91 mustang fuel pump runs continuously with key on but car wont start??

brokenmustang

New Member
Jun 19, 2008
14
0
0
Hello:) My car will not start. If I spray starting fluid into the throttle body it will fire up and stall. When I turn my key on the pump comes on but will not turn off. I have replaced the fuel pump and the same thing is happening. I am ready to begin diagnosing and found some great links on here but a little confused. I have my driver seat out and took my computer out of its location and it is resting on the passenger floor. On one link it says to test a tan/lt green wire to make sure it has not shorted to ground? Where is this wire?on the connector for the computer? Also says that there is a special ground for the computer only by the battery...anyone have a pic of this? Thanks in advance...hope to get my car going again:)
 
  • Sponsors(?)


brokenmustang

New Member
Jun 19, 2008
14
0
0
Checked the power at the 4 wires on the fuel pump relay connection

There are the 4 wires at the fuel pump relay(pink,red,blue and green). I have constant power at the pink wire and the other 3 have no power. When I turn the key to the on position(switched power) I am getting power at the pink and the red only. Are the blue and green wires ground or?? Thanks in advance for the help! :)
 

brokenmustang

New Member
Jun 19, 2008
14
0
0
Update: I am getting power in and out of the inertia switch as well. With the fuel pump relay plugged into its socket I have power at 3 of the wires and with the relay out of its socket I only have power at red and pink. Thanks again
 

Foxfan88

My Grandpa has great wood.
Sep 13, 2004
2,487
3
0
32
Miami, Ok
my brothers bone stock 5.0 car had the exact same problem and it tuned out to be the EEC itself. it would run if you manually put gas in the intake but wouldnt run on its own.
 

brokenmustang

New Member
Jun 19, 2008
14
0
0
Thanks for the replies guys! I am ready to just buy a computer and try it. I just came back from the ford dealer from getting another fuel pump relay. If I turn the key on the fuel pump relay will not click the pump on but if I tap the top of the relay it will turn the pump on and then it stays on. I have tried 4 different relays and the same thing happens. The switched power (red wire) gets power when I turn the key on but wont signal the relay to open unless I tap it. Once I tap it then it allows power through and then I have power on the green wire. The relay ford gave me is a 5 pin and the tech said the fifth pin which I dont have is just a dummy pin and it wont affect anything. My original relay is a 4 pin with the green top(original) and it is discontinued they said. Am I sending some kind of pulse to the computer when I tap it or what on earth is going on?? This car is possesed! :)
 

Sicarius428

Active Member
Jan 6, 2004
2,085
5
49
Not the pump at least. Check the plugs if they are getting wet or pull an injector and see if it will fire into a cup or rag if you turn it over...
Kevin
 

HISSIN50

"How long does it take to get help in here?
SN Certified Technician
Nov 29, 1999
31,179
29
129
Check what's going on with the green wire. It's a controlled ground from the EEC. When it's present with the key-on, you go from 2 wires having 12 volts to 3 wires having 12 volts.

The bottom line is that if the pnk/blk wire has 12 volts, the pump should run.

You can manually ground that green wire at the self-test connector if it helps with diagnostics.

The stock relay is a SPST and the one you got sounds like a SPDT. That middle pin (87A, Normally Closed) does not affect anything (like you were told).

Check the wiring terminals in the relay socket. Your manipulating the relay might be affecting the connection. Otherwise, having two relays fail would be weird (though they can be sensitive to being squished between strong fingers).
 

brokenmustang

New Member
Jun 19, 2008
14
0
0
Thank You for the replies:) I am going to check the fuel injectors tomorrow and the plugs. As far as the 4 wires at the fuel pump relay all I have is a red,pink,green and blue. The pink is live all the time. The red has power if I turn on the key but the fuel pump will not come on. If at this moment with red and pink wires showing power I was to tap the fuel pump relay then the pump goes on and I now show power at the green wire too:) I pulled all the 4 connectors out of the holder that the fuel pump relay plugs into(they slide out of their sockets) and I plugged them individually into the back of the relay and the same thing happens so I can assume the plug is ok. I pulled the tape off of the 4 wires and followed them to the driver side door sill area and they looked fine. I can shake the relay but the pump wont come on but if I tap it then it comes on and the green wire comes alive?? This has happened with a few relays that I bought because I thought the relays were bad. I now have my original and the new ford relay and they both do the same thing... Thanks again guys!! :)
 

