Electrical Fox will crank but won’t prime HELPP

[Tortoler0]

Hello I have a 91 mustang lx 5.0 auto for about a year now and one time I was headed back home from a meet when stopped to fill with gas, and when I wanted to crank I didn’t hear the fuel pump prime. So I started digging in the engine bay to see if something was wrong went ahead and tried to start it again and cranked just fine. Well a couple days go by and the same thing happens but this time it wouldn’t crank til the day after. I waited til the car would let fuel pump prime and it surged its way til it died I heard the relay click off and no more priming but I Still hear the relay click both the fuel pump and eec relays. I pulled the tank down suspecting it was my pump I tested with a power probe immediately shoots gas, so I put the tank back up and start checking the wires I start to check the fuel pump relay this is what I got

IGN OFF
Fuel pump relay :
Red wire 0v
Green/tan 0v
Blue/orange 0v
Pink/black 12v

IGN ON
Red wire 12v
Green/tan 6.5v
Blue/orange 12v
Pink/black 12 v

I would like for someone to verify the readings are right or is there something I need to double check, also went ahead down to the inertia relay/switch and what I got was 6.5 v on both green/tan wire and black/pink wire with key on . Other things I looked over was inside the computer,the capacitors did have some corrosion on them but I didn’t see any stains or leaks on the board but I went ahead and swapped em out also swapped out the relays with new ones and still nothing. I would love to get this over with and get some feed back , I’ve already followed another helpful forum out there and still nothing ALSO I did the test with the plug in the top right corner of the engine bay for the fuel pump and all I get is the click (I think it’s the diagnostic connector ) I appreciate any feedback please

 

[AeroCoupe]

Welcome to Stangnet!

Have you checked for codes? The CEL will only come on with emissions related codes so other codes can exist and the CEL will not be illuminated. Also, next time force the fuel pump at the code readers test leads instead of pulling the tank or just jump the wires at the fuel tank harness to chassis harness at the rear of the car just above the fuel tank. A lot less work than pulling the tank.

I will tell you that if the fuel pump is not priming, you have replaced the fuel pump relay and have no other symptoms then it points to a bad ECU. Best place to send them to get rebuilt is here:

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECU Repair Service

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECU Engine Computer Repair ServicePlease fill out the R&R form here once you've placed your order.Easy repair process1. Purchase repair service (this way helps us keep track of what's coming in for repair)2. Include order number with your computer and mail to...

www.theecuexchange.com

I will stress that the first thing you should do is pull codes when working on these cars as that can stop a lot of unnecessary work and waste of time. Not saying it will always lead to you the problem right away it usually solves the problem or leads to the cause the vast majority of the time. Check for codes and we can go from there. I say we because there will be some other guys jump in here and help out.

 

[Tortoler0]

Welcome to Stangnet!

Have you checked for codes? The CEL will only come on with emissions related codes so other codes can exist and the CEL will not be illuminated. Also, next time force the fuel pump at the code readers test leads instead of pulling the tank or just jump the wires at the fuel tank harness to chassis harness at the rear of the car just above the fuel tank. A lot less work than pulling the tank.

I will tell you that if the fuel pump is not priming, you have replaced the fuel pump relay and have no other symptoms then it points to a bad ECU. Best place to send them to get rebuilt is here:

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECU Repair Service

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECU Engine Computer Repair ServicePlease fill out the R&R form here once you've placed your order.Easy repair process1. Purchase repair service (this way helps us keep track of what's coming in for repair)2. Include order number with your computer and mail to...

www.theecuexchange.com

I will stress that the first thing you should do is pull codes when working on these cars as that can stop a lot of unnecessary work and waste of time. Not saying it will always lead to you the problem right away it usually solves the problem or leads to the cause the vast majority of the time. Check for codes and we can go from there. I say we because there will be some other guys jump in here and help out.

I tried to do the KOEO test and the check engine light did not flicker or flash once and when I checked the cable to see if it had a good connection I read 5 volts. Where do the test leads get their current from?

 

[Tortoler0]

And by cable I meant the ones to jump at the test leads

 

[Mcmahst]

Do you have an OBD-1 tester? If not, I believe Autozone has them for rent.

 

[AeroCoupe]

If you cannot get codes then read this thread over. If you still cannot get them then it’s leaning towards a bad ECU.

Electrical - How To Pull Codes From EEC-IV In 86- 95 5.0 Mustangs

@Noobz347 - The code dump procedure isn't in the sticky - you may want to add it... Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on. Dumping the computer diagnostic codes on 86-95 Mustangs Revised 26-July-2011. Added need to make sure the clutch is...

stangnet.com

 

[85GTStangGuy]

Do you have an OBD-1 tester? If not, I believe Autozone has them for rent.

They're relatively cheap to buy also....

Amazon.com

 

[KRUISR]

Have you checked the connector behind the driver kick panel. I had intermittent fuel pump issues a couple years ago and it was the connector behind the driver kick panel that the fuel pump power runs through (before it gets to the relay). In my case the female connector pins in the plug had opened up and the male pins did not make good enough contact. I took a small screwdriver to bend the two halves of the female connecter more together and have not had an issue since.

