88 mustang fuel pump not priming.. yea.. another one.

Ok, I have an 88 mustang that I know the pump is good (I pulled it out of my running 93) I went through a bunch of the other fuel pump not priming threads and none seamed to fix my problem.
I went through everything in the previous posts and used the wiring diagrams from http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif to do wire color conversions between the 91-93 wire colors.

So here is a list of what I've done.
1. First, when I turn the key the pump does not prime.
2. I first replaced the fuel pump relay (under the drivers seat) with one from napa. Tried to prime the system, and nothing.
3. Checked the fuseable links, I went from the starter solonoid terminal to about 3" past the fuse and all were continuous.
4. I checked the Orange with blue stripe fuseable link all the way to the fuel pump relay and it was continuous.
5. I checked power on the pink/black wires and they had no power (both are grounded to the chassis.
6. I checked the voltages for the wires from the fuel pump relay to the ECU and they are all continuous.

No matter what I did I could not get voltage to the inertia switch.
here is a quick rundown of the voltages:
Key off:
Ground to Orange/Blue = 12V
Ground to Pink/Black = continuous no voltage
Ground to tan/green = continuous no voltage

Pin check to ECU no power
37-Relay Red wire - continuous - .5 ohms on 200ohm scale
57 - Red = continous - .7 ohms on 200 ohm scale
58 - Red - continuous - 15.4 ohms
22 - tan/green - continuous
19 - I do not have a pin 19 on my wiring harness.
1 - Orange/blue - continuous 4.1 ohms

Key On no jumper
ground - pink/black on relay - grounded
ground - red on relay .03 V
ground - tan/green - .03 V
ground - orange/blue - 12 V

Key on with jumper
ground - pink/black on relay - grounded
ground - red on relay 0 V
ground - tan/green - continuous
ground - orange/blue - 12 V

I even swapped out the EEC relay with the spare Fuel pump relay to see if that was bad and still no change.

anyone have any ideas?


I have a bunch of pictures that I can post if that will help also.

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 2-5 seconds and shut off. This on and off again cycle helps to prevent
flooding the engine when cranking. To trick the fuel pump into running, find the ECC
test connector and jump the connector in the upper 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.
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). Notice that
pin 19 on the computer is the monitor to make sure the pump has power.
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.

Diagram courtesy of AutoZone



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.

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.



You will have to drop the tank to inspect the pump power and ground connector
and the pump wiring chassis ground.

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. You should see less than 1 Ohm between the black wire(s)
and ground. The chassis ground is up near the spare tire shell. To get some
idea of what a good reading is, short the two meter leads together and
observe the reading. It should only be slightly higher when you measure
the black wire to ground resistance.

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.

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.
Probe computer pin 22 with a safety pin and ground it to chassis. Make sure the computer
and everything else is connected. Turn the ignition switch to the Run position and observe
the fuel pressure. The pump should run at full pressure.
If it doesn't, the wiring between pin 22 on the computer and the fuel pump relay is bad.
If it does run at full pressure, the computer may have failed.

Keep in mind that the computer only runs the fuel pump for about 2-3 seconds when you turn
the key to the Run position. This can sometimes fool you into thinking the computer has died.
Connect one lead of the test light to power and the other lead to computer pin 22 with a safety pin.
With the ignition switch Off, jumper the computer into self test mode like you are going to dump
the codes. Turn the ignition switch to the Run position. The light will flicker when the computer
does the self test routine. A flickering light is a good computer. No flickering light is a bad computer.
Remove the test jumper from the ECC test connector located under the hood.

Fuel pump runs continuously: The fuel pump relay contacts are stuck together or the Tan/Lt Green wire
has shorted to ground. Remove the fuel pump relay from its socket. Then disconnect the computer and use
an ohmmeter to check out the resistance between the Tan/Lt Green wire and ground. You should see
more than 10 K Ohms (10,000 ohms) or an infinite open circuit. Be sure that the test connector isn’t
jumpered to ground.
If the wiring checks out good, then the computer is the likely culprit.

Prior to replacing the computer, check the computer power ground. The computer has its own
dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to
it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a
black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. You'll find it up
next to the starter solenoid where the wire goes into the wiring harness

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.

