jrichker said:All the fuse links live in a bundle up near the starter solenoid. You have to
unwarp the wiring harness to find them. I would recommend that you use a
voltmeter to check them out before unwarapping the harness.
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
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
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)
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.
Ok, Im not able to get my 1988 to run with either the A9L or the AEM EMS. In both cases I will get just a 3 second prime up of 38 PSI at the shrader which immediately drops off to 15 PSI or so..
Here are the results from both the fuel pump test I did on both computers.....
Test Orange/light blue wire for 12volts Key off....... AEM 12 A9L 12v
Light pink/black wire for 12v, key on, jumper ran.....AEM 12 A9L 12
Red/Black wire for 12v, key on.............................AEM 12 A9L 12
Inertia switch for 12v on both Red/black and Red....AEM 12 both A9L 12 both
Tan/light green for volts, jumper ran, key on..........AEM .109 A9L .74
Tan/light green with test light on batt + , jumper ran,
key on...........................................................AEM lite on A9L lite on
Tan/light green connection at ECM pin #22 with test
light also on ground, key on, jumper ran................AEM lite on A9L lite on
I dont understand why my computers are both suddenly bad? Please Jrichker or someone knowledgeble.. please help me figure this out! Ive been struggling with this a whole week now. Ive learned alot but solved very little. I really am not excited about giving up on this and selling everything off. I have invested almost 12 grand and a large part of my last 3 years into this hobby.