Electrical Q... Engine shuts off when I turn on headlights.

[howie2336]

When my truck is running and I turn on the headlights or park lights, the engine shuts off. Also while it's running, the tail lights are on, even with the headlight switch off. I've been told that the alternator can cause this. I replaced the alternator 8,000 miles ago and I'm not running a sound system or anything. I haven't checked the alternator yet. Has anyone experienced this or has any ideas. Thanks!

 

[jcro61]

Have you hooked up a volt meter to the battery to see where your voltage is at while running and to see how much it drops when you turn the lights on? You might not be charging properly.
The part I have never encountered is the rear lights being on, I don't know what to say about that one.

 

[Sully]

ground issue?

 

[1990GMCSIERRA]

^ thats what im thinking

 

[howie2336]

I replaced the alternator and still have the same issues. I'm going to get out the volt meter later and check wires as well as grounds... The truck is also running/idling very rough like it has a lumpy cam in it. Hopefully it clears up when I replace the fuel filter. If it doesn't, then it cancels out the filter being the cause for poor idling and acceleration.... Are there any relays or fuses in relation to idling and the light switch? The truck is a 2wd 5-speed and has 89,000 miles on it.

 

[dirtridinz71]

If the ground on the passenger side fender comes undone it can cause some strange problems.

 

[howie2336]

So here's where I'm at;
I go to the back and first remove the "T" harness for my hitch and the ends were pretty corroded. I then notice the ground wires between the hitch and frame were secure but there's a little surface rust. Since it's a 2wd truck, I 'm going to pull the hitch off and ground it to a cleaner spot... I will check that passenger side ground as well. Thanks!

 

[TruckinAin4Sissies]

If you have a brake controller, check that too. That was my problem.

 

[SAATR]

When my truck is running and I turn on the headlights or park lights, the engine shuts off. Also while it's running, the tail lights are on, even with the headlight switch off. I've been told that the alternator can cause this. I replaced the alternator 8,000 miles ago and I'm not running a sound system or anything. I haven't checked the alternator yet. Has anyone experienced this or has any ideas. Thanks!

I'm assuming that when you say the tail lights are on, you mean the running lights and not the brake lights. If you're alternator is charging properly, then pull the headlight switch and disconnect it. Inspect the harness connector and the wires for any obvious melting/chafing or shorts. Make sure the connector isn't severely melted or cracked. See if your tail lights are still on when this is disconnected. If they aren't, then it is possible that the problem exists in the switch. You said that the tail lights are on when the truck is running. Are they also on when the key is on, and the engine is off? If they are, then you have a short to an ignition hot, very possibly to the wire supplying the ECM or ignition. This may be being grounded and making the ignition or ECM lose power. If it is a shorted wire, it may help to disconnect the battery and start checking continuity between the light's power wire and various ignition hot wires. Good luck!

 

[howie2336]

I pulled the switch and everything looked fine. I then start checking grounds and noticed that a ground wire located on the passenger side of the engine bay, that runs from the back of the engine. I'm gonna replace it with probably 8 or 12 gauge amp ground wire and hopefully this resolves the problem. I can't believe I've overlooked this for so long. Crazy because I recently replaced the plugs, wires and also installed headers and still didn't notice it.