Need Help Fuel Pump Circuit

[massacre]

Hello,

I am in need of some help, I am baffled by this but I know someone will be able to get me on the right track.

I had been having an intermittent problem with the truck not starting after being shut off. Fuel pump would not prime. I would remove the relay and put it back, wiggle some wires around and it would then audibly prime and start. Only happened like twice maybe 3 times a year.

Well now she will not prime at all. I removed the relay, switched it out with the other replays and no difference. I was able to wiggle the relay and some wires and could hear it trying to prime but then then it went dead.

So here is where I am at-

Turn key, pump will not prime. Check fuses, they check out good.

I get power to the fuel pump relay in 1 location with the key off. If I jump the relay I can hear the fuel pump run. With the key on, I am looking for power at one of the other 2 connections for a couple of seconds but none found (with relay removed).
I checked the inertia switch it is not tripped. I see no voltage at the switch with key on or off but if I use my power probe at the switch I can audibly run the fuel pump.

The EEC fuse is good and that relay seems ok as well. I could be wrong though

The pump seems ok since it seems to run with 12v applied so I am taking that as a good sign.
But it seems like I am not getting power to the relay with the key on for some reason.
Truck is stuck in driveway and I need it to get to work.

Any help would be greatly appreciated. Most of the inquiries I have come across deal with pump failures but mine runs great as long as it primes

Thanks

 

[imp]

Turn key, pump will not prime. Check fuses, they check out good.
I get power to the fuel pump relay in 1 location with the key off. If I jump the relay I can hear the fuel pump run. With the key on, I am looking for power at one of the other 2 connections for a couple of seconds but none found (with relay removed).
I checked the inertia switch it is not tripped. I see no voltage at the switch with key on or off but if I use my power probe at the switch I can audibly run the fuel pump.
The EEC fuse is good and that relay seems ok as well. I could be wrong though
The pump seems ok since it seems to run with 12v applied so I am taking that as a good sign.
But it seems like I am not getting power to the relay with the key on for some reason.
Truck is stuck in driveway and I need it to get to work.
Any help would be greatly appreciated. Most of the inquiries I have come across deal with pump failures but mine runs great as long as it primes
Thanks

@massacre
OK, here goes! I excised your quote to make easy looking back at it as I write. First, how are you checking for "power" being available? Are you measuring for voltage between certain points and ground?

Bold: There should appear 12V between either inertia sw. contact and ground only for the first few seconds after KEY ON, then pump is shut off by PCM. What is your "power probe"? Do you have wiring diagrams? This is a '94? I have them for '96, which is 2nd. Gen., but should apply, but fusing may not be identical. We need to determine whether turning Key On provides 12V to the Fuel Pump Relay; generally, the controlled item, relay, is turned on by PCM grounding the cold side of the relay coil.

Inertia Sw.: If stuck OPEN, pump will not run. If stuck closed, you'd never know it. If push comes to shove, you say the pump runs if you feed it 12V directly; if eng. starts and runs then, you can use the veh. to get to work, but without the safety of emergency crash fuel shut-off.

I see you are on-line now. I'm going to my shop to locate wiring schematics. Maybe you can tell more meanwhile. imp

 

[imp]

@massacre Having a bit of trouble with the fact that "wiggling" some wiring and/or fuses produced result related to the pump issue. BTW, '96 circuits show a warning lamp in the instrument panel which lights if inertia sw. is tripped. Gen I might not have that, would be good to know, though.

 

[massacre]

OK well it’s cold out and dark by now. Wish I had pulled the truck into the garage before all of this would be a lot easier to work on lmao

I am using a test light with incendescant bulb to check for power.
My power probe is like the original Power Probe but you hook it up to the battery and then you can either power up a circuit or ground a circuit for testing purposes.

There is no check engine light present with key in on position or while cranking.

I do have a meter too so I can test for more precise voltages later.
Also bought a new relay but no dice which I kind of expected

 

[massacre]

I do have a diagram going over that now

 

[410Fortune]

can you jumper the relay to power the fuel pump and start the truck, drive it into the garage?

 

[massacre]

Also my internet just went out had to reboot the modem.
Like I need any more weird problem

 

[massacre]

can you jumper the relay to power the fuel pump and start the truck, drive it into the garage?

I will try that. Is there any danger in that?

 

[massacre]

Tried jumping out the relay to start the truck and move it but no dice. Wouldn’t start

 

[massacre]

Thought about pushing it into the garage but would need another person and no one here for that right now. Weather in the teens too haha

 

[imp]

Tried jumping out the relay to start the truck and move it but no dice. Wouldn’t start

@massacre
OK, meaning would not turn over, or cranked but failed to start? Is pump running while trying? Pump must remain running while cranking in addition to initial key-on 2 seconds run.

 

[massacre]

It cranked but failed to start. The pump was running the entire time. There was no "initial key-on 2 seconds run", if I can get it to prime it runs perfect
so weird

 

[massacre]

Well I am calling it a night as it is too dark and too cold to do anything until tomorrow.

I am not against replacing the fuel pump, I was waiting until it got a little warmer to replace it preemptively. Just want to figure out what is causing this, was perfectly fine then nothing

 

[imp]

It cranked but failed to start. The pump was running the entire time. There was no "initial key-on 2 seconds run", if I can get it to prime it runs perfect
so weird

@massacre
OK, personally before blaming the pump, I'd do a few more tests. Is there fuel pressure at the Schrader valve on the fuel rail when the pump is running, and how much? Have you checked for spark? If pressure is present but low, has the filter been replaced? If old, replace it, checkpressure again. An EFI EEC-IV engine will not start with much less than 20 psi of fuel pressure, just not enough to fire. Gradually, we're getting to blaming the pump, or it's relief valve, built-in. imp

 

[massacre]

When I turn the key to the on position, the fuel pump will not prime.

When the pump does prime, the truck starts and drives completely normal.
I will check fuel pressure tomorrow. Filter is just a few months old

Not sure if it would be faster to replace fuel pump myself or pay a shop to do it?

 

[Josh P]

When you jump the fuel pump relay, if the pump is working properly it should prime the engine. My luck would be a bad ground and a failed crankshaft position sensor.

 

[decipha]

failed crank sensor has no affect on the pump priming at key on

it sounds to me like you have either a faulty ignition switch and that circuit isn't getting fed juice or either a short in your fuse box

verify the check engine light comes on with key on every time, a bad ecu relay will have the exact same symptoms

 

[BKennedy]

If you wiggle the relay in the slots, and the pump activates then you have a loose connection. Pull the box away from the fender and turn it over so you can check all of the wire connections. You can pull the box by inserting a flat blade screw driver in the two slots on the mounting bracket.

 

[massacre]

If you wiggle the relay in the slots, and the pump activates then you have a loose connection. Pull the box away from the fender and turn it over so you can check all of the wire connections. You can pull the box by inserting a flat blade screw driver in the two slots on the mounting bracket.

Did this already, nothing found.

 

[massacre]

failed crank sensor has no affect on the pump priming at key on

it sounds to me like you have either a faulty ignition switch and that circuit isn't getting fed juice or either a short in your fuse box

verify the check engine light comes on with key on every time, a bad ecu relay will have the exact same symptoms

There is no check engine light present
Would bad ignition switch let the engine crank though?