Fox 88 foxbody no power to red and black wire at fuel pump relay

[drod88]

I have a 88 5.0. Since I bought it the fuel pump never held pressure. It has a permanent fuel pressure guage on the header panel. Recently the pump stopped priming all together. I figured it finally gave out so I dropped the tank. I noticed my gas tank sub harness was in rough shape so I replaced the pump and the harness. Now the pump still doesn't prime. I replaced the fuel pump relay still nothing. If I pull the relay out little bit the pump runs constantly even with the key off. With the key on I have no voltage to the red and black signal wire. If I apply 12v to the red and black wire the pump primes.

 

[AeroCoupe]

Literally discussing this on another thread here:

S

Thread '1990 foxbody 5.0 fuel system.'

Nov 28, 2025

I have a 1990 mustang 5.0. I can't get my fuel system to work. Just had my ecu rebuilt and still no progress. Fuel pump only comes on if I send 12+ to one wire on relay under driver seat. Ive tried everything i know to do. All fusable links under hood is good. Ive jumped the inertia switch trying that. Please help. Im all out of ideas.

• staceyj
• Replies: 8
• Forum: Fox 5.0 Mustang Tech

Have you checked for codes yet?

Electrical Thread 'How To Pull Codes From EEC-IV In 86- 95 5.0 Mustangs'

Jan 27, 2016

@Noobz347 - The code dump procedure isn't in the sticky - you may want to add it...



Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for...

• jrichker
• Replies: 10
• Forum: Fox 5.0 Mustang Tech

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[drod88]

I have not pulled codes i will do that today. I forgot to mention the eec relay was corroded on 1 of the pins I swapped the relay with a universal and still nothing. And I have no power at all to the inertia switch.

 

[AeroCoupe]

There are wiring diagrams in that thread I linked with some good trouble shooting info. If you cannot pull codes its a pretty strong indicator you need to get the ECU fixed. Most of us are using this place:

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECU Repair Service

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECUEngine Computer Repair ServicePlease fill out the R&R form here once you've placed your order.A Trusted Product with LIFETIME WARRANTY QUICK TURN AROUND TIMEDomestic and International Shipping available! Our Team is Available Monday - Friday 9am to 5pm...

www.theecuexchange.com

Not saying that is it but it very well could be. Run through that thread I linked.

 

[drod88]

I got it running! Apparently the eec relay and the inertia switch went bad simultaneously. I bypassed the inertia switch and replaced the eec relay and shes running. Codes indicate a cylinder 6 misfire. It runs really smooth but does seem underpowered. I've recently replaced the distributor, cap and rotor, plug wires, plugs, fuel pump walboro 255, bbk headers and h pipe, new coil. It runs too smooth for the cylinder to be dead im at a loss with this one. I had timing at 10° i bumped it up to 12 and then 14 still seems underpowered.

 

[AeroCoupe]

What distributor did you replace it with? Auto parts store brand or ???

 

[drod88]

Its a moto craft it is a used one tho im thinking it may be the culprit.

 

[Mcmahst]

Make sure to replace the inertia switch and remove the bypass. We want to keep you alive. Good luck.