Head scratcher Electrical issue

'98 Prerunner V6

Here's a completely crazy one for you all...

When the running lights or headlights are turned on, the following occurs:
1. The Headlights/running lights turn on. (not the problem)
2. The Shift lock relay starts to flutter onand off randomly
3. The brake lights flutter on and off randomly in time with the shift lock relay
4. The engine starts to run a little rough when on the freeway at cruising speed. It kinda feels like its missing.

All these problems go away when
1. I turn off the headlights/running lights
OR
2. I PULL THE TAIL FUSE?!?

Weird.

 

Well, that really is the weirdest thing I've heard of. Definitely sub'd, I'd like to know what ends up being he issue here.

 

Sounds like you have something wrong on the tail light circuit. Have you pulled your tail lights and checked the wiring?

 

x2 on the tail light circuit. Also, I have heard of old trailer wiring causing problems like this. If it has it, disconnect and see if it corrects the issue.

 

Get the alternator tested. It's easy and free to get a test done at almost any auto parts store. I know it doesn't fit the tail light part of this, but that could be an red herring symptom? When headlights cause odd electrical system issues it could be the alternator is failing and can't deliver the power needed to support lights (they take a lot of power). Anyway, it's like a "brownout" and clears up after the power hog lights are turned off.

 

I was having some electrical issues on my 92 involving the brake and turn lights and tach. Turned out am old trailer wiring harness was the culprit. Pulled all the old wire and put in a new harness and all was good.

 

WOW you guys are AWESOME! Thank you all for such quick responses!

For some reason, someone installed TWO trailer light boxes/harnesses I think maybe since this is so strange, I'll start by yanking those out...

Good call on the alternator, I think I'll stick a voltmeter on it under load to see if I get a voltage drop

I'll report back findings.

-Jeremy

 

OK so I removed both tow harnesses but the problem persisted. All the wiring on that side looked ok. For giggles I decided to pull the bulb just to see what would happen... I didn't expect much because I just replaced all the bulbs a few days ago. Low and behold the problem went away. This bulb is the type with two filiments. What is happening is there is a very slow voltage leak between the filiments. Power from the tail light filiment was energizing the brake side. Probably the jumper was heating up and curling away until it cooled enough to curl back and make contact again. (Or something)

Kinda anticlimactic, I know.

 

add some extensive grounding to your battery. worst case scenario, you wasted your time. best case, you have extremely good ground!!!! some performance cars do this just for that reason. better performance.

 

From the battery to the chassis?

 

yes, you might find a good how-to here on TW.

here's a pic for a scion xA

 

Holy cap I never saw so many grounds! How does this help?

 

im not 100% on this. I guess it's just making sure something as small and dumb as bad grounding isn't an issue. But then again, copper has less resistance than iron. so if you're grounding something to the chassis in the rear, it's going to use the copper ground as a shortcut rather than the entire chassis. its a very very slight performance improvement for any electronics.

 

It wouldn't, but I bet he's highly impressed with his work

If you're running a bunch of stereo equipment all the grounds make a little bit (not a lot) of sense but definitely not needed for a regular vehicle.