1985 GMC S15 2.8 Engine won's shut off

[pullmyefinger]

I have replaced the ignition switch on this truck and the engine will not shut off.  I have to put it in gear and stall it, then turn the key off.  Everything does shut off in LOCK position.

My mechanic and I are at the point where he thinks something is supplying current to some part in the engine and that is why it keeps running.
He said start the truck, put it in Neutral, shut it off with the key and then go out and disconnect the inline fuse to the radiator fan.  No luck with that.

He mentioned the distributor, the starter, relays, etc.  He has taught me alot but I don't always understand what he means.

He said it was definitely not the key switch on the column.

What else do I look for, what else might be causing this...??  I don't know what most of you know so please don't assume that I am a mechanic in your replies.  I have learned quite a bit but it does take me time to understand some things.

I have found no burnt wires under the dash, I just am looking for ideas for what to check next.  I can use a voltmeter but am not an electrician.  Any Ideas/Help would be appreciated, this is past the point of a nuisance..

Thank you all...

[bd]

I have replaced the ignition switch on this truck and the engine will not shut off.  I have to put it in gear and stall it, then turn the key off.  Everything does shut off in LOCK position.

In order to assist you, tell us a little more about this ^^^^.

Standard transmission?  Tilt or standard steering column?

Did you replace the "ignition switch" on the topside of the steering column under the dash or the "key lock cylinder" on the right side of the steering column below the steering wheel?

Did the problem begin before or after replacing the ignition switch?

Have you tried "adjusting" the ignition switch?

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See if the symptoms go away when you unplug the two-wire connector from the rear side of the alternator.

[Rapid Roy]

Welcome

[VileZambonie]

did you try pulling your finger?

Gauges or idiot lights? What happens when you unplug the two wire connector at the alternator? Does it shut off?

[Irish_Alley]

did you try pulling your finger?

lol took me a minute to see the reason for this.

[pullmyefinger]

Fixed column steering wheel, standard shift. 

Problem started before installing a new switch topside, not the lock cylinder. 

Only problem now is that it doesn't seem like the switch because both switches give the same result (old and new).  I understand that the new switch could be bad but I am not looking at that right now.

From what I know the positions in the lock cylinder that correspond to the slider clicks are Accessory, Lock, Off/Unlocked, On and Start (spring loaded, returns to On/Run position.  These seem ok and the panel lights and all that crap Seem right, but...  They only other thing I noticed was when I shut the truck off the alternator gauge "dips" (moves toward 0v.) for a fraction of a second and then goes back to charging and the motor keeps running.  Lately though the alternator goes close to overcharging most of the time (a week or two before the truck quit shutting off).

I will try unplugging the Alternator 2-wire relay and get back to you.

[pullmyefinger]

Yes, the truck did shut off when I pulled the 2-wire lead from the ALT..

[pullmyefinger]

did you try pulling your finger?

Gauges or idiot lights? What happens when you unplug the two wire connector at the alternator? Does it shut off?

Truck does shut off if i pull that connector..  Voltage regulator in alternator bad?

[bd]

Replace the alternator.

[VileZambonie]

Or if you're cheap https://www.ebay.com/itm/CHEVY-DELCO-ALTERNATOR-HARNESS-ANTI-FEEDBACK-DIODE-10SI-12SI-15SI-27SI-wire/201247906131?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D40778%26meid%3D871418155a3d4d3fa6c085ebfcf2233e%26pid%3D100005%26rk%3D1%26rkt%3D6%26sd%3D191103912421

[bd]

The fact that there's an aftermarket patch harness available tells you how common the failure was....

For even less (~$1), install a 1N7007 or similar diode insulated with some heat shrink.  The only minor drawback introduced by the diode is it will prevent the red charge indicator from illuminating during a severe overvoltage event (runaway alternator) - a very uncommon type of failure.

[VileZambonie]

Or if you're even cheaper than that!... Get out, unplug it and just tell people there's a special procedure needed to cool down the hyperdrive motivator.

[bd]

Vile, you're forgetting that that works only if the positronic transducer is negatively engaged through the omichron mitigator.

[pullmyefinger]

Y'inz are still using an omichron mitigator?  Those have been out of style since forever..lol

Thanks to BD, Zambonie and the rest of the group for your replies, I have to go fix the warp drive on my go cart..

[bd]