Question for The Chevy Truck Experts

Jeff NWOH · Aug 19, 2016

Brother asked me if I would be interested in looking at one of his work trucks. 1994 Chevy 3500HD, 454 gas. Issue is that the truck wont shut off when the ignition is turned off. The truck will stop running if you step on the brake pedal, but I believe accessories, fuel pump, etc.. still stay on.

Their regular mechanic changed the ignition switch, some module on the trans, and cleaned up some grounds to no avail. He really didn't have any more time to mess with it, so its still broke.

Any ideas guys? Some relay somewhere that could be stuck on? Brother did pull the connector off alternator to see if that was back feeding somehow, but no luck. Also, I have a wiring manual for a 94 topkick. Any idea if it would be similar to the 3500HD?

Google wasn't much help. I'm thinking maybe someone here has seen something similar.

Thanks for any ideas, Jeff.

MarkB_MI · Aug 19, 2016

It's an electrical problem, not voodoo! I'm surprised the "regular mechanic" didn't try sacrificing a goat on the hood when his parts-swapping failed. Find an electrical diagram (your local library might have a Mitchell subscription) and dust off your multimeter.

gajack · Aug 19, 2016

Move the wiring harness connections around at the fire wall and the fuse panel after you have removed the key. Does it cut off? Electrical connections may need a little TLC cleaning.
Just my suggestion, good luck, j

rustred · Aug 19, 2016

are u sure your not talking about "run on" or "dieseling". thats when u turn off the key and it keeps chugging away. but i dont think that one has a carburator anyhow.

Steve@Advance · Aug 19, 2016

Can't explain what the brake pedal would have to do with it, but a good place to start would be the wire harness going to the starter. Possibly the resistor bypass connector is turned around and contacting the battery cable. Also that harness runs close to the manifold, could be melted, wires crossed.

Also try unplugging the ignition switch connector. Best I recall the switch position is adjustable, could be not fully breaking contact with the lock in off position, especially if the column is loose or has been damaged.

MARK ROBKE · Aug 19, 2016

Are you sure you have to step on the brake to make it shut down? And it's not -it won't shut down with your foot on the brake? 'S' series ih trucks, with hydraulic brakes, had a problem with a diode under the dash, that would fail, and the truck would not shut off if your foot was on the brake. It was back feeding thru the relay for the electric motor for the hydroboost brakes. You need a wiring schematic from the ignition switch to the accessories, and the ignition switch. You would need to test the circuit(s) for power when the switch was off in places where there should not be power. I am voting for a stuck accessory relay, or a failed diode in the circuit. Good luck,Mark

jimg.allentown · Aug 19, 2016

I think a good place to start would be to check the power relay to the PCM. Sounds like a stuck relay to me. Stepping on the brake may cause enough voltage drop to cause the relay to release. Just a guess. Even back in 1994, they probably had fuel injection rather then a carburetor. But... if it does have a carb, my guess is not going to work.

Bob · Aug 20, 2016

That's kinda bizarre, as a '94 would have a basic electrical shystem that is simply disconnected from the battery when the key switch is switched to "off".

When you go to '95 or never the 'puter gets free reign as to what shuts off and when, and there are numerous relays involved.

R U sure it's not newer than '94?

caterpillar guy · Aug 20, 2016

Put a tongue on it and pull it with the tractor. Only thing a chevy was good for was a wagon.

MarkB_MI · Aug 20, 2016

I just noticed where you say you have a wiring diagram for a Topkick. I doubt it will be of much use for troubleshooting the one-ton. The C3500 is a member of the GMT400 pickup truck platform family, not the GMT560 medium-duty truck.

K Effective · Aug 20, 2016

My 1989 Silverado had a strange condition one time- I had hired the step son of a coworker, fresh out of mechanics school, to drop the fuel tank and replace the pump and level sender. He crossed the lines putting it back together, and the fuel injection return line filled the charcoal canister with fuel. When running under load,it was barely noticeable, but at idle it ran rough. It was flooding out as the canister vents off to the intake manifold for the vacuum to draw in the fuel fumes, now it was getting raw gas.

I took it to a muffler shop as the exhaust sounded like it had a leak, and when they pulled it on the hoist, the engine kept running even with the key off and removed from the ignition. We watched it run for a few minutes as we tried this and that, even disconnected the wires to the injector- no change. almost ran well enough to move it that way. Once we got it smothered, we found fuel leaking from the canister. That led to the hoses into the canister, and back to the tank. Boroscope showed the crossed lines on top of the tank where he had cut them and spliced back together. Needless to say, he did not get the chance to fix it.

Steve@Advance · Aug 20, 2016

Never mind looking for the resistor connection...

Somehow I read 74 instead of 94! LOL

Jeff NWOH · Aug 20, 2016

I double checked. I was mistaken, it is a 95..

isie · Aug 20, 2016

Are you sure it is electrical, stepping on the brake pulls vacuum from the engine. I would be looking at vacuum first. It would just be a 3500 as the 3500hd didn't start till 2007.

Determined · Aug 20, 2016

Jeff the first thing I would check is any aftermarket items recently installed;
Brake controller
Stereo
Gauges
Command start
Work lights etc
Next I would start pulling fuses until it quits then start going through that circuit to narrow it down.

isie Gm first brought out the 3500HD in 1991, not very common, used mostly by town works departments, contractors, landscapers etc.
The brakes were hydro boost powered by the power steering pump not engine vacuum.

Electrically when the brake is depressed there is switching for;
depending on how vehicle is equipped
-Brake light activation
-Cancel function for cruise control
-Shift from park enable
-Torque converter unlock
-Trailer brake controller activation...

MN Scott · Aug 20, 2016

I was heading into town the other day and there was a wrecker hooking up to a dead new ford Caterpillar service truck about a mile from town. Talk about irony.

Jeff NWOH · Aug 20, 2016

3500HD as in a 15,000 GVW. I believe started around 1991 to 2002 or so. Then GM came out with the 4500, 5500, etc..

MarkB_MI · Aug 20, 2016

>Are you sure it is electrical

If you can explain to me how a fuel injected, gasoline engine can keep running with no power to either the injectors or ignition, I would concede the problem might not be electrical.

Bhunt · Aug 20, 2016

If you've already pulled your codes Turn off everything disconnect the battery and let it reset. When you reconnect it does it throw sparks with everything turned off?

jim buford · Aug 21, 2016

That is just what the inquirer needed.

jim buford · Aug 21, 2016

You might want to check out Youtube for scannerdannerpremium channel. His latest vid is on a veh that keeps running with the key off and the problem was not the ignition module. That is a paid channel but you can get free try for 14 days. Cancel anytime.

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