What all activates the fuel pump?

SOMETIMES when I turn the key on, the fuel pump will not kick on. It just started happening recently. The fuel pump is maybe 6 months old, and the fuel pump relay is 1 1/2 - 2 years old. I can hear the relay click.. I also jumped the "G" port on the ALDL, and the relay clicked, but the fuel pump didn't kick on.

The fuel pump is a loud one, so I know when it comes on, checking the shrader valve also shows there is no fuel pressure, when the fuel pump doesn't kick on.

The car started up once, ran for a second or two, then died.. the fuel pump must have cut out. If I start it, and give it a good bit of gas for a bit, the car will run and not die.. I have to turn the car off. Same thing happend when the old fuel pump crapped out, once it's running and pumping, it runs.

I'm wondering if something else may be causing the fuel pump to not always kick on, or maybe will a relay still not always work, even when it CLICKs?

Is it possibly something else or is the fuel pump just dying... again..

 

Do you have a voltmeter to watch what the voltage is doing on the output side of the relay ? You need to test at the relay and at the rear harness.
If it is (the voltage) good at the relay, you know it's doing it's job. Probe the rear harness and watch it. If you're seeing relay switched voltage here as well, guess what... ? If no, then there is a problem in the circuit somewhere front to back.
The only other component is the oil pressure switch. This is tied into the FP wiring after the relay output. When oil psi hits something like 5, it passes +12v to the pump wiring (forsay the relay dies, you're not stranded). The OPS will not prevent the pump from running though. Just my thoughts...

 

Yes i have a voltmeter, what terminal at the relay do I probe?

And what do you mean by rear harness? the fuelpump connector? Would i have to drop the tank (no way) to access it?

 

The pump wire should be tan/wht out of the relay and the same gauge (16) as the orange or red +12v in.

There is a harness plug (weatherpack connector) just forward of the tank on the chassis sheetmetal. It's the metal the rear seat backrest lays against. The plug connects from chassis to sender on the gas tank. You should be able to backprobe (like when setting TPS voltage) to test the pump side of the harness connection. Hope that makes sense (long day and a cold one away from going comatose for the eveneing )

*edit.. here's a FP diagram to aid..
http://92b4crs.tripod.com/86wiring/dia-pics/22-2.jpg
(drag and drop it into your browser addy bar)

It's for an 86 but, they are all pretty much similar.

 

if there is enouph pressure you wont here it prime when you turn your key forword..... mine does the same thing if you turn it on and let it prime then turn it off and back on it wont prime again but wate about 5 minuites or so and it will prime again cause the pressure more then likely dropped a bit...

 

Deadbird: Thanks!

z28monster: That's what I was thinking, but no fuel comes out of the shrader valve, by pushing it in, at that point.. And the engine simply will just keep turning over, but not starting.

 

Have you thought about checking the ECM? Just a thought!!!!!!

 

Before turning comatose.. one last comment...

The ECM is blind to FP. There is no way for it to know what it is. (Non-sceintific explaination following) There is a 'key on' delay that keeps the ECM from grounding the relay until X time has passes. From my 2 cars and vauge rememberance of reading, the delay is like 10-15 seconds. After the delay, with key off, turning it back on will 'reprime' the pump.
Regardless of the pump priming or not, the pump should still energize during the crank cycle.
That it for me...

 

If it were me I would start hot wiring the pump fom the front and slowly work my way back to see what is not functioning correctly.

1-apply the switch 12V of the relay to power, see if pump jumps on.

2-Pull relay apart and apply power to "E" (I think)

3.- pull weather pack apart at tank and apply power to see if pump turns on.

4-replace and or check the fuse by the battery and relay.

5-check ecm for codes and loose connections.

6-- check fuel filter?

I had a similar problem when I first installed my pump. would turn on sometimes, turns out it was a loose connection in the tank.

 

Confused a bit.. according to that diagram there's a fuelpump RELAY, and fuel pump switch

 

The fuel pump switch would be the oil pressure switch I was referring to above. It's the sender unit located just above the oil filter. (somewhere around '89-up it was the OPS and the oil gauge sender combined, 3 wire instead of 2)

 

Ah, I see. Thanks.