Dumb Question Pertaining to Fuel Pump

[RawbDidIt]

I love Wisconsin but it reeks havoc on vehicles, I had so many issues with my grey truck all had to do with electrical wiring and the rust and corrosion that I had to track down. I get jealous looking at these trucks from south or west that are in such immaculate shape with high miles. Defintely if it sat check all ground connections and clean them up it very well could be culprit but even if its not saving yourself time in the future.

Looking forward to getting my dealers license out here (TX) and getting access to the auctions. A whole new world of possibilities

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[haroldwca]

Hello all,

I have an '88 K2500 that has sat for a bit and I expect the fuel pump is dead, I was hoping to confirm my logic prior to making uncomfortable decisions about it's future.

I've verified full power and a good ground is present at the end of the harness before it connects to the pump. I've applied power directly to said harness via the fuel pump fuse socket, but the pump is perfectly silent and will not create any fuel flow even prior to the filter. A test light was used to apply the current and it does indicate a complete circuit.

I understand the logic of the fuel pump relay and oil pressure switch as the backup, but those should both be bypassed at this point. Aside from the pump shorting to ground, is there anything else I can blame aside from the pump?

Thank you!
Ben

I worked through a similar issue. I bought an ‘88 K1500 last year. It had sat for ten years, and the only prep was rolling up the windows. After experiencing no power to the fuel pump, I dropped the tank, and found the problem was indeed a fuel pump that looked like it had been lying at the bottom of a lake for years.

While I appreciate a methodical diagnostic approach, I think you have exhausted almost all of the electrical possibilities. It sounds like you might be grasping for a reason not to drop the tank. In reality, dropping the tank doesn’t take that long.

Drain the tank, disconnect the filler neck from the bedside, remove the tank straps (an impact wrench is useful here), and lower the tank sufficiently to disconnect the wires and hoses. You can have it on the ground in an hour if you don’t run into excessively rusty bolts. You indicated you are up north, so that will no doubt figure into the process.

I bought a new loaded tank for about $325 from one of the local discount parts places. Since my truck had sat for a LONG time, I also had to replace the fuel pressure regulator and both injectors. ( A new fuel filter is a given at this point.). FWIW, you can test your injectors with a 9V battery. Unplug the connector, touch each terminal of the battery to each contact on the injector. A click means it still works; no click means no good. Injectors were about $40 each, and the regulator (which is basically a new diaphragm and spring) was about $30.

Good luck and keep us posted !

 

[Schurkey]

I have an '88 K2500 that has sat for a bit and I expect the fuel pump is dead, I was hoping to confirm my logic prior to making uncomfortable decisions about it's future.

I've verified full power and a good ground is present at the end of the harness before it connects to the pump.

HOW? A comprehensive test would require that the pump be running; if there's no current (amperage) flow, the voltage test is suspect. Better than nothing, but not all that informative.

I've applied power directly to said harness via the fuel pump fuse socket,

Are you sure the fuse is on the fuel pump side of the relay? I'd expect it to be "upstream" so that putting power to the fuse only puts power TO the relay, not "through" the relay and on to the pump.

A test light was used to apply the current and it does indicate a complete circuit

So the voltage is dropped by the bulb inside the test light? You're starving the pump for power.


There should be four wires on the fuel pump connector--power and ground for the fuel sending unit, power for the pump, and a separate ground for the pump.

Figure out which are the power and ground for the pump, not the sending unit. String a long, 16-gauge or heavier wire from battery + to the power wire of the pump, connect the ground wire of the pump to a known-good ground.

I bet the pump works. (I've been wrong before.)

BEST way to test electric fuel pumps is with a low-amperage probe and an oscilloscope; but most folks don't have that equipment. A voltmeter does a half-assed job: It's only accurate to the point in the wire harness that you connect it to. When testing voltage, you need to be as close to the load (the fuel pump) as practical. Of course, you can't probe the wires IN the tank, and they're known to corrode. A fresh in-tank wire harness is recommended and typically about $25.

Is there fuel in the tank?

 

[AlanE]

Last week I had to drop the tank to clean out bad gas. I replaced the pump also. If I had to do it again, I would either pull the bed back or tilt it. Never dropping the tank again. I had a real hard time with that ground bolt on the frame. If you are replacing the pump, and that is a definite, just cut the ground wire and once the tank is out, you'll have much easier access to the bolt and to clean up the frame.

