Ecu Unplugged Continues Fan And Fuel

I think those guys must have been pulling your leg (or they're bone-headed stupid), but either way I'd stop taking their advise. The ECU runs the fuel pump by providing a ground to the CCRM. It's not going to run with the ECU disconnected.

To eliminate the ECU from the equation, you jump the test port (located behind passenger strut tower) to ground, the fuel pump trigger as in the very fuzzy image below. You can also run the KOEO test which runs / primes the fuel pump, but jumping the port gives you more opportunity to check the running pressure since the pump stays on.

The fact that your fuel pump is running (and at least dribbling fuel) tells me it's much more likely a mechanical problem. Clogged filter or failed pump.

Can you hear the fuel pump running? You could try to disconnect the fuel line from the fuel filter and see if it's squirting out there (obviously have a pan / catch-can on the line and don't set yourself or your car on fire). This will help eliminate a problem with the pump, wiring, and filter.

If it looks like a good, steady stream, then it's possibly a clogged fuel line, but the better possibility is that it's a failed fuel pressure regulator (letting all the fuel just run back through the return line). You 'could' also have a stuck-open injector allowing all the fuel to drain out of the rail and not allowing it to build up pressure. In that case you'll have the strong smell of fuel out of the exhaust.

If you can't hear it running at all, I'd focus on the electrical side of things first. The fuel pressure regulator is on the back end of the passenger-side fuel rail (it has a single vacuum line connected to the top). I wouldn't replace it until you have the fuel pump running properly and can better test things. Merry Christmas to you too!

Unplugging the ECM should indeed turn the fan and pump on.

addermk2 said:

Unplugging the ECM should indeed turn the fan and pump on.

Click to expand...

Nothing surprise me, but it's definitely not the recommended / easiest method to test the fuel pump isolated from the EEC. That would be jumping the test port and ignition on.

The EEC relay gets its power from the EEC fuse (battery +). It's then switched by the ignition switch (switched +). The fuel pump relay gets its power from the Fuel Pump fuse (battery +). The fuel pump relay gets its switching + signal from the EEC relay output. The EEC provides the switching - signal (ground) to the fuel pump relay. You simulate that ground at the test port.

So electrically I don't know why disconnecting the EEC would make those relays close, other than perhaps a property of the EEC connector to ground everything when disconnected? (if the fuel pump relay were normally closed in the diagram below the pump would always run since it gets it power from battery +).

I'm interested in why this is thought to work, because logically / electrically it shouldn't (at least if the below diagram is accurate & complete).

I've given the OP several sure fire ways to diagnose his problem... and he has yet to do ANY of them.

I took a video, but it won't upload through crapatalk. I distinctly remember being able to unplug my PCM and kicking both the fuel pump and fan on. Right now only the fan is powering on, but it might be due to how I wired in my boost a pump.

Anyway, OP is kinda thick headed, and when I told him to simply run a 12v lead to his cut off switch to see if the pump came on, he didn't really understand.

I didn't think it was that complicated.

Fuel Pump Troubleshooting for 94-95 GT 5.0 Mustangs

Revised 29-Sep-2014 to add diagrams for CCRM and under hood fuse boxes.

Clue – listen for the fuel pump to prime when you first turn the ignition switch on. It should run for 1-5 seconds and shut off. To trick the fuel pump into running, find the ECC test connector and jump the connector in the lower LH corner to ground.







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 –Note that all the relays on 94-95 models are in the CCRM box under the hood
C.) Clogged fuel filter
D.) Failed fuel pump
E.) Blown fuse in under hood fuse box.
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.

Diagram courtesy of Tmoss & Stang&2birds


The control path consists of 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 relay coil and then from the relay coil to the computer (light blue\orange 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 the under hood fuse box located between the windshield washer filler and the driver's side shock absorber strut tower.. The feed wire from the fuse (pink/black wire) goes to the fuel pump relay contacts. The fuel pump relay is located in the CCRM box on the passenger side of the car up near the radiator. When the contacts close because the relay energizes, the power flows through the pink/black wire to the contacts and through the dark green\yellow wire to the inertia switch. The other side of the inertia switch with the brown\pink wire joins the pink/black wire that connects to the fuel pump. 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 http://www.autozone.com/servlet/UiB..._us/0900823d/80/1d/db/3c/0900823d801ddb3c.jsp for more wiring help for 94-95 cars

Now that you have the theory of how it works, it’s time to go digging.


Power circuits:
Under hood Fuses
Diagram courtesy of Tmoss & Stang&2birds

Click on diagram to enlarge it



CCRM relays - all CCRM relays are located under the hood

CCRM location
Diagram courtesy of http://ww2.justanswer.com

Click on diagram to enlarge it




CCRM Diagram
Diagram courtesy of http://diagrams.hissind.com/



Power feed: Look for 12 volts at the pink/black wire (power source for fuel pump relay). No voltage or low voltage, bad fuse link, bad wiring, or connections.

Relay: Turn on the key and jumper the ECC test connector as previously described. Look for 12 volts at the dark green\yellow 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.

Inertia switch: Check the brown/pink 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 dark green\yellow (inertia switch input) and brown/pink wire (inertia switch output). Power on the dark green\yellow wire and not on the brown/pink wire means the inertia switch is open. Press on the red plunger to reset it to the closed position. Sometimes the inertia switch will be intermittent or will not pass full power. Be sure that there is 12 volts on both sides of the switch with the pump running and that the voltage drop measured across the switch is less than .75 volts.

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.

Make sure that the power is off the circuit before making any resistance checks.
If the circuit is powered up, your resistance measurements will be inaccurate.

Fuel tank wiring connector



Control circuits:

Relay: The red wire for the fuel pump relay coil gets its power feed from the ECC relay. No 12 volts here, and the ECC relay has failed or there is bad wiring or bad connections coming from it. The ECC relay is located on top of the computer, which is under the passenger’s side kick panel. It is not easy to get to, you must have small hands or pull the passenger side dash speaker out to access it.

Relay: The light blue/orange wire provides a ground path for the relay power. With the test connector jumpered according to the previous instructions, there should be less than .75 volts. Use a test lamp with one side connected to battery power and the other side to the light blue/orange wire on the fuel pump relay. 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. Remove the test jumper from the ECC test connector. 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.

Computer: If you got this far and everything else checked out good, the computer is suspect .Remove the test jumper from the ECC test connector located under the hood . Remove the plastic cover over the computer wiring, but leave the computer wiring connector plugged into the computer. With the ignition switch in the run position, connect a test lamp to the battery and back probe pin 22, the light blue/orange wire with it. The lamp should glow brightly. No glow and the computer has died a sad death. If you used a voltmeter instead of a test lamp, you should see less than 1 volt.




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/ Everyone should bookmark this site.

addermk2 said:

I distinctly remember being able to unplug my PCM and kicking both the fuel pump and fan on.

Click to expand...

I definitely believe you, and I've also seen the same thing happen when I've uploaded a corrupted tune (fuel pump runs all the time). I was hoping you could help me solve the mystery. Perhaps the CCRM has more going on than the diagram shows. I still wouldn't recommend it (I hate working under that kick-panel).

And as always, jrichker has the ultimate guide, it ought to be a sticky! (along with no-start and surging idle checklists)