Results From No Fuel Pump Prime Tests

wyteelx

Founding Member
Apr 17, 2000
285
0
16
Ohio
Hi people, haven't popped in here in a long time. No running car / No problems in years... After about 4 years of no Stang here I got her up this summer, running nice, n on Thanksgiving she just quit. Seemed like the fuel pump relay so I swapped it with the AC cutoff. I worked one more time n then no more pump prime. Another new relay, and nothing still. Way more digging and...

So everything seems to point to the ecm I just wanted any other input/feedback before I order one.

I went through the checklist on this site and allfords site.

Voltages seemed good everywhere. Pump ran full pressure jumped at the test port.

Is there anything else to check if you end up with ecm as the problem? Seems I'm not getting the ground from the ecm on wire 22. Pump ran full pressure, k/o and wire 22 grounded to chassis. Also did not flicker the test light connected from power to pin 22 during jumpered code dump. But it still did dump codes. 21,24,87.

Another test from the allford site said to watch voltage from bat power to the (bl/o) wire when you switch the key on and it should hold for a few seconds. Mine went straight to 0 when I flipped the key.

Did I miss anything? Thanks ahead...

Don't know what's in the sig so. It's a 92 h/c/i, exhaust, n 3.73s. pretty basic mods.
 
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Fuel Pump Troubleshooting for 91-93 Mustangs

Revised 6-Feb-2016 to add fuse link diagram

Ignition switch in the Run position, engine not running tests.

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. To trick the fuel pump into running, find the ECC test connector and jump the connector in the upper RH corner to ground.

Foxbody Diagnostic connector
foxbody-mustang-diagnostic-connector-jpg.586766


Foxbody Diagnostic connector close up view
foxbody-diagnostic-connetor-closeup-view-jpg.586765



attachment.php


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's seat.
On 92 and 93 cars it is located under the MAF. Be careful not to confuse it with the A/C WOT cutoff relay which is in the same area. See the diagram to help identify the fuel pump relay wiring colors.
Be sure to closely check the condition of the relay, wiring & socket for corrosion and damage.
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.

Theory of operation:
Read this section through several times. If you understand the theory of operation, this will be much easier to troubleshoot. Refer to the diagram below frequently.

Diagram of the fuel pump wiring for 91-93 cars.
attachment.php



The electrical circuit for the fuel pump has two paths, a control path and a power
path.

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.

Control 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.

Computer power path
The computer power relay must properly function to provide power for the fuel pump relay. That means you must check the operation of the computer power relay (PCM Power Relay) before chasing any problems with the fuel pump circuit. The computer power relay is located above the computer under the passenger side kick plate cover. . It is not easy to get to, you must have small hands or pull the passenger side dash speaker out to access it.
With the Ignition switch in the Off position, check the resistance between the black/white wire and a clean bare spot on the car body metal. You should see less that 1 Ohm. More than 1 Ohm is a broken wire, or bad connection of the black/white wire and the car body metal.
Check for 12 volts at the yellow wire. Good 12 volts and the fuse link is OK. No voltage or low voltage, bad fuse link, bad wiring, or connections.
With the Ignition switch in the Run position, look for good 12 volts on the red/green wire. No voltage or low voltage, bad fuse link, bad wiring, or connections.
Good 12 volts on the red/green wire, look for good 12 volts on the red wire or any of the red fuel injector wires. No 12 volts or low voltage and the relay isn’t closing, or relay socket contacts are dirty/corroded. Water has been known to run down the radio antenna wire or leak from the windshield and get into the relay and relay contacts.

Fuel pump power path
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.

Fuse links at starter solenoid
64326


Fuse links come with a current rating just like fuses. A clue as to what current they are designed for is to look at the size wire they protect.

Fuse link material is available at most good auto parts stores. There may even be a fuse link already made up specifically for your car. Just be sure to solder the connection and cover it with heat shrink tubing.

Heat shrink tubing is available at Radio Shack or other electronics supply stores.

See the video below for help on soldering and heat shrinking wiring. There is a lot of useful help and hints if you don’t do automotive electrical work all the time.


View: http://youtu.be/uaYdCRjDr4A

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:
The location for the inertia switch is under the plastic for the driver's side taillight.
There should be a round plastic pop out cover over it, remove it to access the switch button.
With the test connection jumpered and ignition switch in The Run position as described above, 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.

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.


attachment.php


Control path:
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 Mustang FAQ - Engine Information

Fuel pump runs continuously:
The fuel pump relay contacts are stuck together or the light blue/orange wire (pin 22) 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.


a9x-series-computer-connector-wire-side-view-gif.71316



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.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972
ground.jpg
 
Fuel Pump Troubleshooting for 91-93 Mustangs

Revised 6-Feb-2016 to add fuse link diagram

Ignition switch in the Run position, engine not running tests.

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. To trick the fuel pump into running, find the ECC test connector and jump the connector in the upper RH corner to ground.

Foxbody Diagnostic connector
?temp_hash=124ae0e661fe7ed48f0bcc9e908d5b73.jpg


Foxbody Diagnostic connector close up view
?temp_hash=124ae0e661fe7ed48f0bcc9e908d5b73.jpg



attachment.php?attachmentid=68357&stc=1&d=1322348015.gif


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's seat.
On 92 and 93 cars it is located under the MAF. Be careful not to confuse it with the A/C WOT cutoff relay which is in the same area. See the diagram to help identify the fuel pump relay wiring colors.
Be sure to closely check the condition of the relay, wiring & socket for corrosion and damage.
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.

Theory of operation:
Read this section through several times. If you understand the theory of operation, this will be much easier to troubleshoot. Refer to the diagram below frequently.

Diagram of the fuel pump wiring for 91-93 cars.
attachment.php?attachmentid=57323&stc=1&d=1235957538.gif



The electrical circuit for the fuel pump has two paths, a control path and a power
path.

