Engine still runs with key turned off

[ChaseBBC]

So I finally have my truck back together and just installed a painless harness. Everything seams to be working fine except when I turn the key to the off position the engine doesn't kill. Been reading a few articles and some say that the alternator could be back feeding the ignition switch. Does that sound right or has anyone else had or even heard of this problem? I installed a later model alt with the internal regulator to avoid more wiring and a cleaner look under the hood. I don't know if that matters. Thanks in advance.

[LT102]

Does the truck not shut off at all or does it shut off after a few seconds?

[Steeveedee]

Make sure that the yellow wire going to the "R" terminal on the starter isn't touching the hot battery cable on the solenoid.

[garyd1961]

How is your alternator wired? There are several threads on here that show the proper way to switch to an internal regulator alternator.

[71cheyennesuperlongb]

Have you installed a electrical fan on your truck? Sometimes they will backfeed till the fan slowes down and stop turning.

[Barnfind46]

I would look in to how the alternator is wired also. I wade this mistake on a rewire and had the same result.

[bcfishon]

X3 on the alternator being wired backwards.

[SoCoC10]

You might also check the wiring on the back of your ignition switch. We had a Blazer with lots of stereo equipment and they had the purple wire pushed in to the wrong place and it did the same thing.

[panhead59]

I doubt this is your problem but replacing the points with a pertronix modual will sometimes cause it not to turn off. A specific diode solves that issue. Just some useful info.

[ChaseBBC]

Truck does not shut off at all (not dieseling). I don't have anything hooked up except for ignition stuff to run the motor. no radio, fans, lights, etc. I doubt the wiring to the alt is backwards cause the harness is clearly labeled but I am def going to double check. I will research the internal alternator swap thread and see if that sheds some light on my situation. I will get some pics this evening and post. thanks again. love the input from this forum!

[bw46]

This is a common problem. Put a dummy light inline (any little interior bulb would work for test) on the wire to the alternator terminal #1, right near the alternator. The bulb is just acting like a little resistor. This test will point you to the problem. If it works, there are countless websites showing how to properly diagnose and wire the alternator correctly.

[ChaseBBC]

Sounds pretty common. I'll get after it this evening and see if I can figure it out. but I might have to take it around the block first for its maiden voyage

[Barnfind46]

Round the block is a great idea. Might just have to do the same with mine!

[ChaseBBC]

After some trial and error I installed a external regulator from my parts pile and it solved the problem. And took it around the block for the first time! hope the pics come out right been a while since I posted anything.

[VetteVet]

Have you thought about just unplugging the alternator to see if it'd shut off. If it doesn't then maybe the key switch is the problem.

[ChaseBBC]

Yea I tested that too Vet. Unpluged the alt after I turn the key to off position and it dies.

[VetteVet]

Originally Posted by ChaseBBC

Yea I tested that too Vet. Unpluged the alt after I turn the key to off position and it dies.

Well then you have isolated the problem to the alternator, which means one of the diodes in the diode trio is shorted and allowing feedback current to return to the ignition switch through the no.1 wire. You might be able to just install a diode in the no.1 wire to prevent this feedback but if you have a charging light it will no longer work, because it relies on some current from the alternator to go out when the alternator starts charging.

It also provides enough resistance to prevent the backfeed to the ignition switch which will cause the engine run on. If you don't have a charging light as with the gauge cluster, you have, or should have a resistance wire running from the key switch to the inside of the cab at the firewall block. It connects to the no.1 alternator wire which excites the alternator.

Bottom line: install a small diode in the no.1 wire which should cure the problem, but be sure it is installed so that the current will flow to the alternator. This symbol I<- is on the diode and the arrow must point toward the alternator. A small 1 watt diode should be fine.

[VetteVet]

By the way that regulator you installed is for a generator so I don't know how it worked.
How were you able to wire it to the SI alternator?