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

ignition source

Is the ignition source to the EEC relay (red/lt Green) supposed to be a switched 12V so when the key is on should that be 12V?

I'm not getting power past the EEC relay, and it looks like when I turn the key to on there is no change in voltage at the red/lt green wire. but if I disconnect the red/lt green and hook a jumper from the radio 12V power on the pump does not prime when I turn the key on, but when you turn it off it primes then shuts off like it's supposed to.

still not working

Ok, so I went through the testing again and here is where I'm at:
when I put the jumper on the fuel pump test port to the computer test ground the pump will run continuously WITH and WITHOUT the key on.

With The key off, and no jumper when I connect the battery I can hear the fuel pump prime. but when I turn the key to ON it does not prime.
I did some testing with this and if I pin out the fuel pump relay during connecting the battery, the Tan/Lt Green wire goes from 12V to ground while the pump primes and the Pink/blk wire goes to 12 V. But it will not do that when I turn the key to on.

I am now getting 12V to the inertia switch and out of the inerta switch to the Fuel pump relay.

When I go to the Pump wiring section of the instructions above I am getting 12V to the pink and blk wire (with jumper) and on the ground wire when I measure from chassis ground (where the interior light connects near the latch) to the fuel pump plug (pump disconnected) I get 18 ohms (jumper on, and key off), and 83 ohms (jumper on and key on)

When I check the Tan/Lt Green wire with the jumper to ground I get less than the .75V.

Computer: when I short pin 22 to the ground (computer hooked up) the pump runs with the key OFF, when I turn the key ON it still runs (both continuously)

I tried the self test with the test light hooked up to pin 22 and I did not get a flashing light on the test light... but I did this with my second computer and didn't get it either so I know it's not a sign of a bad computer.

Any Ideas?
I'm thinking it's got to be a grounding issue somewhere, when I have everything off and hook the battery up it will spark.

This means the ECC relay isn't turning off. Check for power on pins 37 or 57 on the computer. If it is there key on and key off, the ECC relay has stuck shut or is continuously powered on by the ignition switch.



Diagrams courtesy of Tmoss & Stang&2birds




See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 86-95 5.0 Mustang wiring Mustang FAQ - Engine Information Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 5.0 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 91-93 5.0 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 94-95 5.0 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/94-95_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

continuous power to pin 37 and 57

I just checked the power on the ECU Pin 37 and 57 and yes they are 12V between those pins and chassis ground Key on and key off.

more voltage checks

Tonight I checked the voltage at the igniton plug on the steering column, at the FP relay and the EEC relay all with no key and with the key on. Here were the results:
No Key, ignition: / with key on
Yellow - 12 V / 11.6
Yellow - 12 V / 11.6
Gray/yellow - 0 / 11.3
Gray/yellow - 0 / 11.3
Black/blue - 0 / 10.8

black/green - 0 / 11.2
brown/pink - 0 / 11.2
Red - 0 / 11.2
White/pink - 2.62 / 2.64

Fuel Pump Relay
No Key / With key on
Orange/Blue 12.18 / 11.65
Pink/Blk 0 / 0.02
Red/Blk 12.13 / 11.62
Tan/green 12.13 / 11.61

EEC Relay:
Black/orange 12.12 / 11.6
Red 0 / -0.01
Black/green 0 / .05
Black/Orange 12.4 / 11.63

I think the plug may be bad, I was screwing around with the plug and when I dropped the relay against the computer I heard the fuel pump prime (like it does when I hook the battery up) and no key is in the ignition. I think I'm going to order a new harness and another relay (#2) and see what that does.

any other ideas? Tomorrow I'll continue trying to trace the red/blk wires on the EEC relay to find why they are getting 12V.

Posted via Mobile Device

Looks like you are missing the 12v switch to the EEC relay. I'm trying to remember if there is a fuse link of not.

I ordered a new EEC Relay and Harness this morning and it should be in this afternoon, I'm going to swap that out and see if anything changes.

Posted via Mobile Devicewhat harness did you buy?

parts were from NAPA
Relay connector - ECH EC121 - 12.29
Relay - ECH AR242 - 16.99

I just swapped them out and it didn't change anything. Now I know the connections and the EEC relay are good though.

