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Discussion Starter · #1 ·
I have a 1995 Chevy K3500 5.7L 4L80E. It's 4 door with power windows, power locks and AC. I'm having issues with the truck starting for some reason the fuel pump is not coming on at all I've switched pumps switch relays and the pumps and relays are not bad. I've even run direct power with an alternator from the battery to check at the relay and at the plug for the pump and the pump will kick on. I've even bought a new ECU and that still has not made the pump work I'm not sure why the pump isn't working. The truck will try to crank and turn over and turn over or just not start because the pump isn't turning on to send fuel. I've checked all the fuses under the hood and inside the cab none appear to be blown.
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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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Check your fuses again with a multimeter (continuity test) to make 100% sure they're good.

If the fuses check out, verify you have power at the appropriate locations at the fuel pump relay socket with a test light or a multimeter. There should be a power supply pin and a "trigger" supply pin. The trigger supply pin may only be hot at certain times, so a test light you can see while manipulating the key would be best.

If you have power at the relay where you are supposed to, then jumper the fuel pump power supply pins to finish the circuit usually closed by the relay. See if the pump turns on. If it does not, then you most likely have a wire broken between the fuse block and the pump, as you previously verified the pump works with direct power.

I will let someone else interject with the pin numbers. I'm not good enough offhand with a relay pinout scheme to be confident in telling you where to stick direct power. 馃槓
 

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Discussion Starter · #3 ·
Check your fuses again with a multimeter (continuity test) to make 100% sure they're good.

If the fuses check out, verify you have power at the appropriate locations at the fuel pump relay socket with a test light or a multimeter. There should be a power supply pin and a "trigger" supply pin. The trigger supply pin may only be hot at certain times, so a test light you can see while manipulating the key would be best.

If you have power at the relay where you are supposed to, then jumper the fuel pump power supply pins to finish the circuit usually closed by the relay. See if the pump turns on. If it does not, then you most likely have a wire broken between the fuse block and the pump, as you previously verified the pump works with direct power.

I will let someone else interject with the pin numbers. I'm not good enough offhand with a relay pinout scheme to be confident in telling you where to stick direct power. 馃槓
My buddy did use an multimeter and jumped power to the relay socket and the pump kicks on also ran power with the multimeter to the fuel pump plug and the pump will come on.
 

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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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OK. So we're looking at a trigger problem...

Does the trigger get power when you turn the key on during the prime phase? If the trigger gets power, then verify the ground for the trigger circuit is good. If the trigger doesn't get power, you need to walk upstream from the trigger to the computer.

You're probably going to find a bad connector or a broken wire, at this point.
 

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Discussion Starter · #5 · (Edited)
OK. So we're looking at a trigger problem...

Does the trigger get power when you turn the key on during the prime phase? If the trigger gets power, then verify the ground for the trigger circuit is good. If the trigger doesn't get power, you need to walk upstream from the trigger to the computer.

You're probably going to find a bad connector or a broken wire, at this point.
I'm not super good with all the terms all the guys in this group use. I'm just a average everyday guy asking advice. What is the "trigger"?
When I turn the key on you don't hear the pump prime or do anything at all.
 

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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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I'm not super good with all the terms all the guts in this group use. I'm just a average everyday guy asking advice. What is the "trigger"?
When I turn the key on you don't hear the pump prime or do anything at all.
In the relay are 2 circuits.

One is open (switched) and controls the power supply to the pump.

The other closes the switch inside the relay and passes through to ground.

If jumping the prongs of the relay turns on the pump, then I suspect the pass through circuit is faulty. This means either the ground side of the passthrough circuit is bad or the signal side (positive) of the pass-through circuit is bad.
 

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Discussion Starter · #7 ·
In the relay are 2 circuits.

One is open (switched) and controls the power supply to the pump.

The other closes the switch inside the relay and passes through to ground.

If jumping the prongs of the relay turns on the pump, then I suspect the pass through circuit is faulty. This means either the ground side of the passthrough circuit is bad or the signal side (positive) of the pass-through circuit is bad.
So basically the fuse box may need to be replaced. When my buddy used direct power from the battery to where the fuel pump relay goes the pump came on and ran. The relay isn't bad. I put the same relay in my other truck and it cranked up. Do you want me to take some more pics of the fuse box
 

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It may not be the box. It may be a broken or chafed wire. Don't fire the cannon until you make sure. If other things aren't broken in the fuse panel, it's probably more simple.
 

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Discussion Starter · #9 ·
It may not be the box. It may be a broken or chafed wire. Don't fire the cannon until you make sure. If other things aren't broken in the fuse panel, it's probably more simple.
Dang ok..... This is a pain in my butt. I hate wiring I'll try to trace the wires down if I can
 

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By trigger, Jacob is talking about whatever causes the relay to operate.
You've tested the relay secondary circuit, the one that switches the pump on and off, so the problem is in the primary circuit, the one that causes the relay to operate.
I don't have a wiring diagram for an '95, so I may be wrong.
But I think there is a circuit that uses the oil pressure switch to complete the path to operate the relay.
 

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Discussion Starter · #11 ·
Thank you everyone for your help I'm not a mechanic by any means so I'm trying to use my best judgment and common sense to figure some things out for myself so if I don't use the right names of certain stuff my apologies I'm just describing it the best way I know possible but I greatly greatly appreciate everyone's helping input that has way better knowledge than I do.
 

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鈥01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Ray, you鈥檙e right about the oil pressure switch. It鈥檚 a redundant seal in circuit when the engine speed creates at least 10 or so psi while cranking and will fire the fuel pump if the pump relay is bad.
 
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Discussion Starter · #13 ·
Ray, you鈥檙e right about the oil pressure switch. It鈥檚 a redundant seal in circuit when the engine speed creates at least 10 or so psi while cranking and will fire the fuel pump if the pump relay is bad.
So I may need to replace the oil pressure switch and see if that works?
 

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鈥01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Before going through the yoga to get back to the oil sending unit, double check this fuse:

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