 

[JD1964]

You can visually check the ECU circuit board for problems. Most often the capacitors go bad. They leak and cause corrosion on the board. It's not a difficult job to remove the ECU from the car, remove its cover and have a look.

 

[KRUISR]

You can visually check the ECU circuit board for problems. Most often the capacitors go bad. They leak and cause corrosion on the board. It's not a difficult job to remove the ECU from the car, remove its cover and have a look.

Post#1 he said he checked the capacitors

 

[85GTStangGuy]

Post#1 he said he checked the capacitors

And he actually swapped them out, along with the relays, for new ones.

 

[Tortoler0]

Have you checked the connector behind the driver kick panel. I had intermittent fuel pump issues a couple years ago and it was the connector behind the driver kick panel that the fuel pump power runs through (before it gets to the relay). In my case the female connector pins in the plug had opened up and the male pins did not make good enough contact. I took a small screwdriver to bend the two halves of the female connecter more together and have not had an issue since.

Are you referring to the inertia relay?

 

[KRUISR]

Nope. Inertia switch is in the back - driver side rear, behind the trim panel, by the tail light wires.

The 12 pin connectors in the diagram below. If you pull the driver side kick panel (toe area) you should find the two connectors tucked into the door pillar area. From the diagram, you will see the main power for the pump goes through those two connectors - wire 787 goes into the first connector and wire 238 goes into the second one. 238 gets power from the fusible links near battery and 787 takes power to the pump.

If your relay is transferring power, I would check these connectors.

 

[JD1964]

Post#1 he said he checked the capacitors

Just to note, I tried replacing the capacitors on my ECU but my attempt was unsuccessful and the car wouldn't start. Maybe my soldering skills are bad, maybe I had capacitor polarity wrong (it matters). Anyway, I sent that whole mess in for an exchange ECU. When I put that one in it fired up and has been perfect ever since.

 

[Tortoler0]

Okay I just got back my ECU from ECU EXCHANGE and still nothing just clicking from the relays . At this point I’m not to sure what could be the problem I checked all my relays,ecu is new, wires. I noticed someone talking about the driver side panel by the pedals and I see a ton of connections , not sure which connection to look at . Is there anything I’m missing to look at ?

 

[AeroCoupe]

Fuel pump relay wires should get the following with the ignition off:

Red - 0 v
Dark Green / Yellow - 0 v
Light Blue / Orange - 0 v
Pink / Black - 12 v (constant 12 v from fusible link at the starting solenoid mounted on the driver side fender apron)

Should get the following with the ignition on:

Red - 12 v (comes from the PCM power relay)
Dark Green / Yellow - 12 v (comes from the fuel pump relay via the Pink / Black wire when the coil is energized. this goes to the inertia switch and also goes to PIN 19 on the ECU for fuel pump run conformation)
Light Blue / Orange - 0 v (this is a ground generated by the ECU from PIN 22)
Pink / Black - 12 v (constant 12 v from fusible link at the starting solenoid mounted on the driver side fender apron)

Have you checked the Light Blue / Orange wire with the rebuild ECU to see if you are getting ground? Once you have everything confirmed at the fuel pump relay then you need to chase the Dark Green / Yellow wire from under the driver seat to the driver kick panel, and to the inertia switch in the trunk and make sure 12V is getting to that point. If you have 12V to the inertia switch then check the Brown / Pink wire from the inertia switch to the plug under the car at the back of the fuel tank to see if you have 12V there. Back probe the connector on the tank side (Pink / Black wire) and make sure the 12v is carried across this connector. If you have 12V there then you will need to drop the tank and see if you have 12v at the tank connector and then on to to the pump.

Another issue can be the fuel pump ground wire connection in the trunk of the car.

Here is a picture I found on here (thanks @91AOD5.0LX) of the wiring behind the driver kick panel on a '91. You can see a dark green / yellow wire in the connector in the foreground and bottom right. You can pull this connector apart, set the DVM to continuity (https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-for-continuity), and check to see if this is indeed the wire from the fuel pump relay. If so put the connector back together and back prove for 12v.

This is all I have man so hopefully it helps.

 

[Tortoler0]

the KOEO test worked finally with the new ecu and I got code 95 so now I know the ecu works atleast. I will go ahead and check for the green wire on the driver side panel. Is there anything meaning towards that code I know it’s a fuel pump circuit problem but is there something else I should look at ?

 

[Tortoler0]

Also I proved the green wire at the fuel pump relay I get 6.4 v
And light blue wire gets 12v
With key on
Key off everything is good

 

[Tortoler0]

Okay when I turn the key the green wire gets 12v but as soon as the fuel pump relay clicks it drops to 6.4v if I try to crank it it goes up to 12v but as long as it sits ign on position

 

[AeroCoupe]

Bad relay? Do you have another one you can try?