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I have a problem with my 1990 5.0 liter F150. A short history, I bought this truck last year from my fathers estate and plan on using it for my everyday truck. When I got it, just about every gasket was shot, and it was leaking oil out faster than I could put it in. So I decided to pull everything and refresh all of the gaskets and seals. All went well, and yesterday I finally got it all back together and fired it up for the first time since last year. It started and ran for several minutes. I was getting it up to temp, and checking for leaks when it stalled and would not fire up. After rechecking everything, I determined that the fuel pump(s) (dual tanks) were not pumping fuel. It will start on starting fluid.
I checked the voltage at the plug on the front tank and I was only getting about 7.5 to 8 volts. My problem is that I don't have a good electrical manual. It seems that there are a lot of variations of electrical diagrams. I understand that fuel pumps are powered through a fusible link at the starter relay? Is that correct? And the that the fuel pump relay is located near the air box? I'm trying to identify it. I found this relay next to the air box. Can anyone confirm if this the fuel pump relay? I'm getting 12 volts to it, but have not checked the output. Any advice would be appreciated. Thanks
I've followed this procedure to test the fuel pump (green) and EEC (Brown) relays. I think it's a good place to start. You can jump 12v from the battery to the power wire to see if its the fuel pump. If that works, likely bad relay or fusible link coming off your starter solenoid.
Glazer67, Thanks for the link, Thats exactly what I was needing. I already tested the voltage at the plug, and I'm getting 12 volts there, So the drop is in the relay or somewhere between it and the fuel pump.
Edit. That was the problem, a bad FP relay. On my may to go get a new one. Thanks for your help.
I checked the voltage at the plug on the front tank and I was only getting about 7.5 to 8 volts.
Normal voltage after one second with the engine not running.
Originally Posted by KMag1226
I understand that fuel pumps are powered through a fusible link at the starter relay?
That is true for the contacts over a large yellow wire.
Originally Posted by KMag1226
Is that correct?
The fuel pump relay is closed by the Computer grounding the coil of the fuel pump relay.
Originally Posted by KMag1226
Can anyone confirm if this the fuel pump relay?
Yes that is the fuel pump relay in your photo.
When working in this circuit use a loading test light (non LED) and not a meter.
OR you may be replacing good relays and switches.
I would not go by that link.
Wow. Where where you a couple of months ago when I first got ahold of this 1989 F150 4x4 (300ci)... ?
I'm currently bypassing the 'loom through the firewall' by sending the power from the FP Relay's output to the Inertial Cut Off output wire. With a fuse.
<smiley face>
I'm working on this for the actual owner, but we got it running off the back tank so far.
(Gauge and in-tank pump on this truck's rear work OK. Front tank shows 'pegged' full when switched to & the truck continues to run off the rear tank in any case.)
Not to hijack this thread, but it's got good info and I wanted to thank KMag & Bill both.
Subford, Can you tell me where the fuel pump PCM is located in the vehicle? I can't seem to find out where it is. Thanks
Edit: I found these three devices under the dash, to the right of the steering wheel. One is white plastic, one is black and one is cad plated. Can anyone tell me what each of these devices are? Thanks
OK. Thanks. From everything I was reading, it sounded like the FP PMC was a separate unit. So it is in the main PCM. I was looking for another one somewhere else.
I did the step by step procedure checking the relay. Everything seemed to be OK until I got to the step of checking the that the PCM was activating the Fuel Pump Relay. I used the test light from wire #926 to the hot side of the battery. it was my understanding that the light should only be on when the engine is cranking, Correct?
However, when I made the connection, (no key in the ignition or turned on), the light lit up and stayed on until I unplugged the test light. Am I right this should not happen? Am I also right saying that the PCM is faulty?
I pulled the PCM just now, and I cannot see any damage to it at all. There are no leaks in the capacitor, and I cannot see any breaks in the solder joints. Nothing looks like it is damaged. Is there a way to test the PCM fuel pump circuit to verify if the PCM is the problem?
I did the step by step procedure checking the relay. Everything seemed to be OK until I got to the step of checking the that the PCM was activating the Fuel Pump Relay. I used the test light from wire #926 to the hot side of the battery. it was my understanding that the light should only be on when the engine is cranking, Correct?
However, when I made the connection, (no key in the ignition or turned on), the light lit up and stayed on until I unplugged the test light. Am I right this should not happen? Am I also right saying that the PCM is faulty?
That depends on the sensitivity of your test light. The coil of the fuel pump relay is fed by the red wire #361 and the wire #361 also feed all of the fuel injectors and other solenoids. So it is possible that your test light is lighting from that path. You would need to unplug the PCM computer and do the test again and if it still lights up it would be from the grounding as I spoke of above. Sounds like you may have a LED test light and the test you spoke of was for a incandescent bulb loading test light.
Where did you get this testing procedure?
I got the grounding procedure from the link in post #2. There was a 6 step process to check the FP Relay. When I got to test #4, "checking that the computer is activating the FP Relay", it directed me to jumper the test light as described previously. Is that not the correct procedure? Is there a better one?
I have the PCM out right now. I appreciate you help.
I would just ground pin #6 of the self-test connector and tun on the key.
Then the selected fuel pump should run.
This bypasses the PCM and tests the fuel pump pump and circuits.
Note the PCM dose not have to be installed or plugged in to its connector for this to test the fuel pump circuits.
If the selected fuel pump will run this way and does not run for one second each time the key is turned to the run position then you would have something wrong with the PCM or its wiring.
OK, Great. Thanks. I will try that next. I hope that I can figure this out soon. It puzzles me that it ran for ten minutes and then suddenly just shut itself off, and would not start up again.
07-18-2018, 09:45 AM
