Fuel Pump Relay Question - Please Help Wizards

Welcome to the boards!

Is there a chance that the second wire is orange/lt blue? It sounds like the positive relay terminal was being jumped to constant power and controlled via the toggle switch.

It's weird that when you restored the wiring pathways, it would not stop priming. I'd put a meter on the coil of the relay (checking both the wire from the inertia switch and the ground wire from the computer). For the Red/Blk, ensure it falls to zero volts when the key is off.
On the ground, ensure that it goes away after the priming-out. If it doesn't, check the fuel pump terminal on the self-test connector to see if someone jumped it.

I have a hunch that there might have been some issue with the EEC relay circuit and the toggle switch allowed the circuit to be backfed with power. You might end up testing the EEC relay out. More info would be needed for that.

Random thoughts.

Good luck.

Sounds like the PO wired the switch across the R/BK & T/LG wires which would be the control voltage for the relay rather than the output terminal circuit. I would have the same hunch about the EEC relay ckt.

Here's some general help & troubleshooting. Look closely at the diagram- that's how it is supossed to be wired.

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.

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



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.

I thank you for the kind words. The irony is the other guys are much smarter than I am; JRichker and Saleen0679 are the true wizards. They will be able to help you much more than I can but I'll pitch in where I can. Because I'm not as savvy as they are, I tend to speak in very simple (albeit sometimes confusing) terms.

To try and answer some of your questions:
The red and black wire does literally go all over the car. It is the output from the EEC relay and that relay controls many solenoids and circuits. It only supplies enough current to energize a given relay (like your fuel pump relay).

I'll post a slightly different spin on JR's description (kind of the same info, but stated how I understand and think of things).

Here's a quick run-down on how the FP relay works (this holds true for how the EEC relay works too):

Here's the diagram JR posted again:



An SPST relay has a control side and a load side. The little coil squigglies denote the coil (AKA control) side. When one coil terminal has 12 volts and one has ground, the relay energizes. When the relay energizes, the two remaining terminals connect and send power to the device (like a fuel pump). The terminal with the battery power is called the common terminal and the 'output' terminal (to the fuel pump, for example) is called the normally open terminal (it's open, or unconnected, until the relay energizes).

So with the FP relay:
The EEC relay sends 12 volts (with the key-on) through the inertial switch and to the positive relay coil (red/blk). Then the computer sends a ground pulse through the tan/lt green wire. Now your relay is energized, and the orange/lt blue and pink/red wires connect together. Orange/lt blue is battery power and pnk/red goes to the pump. So when the relay is energized, the pump gets battery power.

For testing the FP relay:
red/blk: 12 volts with the key-on.

Tan/lt Green: ground. you have to test this one quickly after turning on the key because after a couple seconds, the computer stops the ground pulse intentionally (this is what makes the pump prime and stop).

Or/Lt blue: should show constant 12 volts.

Pink/red: shows 12 volts when the above-three-elements are present.

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The EEC relay is above the computer. JR has some tips on how to access it if you cant find it (going in through a speaker hole, IIRC). You will test it like you did the FP relay.
This time:

Red/lt green (needs 12 volts with the key turned on).

Blk/lt green needs to show ground. On the EEC relay, this is a constant ground.

Blk/orange shows battery voltage (I think you might find this wire is open, or not connected to the battery. A burned fusible link is possible).

Orange/blk should show 12 volts when the three wires above show what they should. In your case, it sounds like the above-elements were missing so someone 'backdoored' it by essentially connecting 12 volts to the or/blk wire (via the red/blk wire) with the toggle at the FP relay.

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The ground for the computer is on the driver side fender adjacent the battery. Jrichker has a nice picture of it that he often posts.

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The self test connector on an 87-93GT is in the driver corner of the firewall (near the wiper motor). I think JR might have a picture of that too (he and Saleen are great with pics, which is SO much better than me babbling). The deal with the self test connector jumping that JR mentioned: Remember the tan/lt green wire at the FP relay (it supplies the ground pulse to the control side/coil of the FP relay). You know how I said you have to test it fast because it goes away after a couple of seconds? Well, this wire also goes to the self test connector and you can jump it to ground to keep your fuel pump energized. This can be helpful for diagnostics, and is where jumping that wire comes in. When you go to start your FP relay diagnostics, I would test that tan/lt grn wire first thing. If it gets ground as soon as the key is turned on, I would ground the appropriate terminal at the self test connector so you dont have to keep turning the key on and off (to keep making the pump prime, so you can test each FP relay terminal).

Let us know if that makes sense. It's really nothing new but just kind of a spin and different perspective of what was said already. (Joe and Saleen, please correct anything that is misleading or wrong).

Good luck bud.

As Hissin50 has said, the EEC relay is hard to get to. Removing the passenger side kick panel gets you started.
The relay lives on top of the computer. You need to remove the computer from its plastic mount to get to it.
Removing the passenger side dash speaker helps a little.

Small hands and an eyeball on a stalk like a snail help, however, the two are not usually together found in a
workable combination...

weaselman said:

If i bypass and give the red/black 12volts at the FP relay the pump primes normal and the fuel pump works but the car will not start. It wont even try and start (like it is getting no spark) As usual if I jump the the orange/blue and red/black its all good.

Still going to be testing here - not many good days to work on it - living in an ice world here

Click to expand...

If I read ya right, it still sounds like an issue getting 12 volts from the EEC relay to the positive control terminal on the FP relay. There would be a super easy way to circumvent this but fixing it correctly is the way to go. Since this appears to be the only part of the EEC relay circuit acting up, the inertia switch in the trunk might be resistive or broken. Check In/out voltage.

Good luck.

weaselman said:

Inertia switch has 4.9v in and the same 4.9 out.

Click to expand...

That aint good.

Time to work backwards. If you have 12 volts egressing the EEC relay's Normally Open terminal (orange/blk in Jrichker's diagram), that suggests an issue in the wiring from the EEC relay to the inertia switch.