97 GMC Truck Shutting Off Randomly

**Custmx1** · 08-04-2009, 02:11 PM

Well, I've got a 97 GMC Truck and I love it. It hasn't given me any real problems until recently.

About a month ago when I would shut it off there would be something still running, it sounded like a faint buzzing that would come from between the cab and the bed of my truck. Don't ask how but I figured out that if I turned the ignition to almost start the engine it would usually shut the buzzing off after 5-10 seconds. Not sure if it's relevant or not.

But now over the past week my truck has shut off two times while I was driving it, the radio was still on but all the gages dropped to zero and had no power. Not sure what is going on wanted to know what I might be getting myself into as far as repair. Thanks!!

**tattooed4life** · 08-04-2009, 03:22 PM

The buzzing sounds like the fuel pump is still running to me. As for the truck randomly shutting off, is or has your security light been coming on?

**Custmx1** · 08-04-2009, 05:25 PM

I haven't noticed it. However when the truck shuts off a majority of the dash lights come on, air bag, check gage and battery... The two times it has shut off it has been early in the drive, like within the first 1/4 mile. I'm totally at a loss....

**2COR517** · 08-04-2009, 10:33 PM

Sounds like an ignition switch or relay to me.

**vncj96** · 08-04-2009, 11:09 PM

i say fuel pump

**bill190** · 08-05-2009, 10:54 AM

Problems like this are a pain to troubleshoot because they are intermittent.

You can't go tracking down the problem because it goes away!

But there is a method/system to finding the problem. Just might take awhile, but you will eventually find it.

That is to set various test devices up so you can "see" what is going on with various systems.

Now one of the things this could be is the "cam sensor" or its associated wiring. Or a ground or +12 volts provided to that sensor. As the distributor cam rotates, this gizmo sends out pulses of electricity. This tells the engine computer the engine is running. And it is the basis of sparking. Without this, no spark!

And this signal would normally be checked with an oscilloscope. Perhaps you could run some wires to the engine compartment (through an open window from the cab) and monitor this signal with a voltmeter set on AC volts?

Anyway you would need a GM Factory Service Manual set so you could look at the wiring diagrams and see where to tap in your wires to monitor this. Order this from helminc.com or a GM dealer.

Then get someone to drive around with you and keep a constant watch on the multimeter as you drive. See if this signal stops just as the engine dies.

Fuel pressure: Or you could hook up a T to the fuel line and run a fuel pressure gauge through an open window. Keep an eye on that while driving. See if you lose fuel pressure just before it dies.

Or electrical problems: Connect test wires to key points. See if you are losing electrical power to the electronic spark module. See if you are losing electrical power to the engine computer.

For "nasty" intermittent electrical problems where the power may blip off for just a moment, you can get a "recording multimeter" which records the voltage. You can play this back and see if there was a momentary loss of power.

Also just to check if you are losing spark in general, I suppose you could get one of those inductive timing lights and put the inductive pickup on a spark plug wire near the passenger firewall. Then if wires long enough, run it through the window. Then drive around with your passenger constantly watching the flashing light to see if it stops flashing before the engine dies. (And that you have lost spark).

Alligator clip test leads like these can be helpful for setting up test wires...