HELP, Broke down Again !!!

[jchav62]

I had a couple of incidents similar to yours. One was on a 90 GMC 2500 that was actually a Survey Crew truck for the Nevada DOT. It left me stranded a few times...and I would have to get a tow. One time, while waiting for tow truck, I checked the fuses in the inside fuse box, on the side of the dash. There was one blown fuse in there labeled 'ECM' if I remember correctly. I replaced it with a good one, turned on the key and I could here the fuel pump right away. I fired it up, called off the tow truck and drove back to the yard. Worked for me a few times. The second time was on my personal vehicle, a 93 Chevy Shortbed. It left me a few times... got towed home a couple times, other times I would let the truck sit for a few minutes and it would start up again. That one ended up being the fuel pump....I replaced it and the problem was solved for the rest of the time I owned it... only thing is, after I replaced the fuel pump, the gas gauge acted funky...

[ChevyTech]

I used your link, and read it.

Any idea on what the fuel pump pressure should be on a TBI system? My book doesn’t say Squat about it.

The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.

The fuel pressure is tested by using fittings to “T” in a pressure gauge and must be tested with the engine RUNNING.

I don’t have a schematic for your truck but this is my best educated guess at where the wires go.
Orange wire, hot all the time. Power supply from junction block

Red w\white, not hot. To ALDL terminal

Black w\white, not hot. ground

Red w\black, not hot, goes to Fuel Pump Fuse, didn’t expect it to come hot anyway. Out to fuel pump. This wire should have power for at least two seconds when you first turn the key on. It should also have power when the engine is cranking. If it does not get power the relay is most likely bad.

Green w\white, hot Only with starter engaged and turning the engine over. From ECM – controls the relay

The ECM controls the fuel pump relay.

There will only be power to the fuel pump for a few seconds when you first turn the key to the on position. If you are turning the key on then walking around the vehicle testing for voltage to the fuel pump circuit there will be no voltage and should not be power to the pump.

When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through an oil pressure sensor/sender.

Once the engine is running the fuel pump should get power from the redundant circuit and not need the fuel pump relay to keep running.

[MonteSS454]

Originally Posted by jchav62

only thing is, after I replaced the fuel pump, the gas gauge acted funky...

Exac same thing happened to me, including the gas gauge moving back and forth quickly.

[88lowrider]

Thanks guys.
First thing I check was all the fuses and was not able to get power to run the fuel pump until I put a jumper around the Fuel Pump Relay..

Now that I let everything cool down, it started with the FPR hooked back up, like Nothing was wrong???

Does anyone know if the Distributor module is bad that it shuts off the fuel pump circuit ?

Reason I ask this is now that I think about it more, its never acted up until after its been running for awhile.
I was able to move it around the farm, hooking to trailers, moving them but just short running times.

I called the parts store and told them I needed more prices on other parts as replacing the ECM did not fix the problem.
He asked me, how many miles and have I ever replaced the distributor module on it.
After saying no, he said pull it, bring it in and they can test it. He told me its a big seller and they always stock several of them.

Again, the big question would be, can the distributor module not working cause the fuel pump circuit to fail?

The thing that is making me nuts is it "won't" stay broken as to find it.

Now that this truck is a high miler, makes me think I need to get my tail to GMC and purchase the "real" service manual for it.

Thanks again for your help guys...

[JohnO356]

Before you go to the dealer for a manual, check Ebay. Then Google for '88 Service Manual. I got all 4 manuals from different places (Service Manual, wiring diagram manual, electrical trouble shooting, and emissions) off the internet and total was about $150. It took a little while, but you can find them.

[ChevyTech]

Clear the trouble codes that are in the ECM from you unplugging items like the Fuel pump relay. Then see if the system sets a trouble code the next time the vehicle won’t start or the next time it dies.

Originally Posted by 88lowrider

Thanks guys.
First thing I check was all the fuses and was not able to get power to run the fuel pump until I put a jumper around the Fuel Pump Relay..

Now that I let everything cool down, it started with the FPR hooked back up, like Nothing was wrong??? ...

You are going to fight with this thing for a long time if you don’t use a logical plan to determine what the problem is. If you would test if power was getting to the fuel pump fuse under the hood When the ECM is sending the signal to prime the pump for a few seconds when the key is first turned on then we would have information that helps.

Originally Posted by 88lowrider

Does anyone know if the Distributor module is bad that it shuts off the fuel pump circuit ? ...

No it does not shut off the fuel pump circuit. People think is does because they do not understand how the systems are interconnected.

The ECM controls the fuel pump relay.

The truck has a fuel pump relay that supplies power to the fuel pump when:
- Anytime the ECM receives a distributor reference pulse. (Engine running / engine cranking)
- The key is turned on engine not cranking – relay applied for 2 to 20 seconds (most trucks 2 seconds), depending on emissions equipment. Trucks over 8500 GVW and some other vehicles have and electronic Fuel timer module that allows the pump to run up to 20 seconds.
The key must be turned off for a certain amount of time before it will prime the system again when turned back on.

