Dumb Question Pertaining to Fuel Pump

[BWoods]

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

 

[RawbDidIt]

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

If you're getting power to the pump, and the ground has solid connectivity, then the pump is bad. If it were a cutoff issue, the power wouldn't be getting to the pump. Regardless of cutoff, the pump should get power for 2 seconds after the key is switched to the on position. Sounds like you did your homework as far as connectivity, but just for good measure, hook up a multimeter and test for voltage difference at the harness between power and ground. Then test resistance of ground between harness and battery negative. Grab a good long section of decent gage wire (a jumper cable works real well here) just to ensure the connectivity is good. Can't remember what gage wire it is, I think 10ga or 12ga, but IIRC the resistance should be in the 0.01-0.03 range based on length of wire and gage. You can test the power wire resistance against the positive power terminal if you'd like as well.

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[HotWheelsBurban]

Good to know! From what I've read, if you have a GMT400, you'll be needing to do this check eventually LOL

 

[RawbDidIt]

Good to know! From what I've read, if you have a GMT400, you'll be needing to do this check eventually LOL

Yeah the ground on the frame rail gets really corroded after 20 years or so, less if you live in a salty region.

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[HotWheelsBurban]

Yeah the ground on the frame rail gets really corroded after 20 years or so, less if you live in a salty region.

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Seems like so many big electrical problems start out as little bitty issues like that.....

 

[RawbDidIt]

Seems like so many big electrical problems start out as little bitty issues like that.....

Yeah, I've learned a couple things throughout owning this truck. 1: things fail in strange ways, buy testing and diagnostic tools. 2: previous owners do stupid s**t to their truck before it was yours, be prepared to undo their work, and fix anything they broke.

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[BWoods]

If you're getting power to the pump, and the ground has solid connectivity, then the pump is bad. If it were a cutoff issue, the power wouldn't be getting to the pump. Regardless of cutoff, the pump should get power for 2 seconds after the key is switched to the on position. Sounds like you did your homework as far as connectivity, but just for good measure, hook up a multimeter and test for voltage difference at the harness between power and ground. Then test resistance of ground between harness and battery negative. Grab a good long section of decent gage wire (a jumper cable works real well here) just to ensure the connectivity is good. Can't remember what gage wire it is, I think 10ga or 12ga, but IIRC the resistance should be in the 0.01-0.03 range based on length of wire and gage. You can test the power wire resistance against the positive power terminal if you'd like as well.

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Thanks for the reply! Difference in voltage is only .06 volts. The battery is pretty weak, but I would think the pump could still run on 11.5v right? The ground is way out of spec if that's the expectation. On the 20K setting I'm getting .95 from the harness lmao. Still, I went a step further and played with the harness on the pump itself. Grounded it directly to the t-stat housing (which is .01 x20K ohms to battery negative) and applied voltage, no dice. It has to be the pump or it's individual harness that's at fault.

Yeah the ground on the frame rail gets really corroded after 20 years or so, less if you live in a salty region.

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Aside from the fuel filler, what else grounds to the frame rails? I can tell you right now, those grounds are SHOT on both of my trucks in Wisconsin...

 

[RawbDidIt]

Thanks for the reply! Difference in voltage is only .06 volts. The battery is pretty weak, but I would think the pump could still run on 11.5v right? The ground is way out of spec if that's the expectation. On the 20K setting I'm getting .95 from the harness lmao. Still, I went a step further and played with the harness on the pump itself. Grounded it directly to the t-stat housing (which is .01 x20K ohms to battery negative) and applied voltage, no dice. It has to be the pump or it's individual harness that's at fault.



Aside from the fuel filler, what else grounds to the frame rails? I can tell you right now, those grounds are SHOT on both of my trucks in Wisconsin...

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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[HotWheelsBurban]

Yeah, I've learned a couple things throughout owning this truck. 1: things fail in strange ways, buy testing and diagnostic tools. 2: previous owners do stupid s**t to their truck before it was yours, be prepared to undo their work, and fix anything they broke.

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Ain't that the truth!

 

[Wiscoburban]

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.