Hey guys,
Looking for insight and help into an electrical issue I'm having.

I have a 1992 SR5 Pickup, 4x4 manual with the 3.0L. Might add it's a Canadian truck which seems to have an extra relay: dimmer relay in the engine bay.

Been having an issue with my brake lights. Basically when the truck is off all the lights are off as they should be. If you press down the brake pedal the brake lights come on.
As soon as I turn the truck on and it's running the brake lights are on full power all the time. Pressing the brake pedal does nothing, not even change the brightness of the bulb.

Truck does not have a trailer brake harness in it.

I have disconnected the switch on the arm of the brake pedal under the dash and the brake lights stayed on full blast.

The bulb socket in the taillight housing has two terminals. I'm guessing one is for night time running lights and the other is for brake light operation which makes it brighter? And then the barrel of the socket is ground / negative? Correct me if I'm wrong here.



When checking for continuity I get continuity between the White / Black ( ground?? ) and Solid green with red spots. Is this correct or wrong?



Based off this I'm leaning towards a ground fault? Or could it be a faulty relay?

I should also add my headlights work but I have no high beams unless I hold flash to pass. Not sure if this is related but I just want to throw it in incase...

Looking for any help or advice before I bite the bullet and take the truck into a shop. Hour by hour diagnosis isn't cheap

Thanks!!

 

ding ding ding

Add in a possible faulty combo switch, faulty alarm system (if installed), or faulty integration DRL circuit (driver's kick panel, behind the fuse box). These older trucks use good quality circuits and contacts, but a lot is not shielded from moisture. A little bit of water gets in and corrodes the traces, making or breaking circuits. Start with the easiest/free-est areas and go from there.







Colors might be different, this is from a '93. White with black stripe is always ground.

 

Dude thank you so much! This will be a huge help!!

 

No problem. I have all the writing diagrams for my 93, so if you need another area, let me know. Canada circuits are a hair different, but still have all the similar components.

 

This last measurement is (probably) flawed. Look at the diagram, the bulbs are wired in parallel, what you are seeing here is the resistance of the front (other) bulbs.

What you are looking for is a short between the marker light and brake light. In your picture is the rear body harness, did you test the other side of this for shorts?

 

So after spending Saturday chasing circuits around I was turning up blanks. Everything seemed to be functioning properly and I wasn't turning up any shorts.
Later that night I was on 4crawler reading info about head light conversions and he mentioned that LED's don't work with the switched ground system in Toyotas. Well low and behold the next morning I replaced the LED bulbs with standard dual filament bulbs and the brakes lights worked perfectly.

So somehow the LED was causing power back feed into the circuit or some sort of black magic ˟˟˟˟ery...

Just happy the lights are working again!