The Stovebolt Site Forums The Shop Area The Electrical Bay Won’t shut off᠁᠁sometimes

This has me baffled, totally and completely but I’m probably not that smart either!!!

57 GMC 1/2T with a 261 and a push button start

After starting and if it only runs for a very short time the engine can be shut off by turning the key to the off position, if it runs for any amount of time it won’t turn off and I have to choke it to kill it


Any ideas are most welcome

Thank you so much!!!

Maybe the generator or alternator is wired to the wrong side of the ignition
And once it’s running it provides power to the ignition even with key off / battery disconnected

Disconnect your generator / alternator and see if the problem persists
-s

Yep sounds like the ignition coil is still getting power when key is off.
I would start with the minimum wiring to start and run the engine. Like remove hot from battery and run a jumper to coil and jumper to starter.
Get it running then pull the coil wire and see if she stops. then just start hooking stuff up until you find where its getting power from generator/alternator and not battery.
also you can start it up warm it up unhook the battery and then turn off the key as well. That eliminates the battery side and you will know its on generator side.
Had this problem once when putting some aftermarket stuff on the 56 had a hot wire on wrong side of switch and once it was running never shut off.

The running only after "warm" does have me scratching my head.
Now I know on some alternators the field does not kick on till a few seconds after starting. Generator maybe never had one.
OR you got something getting hot and expanding enough to touch something else and making a circuit on the coil side of things bypassing the key.
Either way I would start pulling some fuses and minimizing what needs to be HOT to run and test this out.

In the back of my feeble mind I seem to remember this symptom occurring on alternator-equipped cars and being solved by putting a diode in series with the idiot light. Can't remember the details though.

The "sometimes" of your problem may have something to do with whether or not your alternator is providing a charge when you shut off the key.

It just that the idle is too high. Turn the screw that hits against the stop on carb.

That`ll do it.

I have traced all my wiring and am still not seeing anything incorrect!!

My idle is set at about 650rpm, got the out of province inspection done today and drove the truck about 50 miles, first time I’ve ever really driven it and magically the key started working to turn the truck off

Is the ignition switch a new replacement or is it the original one?

I've seen this happen with ignition switches. They either wear out to the point that they no longer lose the completed circuit once they're turned off or somebody adds HEI to their engine and doesn't upgrade the switch or (best answer) put the HEI on a high amp relay. That will kill these old switches tout de suite as some of them were only built for 20 amps...maybe less on the older ones. Good luck.

Ignition switch is original, so is distributor

Fired up today and back to square one, won’t shut off with the key!!

That sounds like the switch isn't turning off occasionally. Not at all uncommon. If you ever disassembled one, you'd see why. Have a friend, wife, somebody start the truck and then turn it off. When it continues to run, disconnect the wire leading to the coil...either one of them. There'll be + coming from the ignition switch and - heading off to the distributor and when either of those is disconnected, the engine will stop running. Personally I'd disconnect the + wire. If the engine dies when you do that, then disconnect your battery and check your ignition switch with a handheld tester (one with a broken circuit checker). Test between the center wire (the hot wire) and the wire going off to the coil. You'll see it turn on (or hear the little buzzer in the tester). That shows a completed circuit. When you turn the key off, you will still hear the little buzzer...maybe. Let us know.

When the engine continues to run with the key in the OFF position and I try to use anything that consumes power, headlights, horn, heater, it kills the engine instantly

questions:

1. Original type generator or does it have an alternator?

2. Did the problem happen immediately after some work was done to the truck or did it just start doing this without provocation?

3. US 1957 GMCs have an idiot light (not an ammeter). Assuming yours does:
.........3a. When you first turn on the key to the run position does the light come on?
.........3b. When you start the engine and rev it a little does the light go out?
.........3c. When you turn off the key and the engine continues to run is the light on?

This is why some of the guys are thinking bad switch or you got a small touch point going on somewhere.
Its providing just enough power to the coils with barely enough amps. Soon as you put power on something else the draw is enough to pull the load off that cross going on somewhere.
Since its most likely not a true hard contact or full wire it does not have the capacity to provide enough when the switch is off and your pulling load somewhere else.

Original generator, idiot light works like it should

I had to wire everything behind the dash as almost every wire had two or 3 splices that were loosely twisted and lightly tapped as well as at least 6 wires that were fried, easier and safer to just start over.

I’m starting to lean towards the switch as well, the outer trim is a bit torn up like someone tried removing it without knowing how, might have to splurge for a new one

Thanks for the update. Having a generator leaves out my idea of what the problem is.