Remenber, there will only be power to the fuel pump for a few seconds when you first turn the key to the on position, so if you are turning the key on then walking around the vehicle testing for voltage to the fuel pump circuit there will be no voltage and should not be power to the pump.

The ECM should store a code 54 if the fuel pump relay fails.

Originally Posted by ChevyTech

When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through an oil pressure sensor/sender.

Once the engine is running the fuel pump should get power from the redundant circuit and not need the fuel pump relay to keep running.

Originally Posted by 88lowrider

… I called the parts store and told them I needed more prices on other parts as replacing the ECM did not fix the problem.
He asked me, how many miles and have I ever replaced the distributor module on it.
After saying no, he said pull it, bring it in and they can test it. He told me its a big seller and they always stock several of them.

Again, the big question would be, can the distributor module not working cause the fuel pump circuit to fail?

A bad module will not stop the ECM from sending power to the fuel pump relay for a few seconds when the key is first turned on.

A bad module will not stop the redundant circuit from supplying power to the fuel pump once the engine has oil pressure.

A bad module can cause the ECM the receive no distributor reference pulse during cranking, so after the prime timer has expired the fuel pump relay would not be energized.

[ChevyTech]

Keep in mind, on a 20 year old truck, you could have 2 or more failing parts causing problems.

The redundant fuel supply circuit may not work so the fuel pump relay is the only item supplying power to the fuel pump.

Sometimes the fuel pump current flow increases when the pump is getting bad. The high currant flow can damage the fuel pump relay, oil pressure sending unit and the terminals for the fuel pump wires on the fuel tank sending unit.

It is common for the distributor module to fail. The module could be the cause of your vehicles problem, and not knowing how the fuel pump circuit works could be leading you astray.

[88lowrider]

Originally Posted by ChevyTech

Clear the trouble codes that are in the ECM from you unplugging items like the Fuel pump relay. Then see if the system sets a trouble code the next time the vehicle won’t start or the next time it dies.

I can do this but I don't have a tester to test it with. Might be able to rent one as the I live in on the Olympic Peninsula of Washington - which seems to be a geographical oddity for parts stores. I have to drive two towns over to get to the nearest parts store or auto shop. There is a big rental place in the next town so it it happens again, I will try and let it cool down and try to start it again to get it home.
Last time out with the truck, it broke down, after waiting for the wife to come and get me, I was able to jump start it (battery was low from the last time me grinding away with the starter like an idiot)
After jumping it I was able to drive it home and Almost get it back to the spot were I drove away from that morning, It broke down in the driveway.
After waiting about 20 minutes, it started again but I had jumped the fuel pump relay - or, it was cooled off enough?


You are going to fight with this thing for a long time if you don’t use a logical plan to determine what the problem is. If you would test if power was getting to the fuel pump fuse under the hood When the ECM is sending the signal to prime the pump for a few seconds when the key is first turned on then we would have information that helps.

When it died the first time, I checked that fuse and there was no power to it.
I might have forgot to say that.
When you mention logical plan, it would be nice to know what this stuff does, how its supposed to work and when, that way I would have a clue on what is going on. Its really hard to find good information.

When I take it out in the morning, I have the light tester and tools with me so I will be able to check that right away.

No it does not shut off the fuel pump circuit. People think is does because they do not understand how the systems are interconnected.

The ECM controls the fuel pump relay.

The truck has a fuel pump relay that supplies power to the fuel pump when:
- Anytime the ECM receives a distributor reference pulse. (Engine running / engine cranking)
- The key is turned on engine not cranking – relay applied for 2 to 20 seconds (most trucks 2 seconds), depending on emissions equipment. Trucks over 8500 GVW and some other vehicles have and electronic Fuel timer module that allows the pump to run up to 20 seconds.
The key must be turned off for a certain amount of time before it will prime the system again when turned back on.

Remenber, there will only be power to the fuel pump for a few seconds when you first turn the key to the on position, so if you are turning the key on then walking around the vehicle testing for voltage to the fuel pump circuit there will be no voltage and should not be power to the pump.

The ECM should store a code 54 if the fuel pump relay fails.

I will see if I can rent a code reader and find out..

A bad module will not stop the ECM from sending power to the fuel pump relay for a few seconds when the key is first turned on.

A bad module will not stop the redundant circuit from supplying power to the fuel pump once the engine has oil pressure.

A bad module can cause the ECM the receive no distributor reference pulse during cranking, so after the prime timer has expired the fuel pump relay would not be energized.

Interesting....
Seems I messed up the Quote some how?
This might make it next to impossible to see my answers.

Thanks...
Joe