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Fuel pump wiring issues...Please help!

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Old 05-08-2018, 01:50 PM
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Fuel pump wiring issues...Please help!

I recently picked up a '93 silverado pickup that had issues. It was purchased as a Crank / No Start when I got it. Normally i'm not bad at diagnostics. The problem is that it had the TBI 305 engine swapped out for a 1995 Impalla SS LT1 350 5.7 motor. They took the harness out of the SS and amazingly enough everything works. HERE IS THE PROBLEM:
When I got it there was no spark or fuel. The fuel pump had been replaced. The Fuel Pump Fuse was blown. I initially swapped out the Opti because of corrosion inside the cap. While doing that I replaced the timing chain and water pump. After buttoning it up I still had no fuel. I replaced the fuel pump relay and cleaned up a couple of wiring connections that were related to the oil pressure switch in the fuel pump circuit and it started and ran great. I drove it around a bunch and it seemed to be "fixed". Yesterday I was driving and it shut down. Checked the fuel pump fuse and it was blown. replaced the fuse and it fired up and ran perfect for a dozen miles and then it shut down again. Same deal so I replaced the fuse and it started and ran perfect again. About an hour later it died again. I replaced the fuse and as soon as I turned the key to the on position is blew the fuse again. I went under the truck und unplugged the fuel pump connector at the tank, turned the key and again it blew the FP fuse. I then pulled the FP relay out, jumped the two outside leads on the oil pressure FP switch and could hear the pump kick on. Again this is with the relay out. I then turned the key and it started up fine. I then removed the jumper and replaced the relay and turned the key and it started and ran fine. No blown fuses. Im at a loss and dont know what to try next. Obviously a short. but where? Any ideas on a fix? Could I run a new fuel pump circuit? Wiring is not my strong suit. Im open to all suggestions! Thanks! Ray
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Old 05-10-2018, 10:34 AM
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Re: Fuel pump wiring issues...Please help!

Just to clarify what you are working with:

- 95 Impala SS LT1

- 95 Impala SS LT1 engine harness

- 95 Impala SS LT1 PCM (?)

- Impala engine harness has been interfaced to the 93 Silverado body harness

- you bypassed the oil pressure switch in the fuel pump wiring


Did you use the PASS-Key II (VATS) system from the Impala - resistor key, theft detterent relay, and theft detterent relay? Or did you bypass the system?

Sounds like the issue is in the fuel pump wiring. Unless someone here happens to have done a similar swap, almost impossible to troubleshoot without the 93 Silverado wiring diagram(s).
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Old 05-10-2018, 01:36 PM
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Re: Fuel pump wiring issues...Please help!

Thanks for the reply! Yes it is a '95 LT1 engine, 95 LT1 harness, and a '95 LT1 PCM. The oil pressure switch is wired in and working. The vats was disabled. Basically they took the entire setup from an SS and dropped it in the Silverado. Its a weird problem for sure. If its blowing the Fuel pump fuse and you unplug the fuel pump and it still blows the fuse then its not the pump. If I unplug the 3 wire plug on the oil pressure switch and jump the two outside wires it kicks on the pump. If I remove the FP relay and put a 12v light on the wire going to the FP and turn the key on it powers the pump for 2 seconds as it should. When its not blowing the fuel pump fuse everything works perfectly. So basically the wire from the PCM to the relay works as it should, the relay does what it should but when it energizes the fuel pump and blows the fuse it must be shorted to ground somewhere. One question I do have is when the ECM sends voltage to the relay, does it go first to the relay or to the fuse block with the 15a FP fuse?
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Old 05-10-2018, 09:01 PM
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Re: Fuel pump wiring issues...Please help!

Here's the 95 LT1 fuel pump wiring (at least for a Camaro)

http://shbox.com/1/1995_pcm3.jpg

What did you do with the fuel pump prime connector? It shouldn’t have any 12 volt power input to the connector.... just hangs free, for use when testing the pump, bypassing relay and signal from PCM.

Sounds like somewhere in the body harness there's 12 volts being fed intermittently into the ground side of the circuit.

Just to clarify - I have no idea if the Impala SS LT1 engine side circuitry is the same as the Camaro, and I don't have a clue what the Silverado body side circuitry looks like. Just throwing out a few random thoughts to try and help out.

Last edited by Injuneer; 05-14-2018 at 07:08 AM.
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Old 05-13-2018, 01:50 PM
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Re: Fuel pump wiring issues...Please help!

I appreciate the ideas! The prime connector is just hanging out as it should be. I want everyone to know that I didn't do this swap. I don't know why it was done or what happened to the SS but I would assume it was totaled. The only reason I bought the truck is that its a totally rust free '93 chevy 2 wheel drive stepside with decent dark blue paint and even the interior is decent and rip free. I was looking for a first truck for my 16 year old son and thought that with a 2" drop it could be a sweet ride. After realizing that there has to be a short to ground, I looked over and under and found a wire that is in the oil pressure side of the fuel pump that had a scuff of the sheathing and a small section of bare wire exposed. It wasn't up against the block but it was close enough to where at speed its possible it would blow around enough to make contact. I covered it with liquid electrical tape and took it for a ride. Drove about 20 miles without issue. That doesn't mean it's fixed but I slept good last knight thinking it might be fixed. I will keep you posted. Thanks for all the help!
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