1991 GMC C1500 Parking Lights/Blinker Problem

NEWMANHARVICKNATION

MEMBER
1991 GMC C1500
4.3L
V6
RWD
MANUAL
101,000 MILES

For my 1991 GMC Sierra, I bought new headlight and parking light assemblies, that have bulbs already installed in them. The headlights work fine-no complaints. The parking lights, however, are acting funky, and have sparked some other problems.

The new parking lights I got are LED lamps, and have a plug-and-play adapter to the previous style bulbs. When I turn the parking lights on, both work, but the right interior blinker light on the instrument panel is on. When I turn my headlights/parking lights on, my left parking light doesn't come on. When I signal to turn left, both blinker and parking light work. When I signal to turn right, the parking light works, but not the blinker, and the signals are at double the speed. At random, one of the interior lights will blink when I signal either right or left. When I turn on my flashers, both work fine.

Blown fuse? Cheap lights?
I DID buy these lights on eBay, for $85 total.

Thanks, Chris

1 Reply

CARADIODOC

MECHANIC
33,871 POSTS

It sounds like you have two different problems, and both are related to the LED bulbs. First of all, whenever you have bulbs doing the wrong functions, it is usually due to a broken ground wire. The glaring and common clue is current can't get to ground so it looks for an alternate path, and that is through a different filament, which can be on the other side of the truck, and through the signal bulb. The signal bulb is sharing the 12 volts with other bulbs, so it will be much less than full brightness.

The second thing has to do with LED bulbs not drawing enough current. With the older thermal / mechanical flashers, too little current causes the flasher element to not get hot enough to flash so, depending on the flasher style, the signal either stays on or stays off, or it might just flash very slowly.

If you have the newer electronic flasher, too little current flow, (as in a burned out bulb), causes them to flash too fast. Since LED bulbs also draw too little current, the flasher acts the same way. The fix for that is to add a resistor to each bulb. I haven't looked into that yet, but a typical 1157 signal bulb has about 12 ohms of resistance. I would guess a 15 to 20 ohm resistor, coupled with an LED bulb, would draw the right amount of current to make the flasher work properly.

I would plug one of the original headlight assemblies back in and verify that side works properly. If there is still a problem, look for a wire in the truck's harness that was brittle and broke when you flexed it. If the lights work properly, the next suspect is your new light is wired incorrectly. That isn't something that can be easily examined or described over a computer.

You can also buy an electronic flasher made for LED bulbs. I bought a few of those, also on eBay, for less than four bucks, and they work fine. If everything else is wired correctly, plugging in one of those flashers is a lot easier than soldering in a bunch of resistors. Those are meant to replace the older thermal flashers.