Dumping Codes/Testing Computer/Bad Wiring? Etc.

You have two separate problems , so we will attack them one at a time.

No computer self test.
The self test ground seems to be good from your description. It sounds like you did not do an
end to end test of the self test input wire. Remove the passenger side kick panel and disconnect the computer
connector. There is a 10 MM bolt that holds it in place. Measure the resistance between the STI computer self
test connector (red/white wire) and pin 48 on the computer main connector: it should be less than 1.5 ohms.
More that 1.5 ohms is a wiring problem.



The following is a view from the computer side of the computer connector.


If there is a wiring problem, you get to run a new wire, or chase the wire and find the break.


Fuel pump does no run:

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.

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. With power off, check the pump ground: you should see less
than 1 ohm between the black wire and chassis ground.



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.

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.

Fuel pump runs continuously: The tan/green wire has shorted to ground or the relay contacts are sticking.
Disconnect the computer and use an ohmmeter to check out the resistance between the tan/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 resistance check is good,
replace the relay.

It will be fine to use a piece of wire to extend the STI connector so you can test the contuinity of pin 48.

tsgman01 said:

OK: While running the above tests we got mixed results
as it relates to voltage readings, we read 12v on both sides of the fuseable link as indicated,
read 3.9v on one side of the inertia switch and all others areas no voltage at all..
Finally performed the test for pin 22 on the comuter and no flicker. BEOFRE WE STARTED
ALL OF THESE TESTS THE BATTERY WAS CONNECTED AND WE FOUND THE GROUND
FOR THE COMPUTER WAS NOT CONNECTED, Is the computer bad?? Need some guidance, thanks...

Click to expand...

The computer power ground is two 14 gauge black/green wires 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.
Without this ground working properly, the car will not run.

The single flat braid that comes from the computer connector to the floorboard
is the TFI wiring shield. If it is not connected, everything will work, but it may miss or sputter.
I haven't tried running the car without it connected, so I can't say for sure.

Spend some time looking at this diagram and tracing out the computer and
fuel pump power feed circuits so that you have a mental picture of what connects to where.

If you have the power ground connected, check the diagram: Notice that the interia switch
has a red wire (power feed) and a red/black wire (power to the relay). No power at the
red wire, or less than 12 volts and you have problems with the power feed to the computer.
Notice that all the red wires join together and become an orange/black wire
that goes to the EEC or computer power relay. The computer power relay is
mounted on top of the computer. It is hard to get to unless you have small hands
and an eyeball on a stalk like a snail. Removing the passenger side dash speaker may
help give you access or eyeball room. The black/orange wire feeds power to the computer
power relay and should have 12 volts on it at all times. It connects to the starter solenoid
via a blue fuse link wire. No full 12 volts and the blue fuse link is blown open.
You can subistute a 15 amp inline fuse for the blue fuse link if you
can't get any fuse link wire.

Diagram courtesy of Tmoss & Stang&2birds

Go back and check out the fuel pump troubleshooting section in my first post for your problem.
That will help you find the problem with the fuel pump circuit.