I'm lost as to what it can be. I disconnected my electric fan, and I'm going through and disconnecting anything aftermarket to make sure something isn't shorting out and causing the issue. I'm even resorting to going through and removing the alarm system to see if it's part of the problem.

I'm going to try tracing the wires one more time and see if I can find anything.

can anyone tell me what each wire on the EEC relay and the fuel pump relay is supposed to read in the key off (ie not in the ignition at all) and Key on positions?

I've done everything I know to do from all the instructions and checking voltages, and to me it looks like the tan/green wire on the fuel pump relay is supposed to be going to ground when the key goes to on but it doesn't. And I know my computer is not bad, so some how the computer isn't getting the signal from the ignition to switch pin 22 to ground.

Is the red/gr wire from the ignition to the EEC relay supposed to be 12V when key is on?
Is the EEC blk/Lt Green wire supposed to be ground (or close to zero volts)?

Posted via Mobile Device

The red/light green is the 12v switch source to the relay. I believe I already mentioned that it looks like that's what your missing.

The tan/light green on the Fuel pump relay is the ground for the relay.

Black/ green is also a ground.

First thing you need to do is go to the EEC relay. And make sure three wires show 12v with key on. I'm thinking you will only have 2....

Last night I after I replaced the EEC relay and harness I measured the voltages everywhere again:
here are the voltage measurements after replacing the EEC relay harness and relay, and the fuel pump relay:
EEC Relay: all referenced to chassis ground
no key / key on
Red/Lt Green - 0 V / -.01 V Ignition harness to EEC relay
Orange/Blk - 12V / 11.57 V EEC Relay to Inertia switch
Black/Lt Green - 0 V / .04 V EEC Relay to ground
Black/Orange - 12 v / 11.6 V Starter Solonoid Fuseable link to Relay

Fuel Pump Relay:
Orange/Lt Blue - 12 V / 11.6V Starter Solonoid Fuseable link to Relay
Pink/Blk - 0 V / 0.02 V fuel pump Relay to fuel pump
Red - 12 V/ 11. 66 V Inertia switch to Fuel Pump relay
tan/Lt Green - 12 V / 11.66V Fuel Pump relay to Test connector AND pin 22 on computer.

just for fun I checked the test connector voltage:

tan/Lt Green - 12.1 V / 11.6 V Fuel pump test port
Blk/white - .1V / .6 V Test connector ground.

I also checked the TFI pin 4
red/Lt Green - 0 V / .05 V

and the coil signal:
red/Lt Green - 0 V / .02 V

yea, it looks like somewhere between the ignition switch and the EEC relay the power is lost, but power is s still being passed to the inertia switch.

I'll look at the ignition more tonight and see if I can apply 12V to the red/gr wire and see if that does anything different than what it is doing now.

Posted via Mobile Device

That's your problem. The EEC relay 12v switched is the same circuit as the TFI start/run and Coil start/run power.

If its not the ignition switch, its the fuse link for that circuit. Its a 20gauge if I remember right.

Posted via Mobile Device

You still have something funny going on though. Your missing a power source at the EEC relay. But by your test on the Fuel pump relay, it shows the EEC relay as powered up. But worry about getting power to the Red/light green wire first.

awesome, I'll concentrate my efforts on looking at that stuff tonight.

My wife finally let me back out into the garage on sunday to work on the car and here is what I found out.
The signal wire from the ignition was not 12v at the EEC relay, coil or distributor.
I was getting 12 V at the harness on the column and at Fuse 1 on the fuse box, but not on the engine side and not at the EEC relay. I spliced a new wire into the area where I saw 12V and ran it to each location that was not getting 12V before. Then, I was still seeing problems with my EEC Relay allowing power when it should not have been. So I took another standard 30A relay that I had from another project (installing an electric fan) and wired it up like a relay would normally work (I wired it in parallel with the existing EEC relay and unplugged the EEC Relay). I turned the key and on the first try the car started.

Good Job.

It's BACK!

well the day before I went road racing my problem returned. I haven't had time to go through and see what is wrong yet but my pump isn't priming and I tried jumping the fuel pump test port to ground and it didn't run.