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.

Control 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.

Computer power path
The computer power relay must properly function to provide power for the fuel pump relay. That means you must check the operation of the computer power relay (PCM Power Relay) before chasing any problems with the fuel pump circuit. The computer power relay is located above the computer under the passenger side kick plate cover. . It is not easy to get to, you must have small hands or pull the passenger side dash speaker out to access it.
With the Ignition switch in the Off position, check the resistance between the black/white wire and a clean bare spot on the car body metal. You should see less that 1 Ohm. More than 1 Ohm is a broken wire, or bad connection of the black/white wire and the car body metal.
Check for 12 volts at the yellow wire. Good 12 volts and the fuse link is OK. No voltage or low voltage, bad fuse link, bad wiring, or connections.
With the Ignition switch in the Run position, look for good 12 volts on the red/green wire. No voltage or low voltage, bad fuse link, bad wiring, or connections.
Good 12 volts on the red/green wire, look for good 12 volts on the red wire or any of the red fuel injector wires. No 12 volts or low voltage and the relay isn’t closing, or relay socket contacts are dirty/corroded. Water has been known to run down the radio antenna wire or leak from the windshield and get into the relay and relay contacts.

Fuel pump power path
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.

Fuse links at starter solenoid
64326.gif


Fuse links come with a current rating just like fuses. A clue as to what current they are designed for is to look at the size wire they protect.

Fuse link material is available at most good auto parts stores. There may even be a fuse link already made up specifically for your car. Just be sure to solder the connection and cover it with heat shrink tubing.

Heat shrink tubing is available at Radio Shack or other electronics supply stores.

See the video below for help on soldering and heat shrinking wiring. There is a lot of useful help and hints if you don’t do automotive electrical work all the time.


View: http://youtu.be/uaYdCRjDr4A

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:
The location for the inertia switch is under the plastic for the driver's side taillight.
There should be a round plastic pop out cover over it, remove it to access the switch button.
With the test connection jumpered and ignition switch in The Run position as described above, 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.

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.


attachment.php?attachmentid=49675&d=1175374071.gif


Control path:
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 Mustang FAQ - Engine Information

Fuel pump runs continuously:
The fuel pump relay contacts are stuck together or the light blue/orange wire (pin 22) 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.


a9x-series-computer-connector-wire-side-view-gif.71316.gif



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.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972
ground.jpg


Yes this is the list I used. My post was the results of all these tests. Would the pump run jumped if the connector for the pump n sender was bad? That's the only voltage I missed getting. But it ran jumpered by test connector and with pin 22 grounded from the pin to chassis. I've read that about 1000 times trying to learn how to troubleshoot this problem.

Aside not getting that one voltage reading at the connector. I did ring it for continuity. Just didnt get a voltage reading. Ill do it later if I should. When the pump ran jumped or with 22 grounded to chassis, it ran at full pressure, just figured the connection was good.
 
Yes this is the list I used. My post was the results of all these tests. Would the pump run jumped if the connector for the pump n sender was bad? That's the only voltage I missed getting. But it ran jumpered by test connector and with pin 22 grounded from the pin to chassis. I've read that about 1000 times trying to learn how to troubleshoot this problem.

Aside not getting that one voltage reading at the connector. I did ring it for continuity. Just didnt get a voltage reading. Ill do it later if I should. When the pump ran jumped or with 22 grounded to chassis, it ran at full pressure, just figured the connection was good.
Your tests are pointing to a problem with the computer not providing ground for the fuel pump relay.

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.
 
That's what I thought too. Nothing else aside the ecm can make that ground from what I read. Right? Is there anything else I can do to test it?

I read about grounding pin 22, and starting the car. if that ground makes the pump run, (Already know mine does), start it. If the car does start, the ecm is bad. Should I try this too?

Found a video on what to check inside the ecm. I'm gonna go through it once more with the checklist, n then have a look inside the 'puter
 
Well, I tink dat be dat. Glad to know the problem. Hope it's the only one. Now for the fixin. I've never soldered on a board, any suggestions on where to send it?

Thanks as always folks!
 

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That's what I thought too. Nothing else aside the ecm can make that ground from what I read. Right? Is there anything else I can do to test it?

I read about grounding pin 22, and starting the car. if that ground makes the pump run, (Already know mine does), start it. If the car does start, the ecm is bad. Should I try this too?

Found a video on what to check inside the ecm. I'm gonna go through it once more with the checklist, n then have a look inside the 'puter

From the checklist...
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.
 
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Reactions: 1 user
Well, I tink dat be dat. Glad to know the problem. Hope it's the only one. Now for the fixin. I've never soldered on a board, any suggestions on where to send it?

Thanks as always folks!
Cheap route; fix a shop that actually fixes TV's and home electronics. They can repair the burnt trace in less than15 minutes once the computer is opened up so that they can see the damage. Have them check for bulging or leaking capacitors while they are at it.
good_bad_ugly_capacitors.jpg


Capacitors also develop electrolytic fluid leaks around the base, which often damages the PC board.
article_02_allsolid_007.jpg
 
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Reactions: 1 user
Yup, blown capacitor. Thanks for the link. Only the one looked bad. You can see its leaking in that pic. Ill have the 3 that are common done though. Hopefully I know someone that can teach me while doing it, I'll tap some resources tomorrow. I always doubt myself, gotta quit that, lol. thanks again for the help. Ever in NE Ohio beers on me. .

I'll update when the boards done.
 
Ended up having to get mine repaired. Cardone was out of stock and that's apparently who every parts store I tried gets thiers from. 100 bucks, 1 week round trip. Started up n ran fine 1st try. Now it's gonna rain every day for the next week... dammit!
Thanks for the help again good people