Seeking 88-98 "Hot Fuel Module/ fuel pump driver" answers.

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dwragon

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Hi all, OK, so the frame cracked at the drivers t on my 94 K3500. I had on standby a 92 C3500 that I knew had fuel issues. I replaced the one inch tube above the fuel pump, filter, sock and cleaned the tank. I drove the truck for over a week with no problems on the farm, then headed to town 45 miles away. It acted like the cat was clogging up, so I stopped at a friends and removed it as it was behind the muffler, and welded a temporary pipe splice inline.
After that I went to start the truck and it would not start. I installed a new fuel pump assembly with strainer sock. The truck started up without any problems, then died after 30 to 60 seconds of running, when idling down to 1000 RPM. I drove it to the local parts store and replaced just about everything electronic on the engine (Ignition module, fuel relay, IAC, MAP, so on that could cause an engine shut off, with no change.
The Chilton's wiring diagram for 88-98 C/K series shows a "Hot Fuel Module", (also called a fuel pump driver) or control module. I found mine behind the glove box. When I disconnected this module, the truck did not run. The fuel pump control modules regulate the voltage supply to the electric fuel pump. These modules receive an electronic signal from the engine control module that determines the voltage applied to the pump, altering pump motor speed and pressure. This is about the only thing I have not replaced, and while it seems to operate properly, it may be shutting off the fuel after the 30 to 60 seconds if it is defective.

If anyone has had the same problem I would appreciate knowing how you cured it, or if/how it can be bypassed.
 
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dwragon

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Well, I looked at it, at the moment there is no damage to my module, in fact I think it is what is keeping my truck starting. In searching, I found that on another site someone stated,

One problem was located in the fuse box, the tan wire jumped over to another pin splitting into two one going to the fuel module another going to the fuel pump, it had a bad connection on the crossover failing to provide power, fix was to remove and repair the wire.

My truck does have the vapor lock prone big block, so I now see the need for this module to do what it is supposed to do, purge the vapor lock by extra pressure, so I wont be bypassing it.

From another site, (The hot fuel module was added to trucks that were equipped with the big block or heavy towing package, as under hood temps may be enough to cause vapor lock, hence the need for extended fuel pump run time.
This was GM's way of work around. GM could have programed the computer but it would have meant stocking a larger selection of computers for v30's and big blocks to keep the fuel pump on longer to prevent vapor lock. This was the cheaper solution, since the ecm didn't care whether the engine was in a v30 or not.)

Just need to figure out why the regular system is not working after a new pump assembly install, and looking back at all the wiring snicks, this is not a new issue with the truck, so I am going to look for bad connections in the fuse box and wiring harness.
 

dwragon

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This is the simplified wiring diagram everyone should have, it doesnt show dual fuel tanks, but most dont need it, and that diagram is out on the net.
 

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dwragon

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Well, I guess I moved too fast for anyone to comment, but my problem was, after careful study of the wiring diagrams and the use of a test light, cured by bypassing the grey wire (From near the connector oil pump sending unit straight to the connector of the fuel pump.) bypassing an open in the circuit.
 

FourEightZero

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Put a new oil psi sender/switch on. The connecter is the original 30 year old one and all 3 wires have sections that are exposed (insulation worn away).

Just wondering if it's time to buy the new connecter and splice it in (get rid of the exposed wiring).

Having trouble keeping the truck running.
How about your Oil Pressure Switch? Is it working? It should bypass the Fuel Pump Relay completely for redundancy.
 
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