Electrical Troubleshooting

Hi Everyone,
Ever since I bought my truck (a year and a half ago) the Passenger Rear brake light and tail light don't work. Driver side lights work perfectly. The truck came equipped with normal, incandescent bulbs (not LEDs). Turn signals work perfectly on both sides. Reverse Lights work both sides. Hazards work on both sides.

I've put a new bulb in. No change. Passenger side brake/taillight still doesn't light up.

I even put in a confirmed GOOD bulb, no change.

I probed the taillight bulb socket with my multimeter, and I have 12volt difference across 2 pins with the lights on. and a 12volt difference across another 2 pins when I hit the brakes. They share a ground.

I'm a bit confused, how can I have voltage, but the bulb doesn't light up?

Thanks
Kyle

 

Have you used a test light to see if you have the ability to carry current?
I don’t like using voltage meters on lighting diagnostics.

Green wire is Tail
Blue wire is Stop
White/Black is ground

 

You need to check voltage with the bulb installed so the circuit is loaded or use a test light, with no load on the circuit you can have voltage but it may not be able to carry a load.

Blue wire is Brake
Green wire is Tail
White/Black is Ground

 

You beat me to it

 

Actually I just thought of an easy test to try to see if its a problem in the tail light or the truck, take the drivers side light assembly out, plug it into the passenger side plug and see if it works. If it does you have a problem in the passenger side light assembly, if it doesn't you have a wiring issue on the truck.

 

Yeah, I know the truck go rear-ended at some point in its life. It came up on the carfax when I bought it. The bed sides are a little wavy, like they've been pulled out. That's sort of my fear, is that it got hit and no one has bothered to figure it out since.

But I'll try swapping harnesses left/right and see if the problem "jumps."

I also have a cheapo test light. So I can test the bulb socket with that too.

 

What do the license plates lights do? They're on the same circuit as the taillights.

If I recall correctly, the turn and tail are combined inside a single lamp, maybe an 1157, on that truck. If not, ignore this.

If the license lights work, the problem is most likely a bad ground, a bad connection to the shell of the bulb. That would kill both turn and tail.

Look around under the back of the truck to see if someone has been into the wiring to hook up a trailer. They might have disconnected something.

The fuses are good. Tail and turn each have their own fuse. If one side lights up as you described, both fuses are good.

 

License plate lights work.
And yes, my taillight and brake light use the same bulb. Think it's a 3157 though. Its a dual-filament bulb.

 

Was gonna suggest the swapping as well

 

I’m not super familiar with the incandescent assemblies but maybe it’s an issue between the connector and the bulb. Try swapping the drivers side tail light assembly with passenger if they connector is the same and see if the issue follows the tail light.

 

Crap looks like I never refreshed the page and am late to the suggestions game.

 

I forgot to say beware of the phantom ground, particularly since your truck has been rear-ended in the past. Sometimes, you have an intermittent ground that works sometimes and doesn't work sometimes. Vibration of driving the truck makes and breaks the connection.

I had the same problem with a VW Bug at Fort Bragg. It had been rear-ended and repaired. It passed state inspection, then later an MP wrote it a ticket for bad taillight. I took it home, cleaned the connections, taillight worked. Took it to the MP station, taillight didn't work. Took it home, dug some more, found body shop forgot to reconnect a ground wire to the frame further down the harness. That wire grounded the taillights. Without it connected, sometimes enough current would flow through the rest of the car to light the taillight, sometimes not.

Years ago, I used to rent a U-Haul trailer every weekend for business. They never had the same trailer twice. I got tired of being stopped for bad trailer license plate lights, so I started fixing the trailer every weekend. By bad design, the license plate lampholder was grounded to the aluminum trailer by a steel to aluminum bolted connection. The connection didn't keep water and air out, so the steel rusted and the aluminum tarnished quickly. Aluminum tarnish is an insulator. To fix it, I would remove the license plate lamp holder, sand both sides of the connection shiny, then put it back. That would work for the weekend. There was no low-cost way to permanently fix it or I would have.

So what I'm trying to say is if it magically starts working, keep looking until you're sure you fixed it or it may magically stop working again.

 

Look down in the socket and see if the contacts are fried

 

Well boys, I think I got it pretty well figured out now. I did some poking around tonight and made some interesting discoveries. First off, the brake light on the passenger side started working. Completely randomly. But still no taillight on the Pass.rear corner.

I tested the bulb socket with a test light; brake light worked. But the tail light pin didn't supply enough current to illuminate the test light. Checked my multi-meter and it showed 11.5volts, but clearly not enough current to actually power the bulb.

So I swapped taillight harnesses L/R and the issue was still present. I even checked for continuity from the bulb socket -> harness connector. That all checked out, so I dug a little further:

Clearly someone was in here before me. All the black wires going into the harness connector. I cut out the crimp connector for the taillight and added the alligator clamp, and she worked!

I added a new crimp and some fresh electrical tape and called it a night. I'll make a more permanent repair during the week. The brake light on that corner works intermittently, depending on how the wires is positioned. So I'm definitely going to re-do this to my standards.

Thanks for all your help!

 

Good work, glad you figured out. I had something similar happen and it was due to aftermarket trailer brakes shorting out.

 

Everyone has loosely tried to do a voltage drop test. Anyone reading this in the future can save a lot of headache by leaving everything in circuit, throw your probes before and after the light, and see what voltage you get. Above 1-1.5v, you have a bad connection.

 

Just as a final write-up here, I took the time and re-did the crimps on the harness. They actually make step-down crimps, for crimping a "small" wire to a "medium" sized wire; which came in handy for my application.

Then added some heat-shrink on top of each crimp, as compared to the electrical tape that WAS there previously.

Then topped it all off with some F4 tape, and a layer of Tessa tape for some abrasion resistance. And everything works 100% now! And it's gotta be a better repair than the first guy that did this. I always try to put stuff back together better than I found it.