Fuel Pump Troubleshooting for 91-93 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 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 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 (pink/black wire) goes to the fuel pump relay
contacts. 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.
diagram of the wiring for 91-93 cars.
Now that you have the theory of how it works, it’s time to go digging.
Power circuits:
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.
Remember that on 92 or later models the fuel pump relay is located under the
Mass Air meter. Watch out for the WOT A/C control relay on these cars, as it is
located in the same place and can easily be mistaken for the fuel pump relay.
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.
Control circuits:
Relay: The red wire for the fuel pump relay coil gets its power feed from the EEC
relay. No 12 volts here, and the EEC relay has failed or there is bad wiring or bad
connections coming from it. The EEC 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.
Also check the black/orange wire EEC power feed at the EEC relay - you should
have 12 volts there at all times.
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 battery voltage, whatever that may be…
Fuel pump runs continuously: The light blue/orange wire has shorted to ground.
Disconnect the computer and use an ohmmeter to check out the resistance
between the light blue/orange 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.
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/
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
