94 pontiac transport - direct shorting of blinker / brake light circuit - fuses blowing

Question

Symptom - No rear brake lights. Left turn signals not working but blinker relay "Sounds" like only one way contact movement. However - Right turn signal lights work fine and blinker relay "sounds" normal.

Diagnosis so far: Purchased new turn signal relay and fuses.

I pulled the left rear running Lt/Turn indicator apart and tested the harness and the bulbs on an independent 12 volt source. All tested fine.

With new equipment installed and through repeated testing...

There are two different fuse locations. Turn indicator and Brake/Hazards

1. Pushing on brake pedal causes instant blowing of Brake/Hazards fuse

2. Key on, tuning on left turn signal blows turn signal fuse instantly

3. Turning on emergency flashers causes instant blowing of Brake/Hazard fuse

4. I noticed a transistor (3 Prong clip) in same harness and on same mount block as the signal relay so thinking this might be causing a direct short I disconnect the clip and run all the same tests with the same outcomes. (No solution or benefit)

I don't know if this transistor is part of the lighting circuit or not but disconnecting it did not cause any solution to the problem.

So now I still have the right blinker but no left blinker, no emergency flashers and no brake lights. If I put in fuses and press the left turn signal before I press the brake pedal, then not even the right turn signals work because the turn indicator fuse blows before the brake fuse blows and so there are no lights at all ( brake or turn indicator)

All other lights in the vehicle seem to be working fine.

All other electrical circuits in the vehicle seem to be working fine.

Do you have any ideas or knowledge about my dilemma?

Answer 1

Blowing a fuse is a pretty good indication of a short circuit. If you have a DMM (digital multi-meter) try some basic tests:

• With the fuse out, read between the prongs of the fuse socket and ground (a nearby screw or other bare metal) with the meter on a DC Volts scale (20 VDC is common) to identify the hot side of the fuse. Once you know which side is hot,

• Switch to reading resistance (Ohms, the Ω symbol) and measure between the "cold" side of the fuse and ground. I would expect a reading of close to 0 (maybe a few tenths of a ohm) since you seem to have a short. With the bulb in, a normal reading would be about 28 ohms (assuming a 5 Watt bulb).

• If the reading is close 0 Ohms, try pulling out the bulb and looking closely at the socket. Corrosion in the socket might be the culprit. Check again with the bulb out. The reading should be infinite (no path for current to ground without the bulb). DMMs don't usually have away to say ∞ so they do it by a cryptic reading like 'OL' or an obviously "wrong" display.

• If the reading is not infinite you have a short (or another load connected to the circuit). You can use Ohm's law (I = V/R) to calculate the expected current, if it is more than the fuse rating. That's your problem.

Now you just have to figure out what is wrong. A few suggestions:

1. Focus on the left hand tail light fixture since you already have one problem there. Check to see if there is any damage to the wiring near the fixture.
2. If you have a connector for trailer lights, check it for corrosion and/or damage.
3. Think about recent work or "events" that might have damaged wiring.
4. Start tracing the wiring to (or from) the left tail lights.