HISSIN50

"How long does it take to get help in here?
SN Certified Technician
Nov 29, 1999
31,179
29
129
Some theory on relays:

There is a relay coil, consisting of two terminals. It's the switch portion of the relay. When one terminal receives 12 volts and one receives ground, this turns the relay 'on'.

When 'on', the common and normally-open terminals connect. In this case, the power feed from the solenoid (which always shows 12 volts) connects to the wire that goes to the FP itself.

It sounds like you might have an issue with energizing the relay coil, and it sounds like the ground portion is what could be acting up. That's why I'd manually ground this wire at the self test connector to remove that one variable. Then retest.
 

brokenmustang

New Member
Jun 19, 2008
14
0
0
Forgot to ad that I grounded out the port on the eec test connector to trick the fuel pump to coming on and it did. I simply had the key on and when I grounded it out the pump kicked on and I removed the ground but pump just keeps on running... Thanks again :)
 

HISSIN50

"How long does it take to get help in here?
SN Certified Technician
Nov 29, 1999
31,179
29
129
If the pump runs on after the prime-out and with the FP port on the self test connector ungrounded:
After the key has been on for 10+ seconds, ohm out the ground wire from the EEC to the FP relay (which also goes to the Self test connector). If the value is low, that wire has a short to ground.

If that wire shows a very high resistance value or infinite value, that would indicate that the relay is latched.
 

brokenmustang

New Member
Jun 19, 2008
14
0
0
Hello, You said to ohm out the ground wire from the eec to the fuel pump relay. Is this the blue wire at the fuel pump relay connector or the green wire? The green wire is the one that gets power when I tap the relay and it then turns the fuel pump on. I will buy a multimeter today and attempt this. Do I ground the black probe on the multimeter and test the wire with the red probe...cant remember the last time I used one of these:)...Im used to my light tester which I just ground the one wire and probe. Thanks again guys!
 

HISSIN50

"How long does it take to get help in here?
SN Certified Technician
Nov 29, 1999
31,179
29
129
lt blue/or is the ground from the EEC.

Dk Green/yellow is the output wire to the pump itself. It should also run to the EEC (Pin 19) as part of the FPM.

Using the meter:
For resistance testing, how you noted doing it is fine. It'll be good to have a meter because while a test light is nice, the draw of the bulb can overload some EEC circuits (in case you need to do some testing on other parts of the car or a newer car).

FWIW, Harbor Freight puts meters on sale for 5 bucks all the time, and they're just fine for this type of use.
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,318
2,650
234
74
Dublin GA
lowendmac.com
Follow the checklist and don't skip around. You will find and fix the problem if you do this.

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 Relay:
On 91 cars, it is located under the driver seat.
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.

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.

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.

Control circuits:

Relay: The red wire for the fuel pump relay coil gets its power feed from the ECC relay.
No 12 volts here, and the ECC relay has failed or there is bad wiring or bad connections
coming from it. The ECC 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.

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.
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.

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/

Fuel pump runs continuously: The fuel pump relay contacts are stuck together or the light blue/orange 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 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.
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
 

brokenmustang

New Member
Jun 19, 2008
14
0
0
Thanks again for the help! I will be working this weekend so I will have no time to play with the car. Will write back on Monday after I do some testing. :)
 

brokenmustang

New Member
Jun 19, 2008
14
0
0
Tuesday morning and playing with the car:) I probed the 4 wires that go to the fuel pump relay and the red,pink and green all light up my tester really bright. The blue wire which I think? is supposed to be ground is also lighting up but not as bright? Is this normal? Thanks
 

brokenmustang

New Member
Jun 19, 2008
14
0
0
I forgot to add that if I ground that blue wire that is at the fuel pump relay connection the pump kicks on but does not turn off...