What could be wrong with my 1992 suburban 454?

Question

We just had a new motor and transmission put in the truck a few months back. It has been doing great since then, until the other day. We were driving it, it backfired, and then the motor died. It wouldn't restart. Fuel pump was no longer getting fuel. Found that fuel relay was getting no power yet fuse to relay was getting power.Got a new relay and installed. Truck started up and then died again. now no power to relay, fuse, or coming into fuse. Fuse, relay, and battery tested, all still good, just no power going into the fuse that goes into the relay and then to the fuel pump. We can hard wire into the battery and truck will run and can be drove all day with no problems. I think the next step would be the computer, but, not sure that that would be the problem since it works with being wired directly into the battery. If I need, is there a way to test the computer? So, what do the machanics on board think it could be?

Thanks to anyone who even considers answering!

Checked all wires and replaced the oil presure switch too. It still isn't getting power to fuel relay or fuse to fuel relay. I can still hot wire it and get it to run. Anyone know what else I can check or how to check the computer? Or where the fuse is at the computer?

Answer 1

Eautorepair.net has online access to factory service manuals and wiring diagrams for a fee--also Wirediagram.com....Helminc.com has factory book manuals you can order.
As others have stated, it sounds like a bad power wire problem and may be best to bypass it if nothing else is bad.
Some models have a low oil pressure shutoff feature that kills power to the fuel pump in the event you lose oil pressure---would be a bad switch or wiring---may be something like that on yours--not sure. Computer may not control the fuel pump for that year truck either. Check a manual to follow the circuits and diagnosing.

Answer 2

There will be a safety circuit either in the form of a tachometric relay or inside the ECU. It is this that is at fault.

A tachometrically controlled fuel pump needs an input from the coil negative (sometimes the alternator on some vehicles) so that the ECU knows that the engine is running before supplying a constant voltage to the pump.

This is necessary because in a collision where the engine has stopped and the driver is incapacitated, high pressure fuel could be flowing all over from ruptured fuel lines at the rate of 2 litres per second. Having tachometric control means that when the engine stops the high pressure fuel pump stops.

On initial start up this circuit is by-passed and the engine will run for between 5 and 10 secs and then shut down if it doesn't see an engine running signal or the circuit is faulty, as in your case.

Autotech Australia.

Answer 3

You have an earth problem or a broken wire somwhere along the line..You will need to get a workshop manual with the wiring diagrams in the back or if ya good follow the wiring back.It wont be the computer because your car wouldnt run otherwise.

Answer 4

You probably have an open or short wire before the fuse, since these can be hard to locate your best bet may be to run a new wire to the fuse and see if that solves the problem.

Answer 5

Is your check engine light lit?

If so have it scanned for codes

Buy a Manual with a wiring diagram, it seems you have enough experience to know how to trace the power back to the relays and pump they probably have a procedure listed to check for certain things in your predicament

Answer 6

I do not know much about cars, but perhaps the electrical lines are corroded or damaged some how. Have the electrical cords checked. I doubt the computer is messing up, but have them check it as a last resort.