Engine won't turn off with instrument panel connected

[Waggymcgee]

Had some loose pins so soldered all of them on the back. Put it back in and the engine would not turn off until I pulled the ign lps fuse. I unhooked the harness and now she shuts off like normal. Could it be a ground issue? I have a 15 amp fuse in there but I occasionally see pics where there is a 3 in there. The 84/85 diagram shows 15. I did leave the red wire unhooked from heater controls since it was all corroded and I have a replacement panel coming in the mail. Thoughts?

[babywag]

Most common culprit is the alternator is backfeeding the ignition circuit.

Could be bad wiring somewhere, a PO modification, or even a bad alternator.
Easiest way to "fix" is usually adding a diode on the alternator wire.

Is this thing stock? Aftermarket ignition modules are also known to cause this.

Really hard to diagnose via internet.

[Beek]

Most common culprit is the alternator is backfeeding the ignition circuit.

Could be bad wiring somewhere, a PO modification, or even a bad alternator.
Easiest way to "fix" is usually adding a diode on the alternator wire.

Is this thing stock? Aftermarket ignition modules are also known to cause this.

Really hard to diagnose via internet.

2nd the diode suggestion. I cleaned up the wiring under the hood and had the same problem until I added a diode.

[nograin]

Just saw this.


GM 'SI' alternators internally redirect power to create the electromagnetic field. When the alternator is just starting or turning too slowly power for the field comes from the battery through the number 1 terminal on the connector.

Common practice was to power the alternator and ignition from the 'run' circuit.

On the '85 wiring diagram notice the wire to the alternator's #1 terminal is actually a resistance wire. Electricity always takes the path of least resistance to ground. When the key switch is turned off, the 15 Ohms of resistance insures internally redirected current continues to flow through the rotor. Without that resistance it 'backfeeds' to the coil and ignition module.

[Waggymcgee]

Will a diode on alternator wire fix that, per the couple of suggestions? I planned to do that next week.

[nograin]

In theory yes.
In reality. Check how much voltage can it resist, how much current it can flow, and whether it can operate and survive at engine compartment temperatures.



I'd first find out what happened to the 15 ohm resistance wire. It's very long and folded back at least one in the harness.

The 1.35 ohm resistance wire spliced to it is critical for the coil to survive.


This one of the splice junctions. I can't imagine why you'ld have to trace it this far back but if you do...
IMG_3329 by Mathew G's photo page, on Flickr


IMG_3330 by Mathew G's photo page, on Flickr


Difficult to solder. I'd use an open barrel crimp splice.


Again I suspect something else is going on and would want to be sure of identifying the problem. Then the right solution can be applied.

[Waggymcgee]

Thanks again for the info and detail nograin. Us no grains gotta stick together

[tgreese]

...
Difficult to solder. I'd use an open barrel crimp splice.
Again I suspect something else is going on and would want to be sure of identifying the problem. Then the right solution can be applied.

What Ive seen, the resistance wire is either carbon-filled fiber (Ford) or single-strand nichrome (Jeep). Either will be impossible to solder.

My opinion - there's nothing wrong with replacing the resistance wire with a diode. I can link to suitable diodes you can buy online, if you want. I have had similar problems before, used generic electronics-type diodes; they work well and will last indefinitely.

You still fighting this Kerry? Do you have the wiring diagram for the '85 GW?

[Waggymcgee]

Thank you!

Thanks for all the replies and information. Given the long list and weird electrical issues that were stacking up and a short amount of time to do the work (and nervousness), I took it to an electrical auto place. I put this on the list and provided them with this thread and a couple of others. They were very receptive which I was glad because I didn’t know if it would ruffle feathers. I expect it will save time and money. You guys are the s**t!

[Waggymcgee]

Hey guys. As a follow up to this thread. The resistance wire checked out. They (the electrical shop) found that the condenser, pictured in the link, had exposed wire that was touching the air cleaner housing. Since being replaced, it has not had a problem with turning off. Sorry if I wasted your time on something you guys would probably have noticed in 2 minutes, but I thought you might like to know the cause. The instrument panel connection I believe was coincidental as it never happened when it wasn’t connected. Who knows, maybe it’s related some how. The PO had it wired all kinds of wrong which is baffling to me. If you don’t know what you’re doing, pay someone else to do it, like I do!

https://photos.app.goo.gl/eouaCh8fQAkWJSVJA