stains coming out of it. I have a 88 lx convertible and I guess the relay had got wet at some point in time. But the relay can just go bad too. Hope this helps.
I was unable to find a relay near the ECM. :(

I did find the ECM and all of the connectors but no relays around it at all. There are relays under the drivers side dash area, one thats just hanging there with about 4 wires, the other has a ton (both are same size) and is hooked up behind the A/C and radio controls.
Dr_EluSivE said:
i believe on 1990's cars the FP relay is in the "black box" on the strut tower.i cant remeber what year exactly they changed though. Also check under the Drivers seat.


The fuel pump relay was underneath the drivers seat and the relay wasn't the problem. Here is an update of what I've traced today.

Found out the inertia switch is ok, the red/black and red both read 12v when its not tripped. If I trip it, the redblack is dead and the red still reads 12v.

I also tested the wires in the EEC test connector, everything is fine. I was unable to find the 22pin connector on the ECM to actually test it.

Here are some more questions:

1)If the Inertia switch is functional, that means power gets all the way back there and I know its the last gasp cut off for power to the pump. However, that must mean that the ECM is not sending a pulse signal to the tan/black wire to kick on the fuel pump, right? This means that the ECM must be defective. Correct me if I'm wrong.

2)Is the alternator supposed to get warm? Because it is. I've noticed this since sunday (read original post closely). I don't know about Ford cars that much, but if this was a GM, I would think that means something is shorted causing the alternator to heat up.

3)With the key in the on posistion, is it normal for the ECM to also get hot?

I am going to the yards tomorrow to try and find a 1990 ford 2.3 ECM to put in this thing and see if the ECM is fried. If a known working ECM doesn't fix the problem, I have no idea what to do. There has to be a way to just hot wire the pump and by pass the inertia switch to provide 12v to the pump while the key is in the on posistion.
Just bought a working ECM...nothing. :bang:

So I put the original ECM back in, said screw it. Clipped the inertia switch pigtail off, spliced the red wire into the pink/black wire going down the fuel pump and voila, she turned on.

I am going to have to get his help to put the tank back in etc...and I hope to god it starts and runs now with the inertia switch by-passed.

I am still very perplexed as to why that alternator stays warm when the key is off. :shrug:

EDIT - Alternator was bad, so a new one was purchased. The new one no longer gets hot. The car does get fuel now because I have just by-passed the inertia switch.

Now it still does not get spark! I just put that new fuel pump relay into the EEC relay pig tail and still nothing. What the hell is going on? :bang: :bang: This thing is an electrical nightmare!
only 6.8 volts at the fuel pump connector

only 6.8 volts at the fuel pump connector....I have the same problem except I have 6.8 volts at my connector. Pump wont run, but when i straight wire it to 12 volts it runs like a top. So is it my pump cause it wont run at 6.8 or is the wiring because it only has 6.8V. My car will start run for while then die...sometimes run for days then die. Sometimes the fuel pump primes at start and sometimes it does not.