 

[BWoods]

HOW? A comprehensive test would require that the pump be running; if there's no current (amperage) flow, the voltage test is suspect. Better than nothing, but not all that informative.

So the voltage is dropped by the bulb inside the test light? You're starving the pump for power.

Are you sure the fuse is on the fuel pump side of the relay? I'd expect it to be "upstream" so that putting power to the fuse only puts power TO the relay, not "through" the relay and on to the pump.

I cannot verify how many amps are present. I verified voltage with a multimeter on the hot wire at the end of the harness, on the connector that will attach to the pump's. If the fuse is indeed upstream, then I guess my relay works lol. The difference in voltage between across the battery terminals and what it have at the tank is only .05v, so yes the bulb uses some.

There should be four wires on the fuel pump connector--power and ground for the fuel sending unit, power for the pump, and a separate ground for the pump.

Figure out which are the power and ground for the pump, not the sending unit. String a long, 16-gauge or heavier wire from battery + to the power wire of the pump, connect the ground wire of the pump to a known-good ground.

I bet the pump works. (I've been wrong before.)

I somehow have three? I did isolate the third wire, which is assumed was the sender, and simulated this very condition to no avail. The tank is very full, I assure you of that because I topped it off, fearing I was siphoned.

It sounds like you might be grasping for a reason not to drop the tank. In reality, dropping the tank doesn’t take that long.

You are absolutely correct lol. To be perfectly honest, the only reason to fix it would be for the sake of selling as a runner vs cannibalizing it for a personal parts stash and offering the rest.

 

[Schurkey]

I somehow have three? I did isolate the third wire, which is assumed was the sender, and simulated this very condition to no avail.

I have an '88 K1500. Unless there's some difference between half-tons and Three-Quarter-tons for fuel pump wiring, you should have a three-wire molded connector, with a fourth ground wire. I'm too lazy right now to look up the schematic. Fairly certain that there's a 3-wire molded connector. + and - for the sending unit, and + for the fuel pump. The fuel pump ground wire is separate, and goes to the frame rail.

You must be registered for see images attach

However the four wires are divided-up, the four wires are:

+ and - for the sending unit, + and - for the pump.

If the sending unit works the dash gauge; you're missing a wire for the fuel pump. No wonder it doesn't work.

 

[BWoods]

I have an '88 K1500. Unless there's some difference between half-tons and Three-Quarter-tons for fuel pump wiring, you should have a three-wire molded connector, with a fourth ground wire. I'm too lazy right now to look up the schematic. Fairly certain that there's a 3-wire molded connector. + and - for the sending unit, and + for the fuel pump. The fuel pump ground wire is separate, and goes to the frame rail.

You must be registered for see images attach

However the four wires are divided-up, the four wires are:

+ and - for the sending unit, + and - for the pump.

If the sending unit works the dash gauge; you're missing a wire for the fuel pump. No wonder it doesn't work.

Wait a minute, that adds some serious context. My whole understanding of the harness is wrong. I thought it was fuel pump +&- and sender - . This throws my verification of ground out the window. I’ll revisit this and post back in the light of day and after work tomorrow. Thank you!

 

[alpinecrick]

Yeah, a Dremel with a wire wheel, a new screw and some grease should do the trick. The main frame rail ground is on the passenger side next to the starter, the rear lights, and I believe the headlights and turn signals are on the frame rail up front. If you have trailer lights hooked up, they'll likely be grounded to the frame as well.

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All of that and the taillight ground on the last foot of drivers side frame rail. That one catches a lot of road grime.

To the OP--when you change the fuel pump, change the fuel filter and keep changing it every 15k-20k miles. Your fuel pump will then last, like.......forever.

Delphi or AC Delco fuel pump is my recommendation.

 

[alpinecrick]

While I appreciate a methodical diagnostic approach, I think you have exhausted almost all of the electrical possibilities. It sounds like you might be grasping for a reason not to drop the tank. In reality, dropping the tank doesn’t take that long.

Drain the tank, disconnect the filler neck from the bedside, remove the tank straps (an impact wrench is useful here), and lower the tank sufficiently to disconnect the wires and hoses. You can have it on the ground in an hour if you don’t run into excessively rusty bolts. You indicated you are up north, so that will no doubt figure into the process.

This^^^

You can have the tank out in an hour if it's your first time.

 

[Schurkey]

You can have the tank out in an hour if it's your first time.

Good luck with that in the Rust Belt.