Both tail lamps and break lights are out

Tiny
THOMAS GREY
  • MEMBER
  • 1994 CHEVROLET SILVERADO
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 180,000 MILES
I noticed last night both break lights are out as well as running lights in rear. I checkec the fuses inside of the truck and couldn't find any that were blown. There were two fuses in that box that were metal so was not able to determine if was blown or not. Where do I start in the hunt for finding out why both lamps are out?
Do you
have the same problem?
Yes
No
Sunday, January 26th, 2014 AT 1:24 PM

3 Replies

Tiny
CARADIODOC
  • EXPERT
The two circuits are totally independent of each other except for the bulbs. The only thing they both have in common, sort of, is the wiring harness. A good suspect would be broken wires, especially if someone added a trailer wiring harness and used Scotch-Lok connectors to splice the wires. Those don't seal out moisture.

First look for a few additional clues. Check the operation of each turn signal, and check to see if the license lamps are also dead. Of course, don't overlook burned out bulbs either.

The metal units in the fuse box are auto-resetting circuit breakers. Instead of testing them, just swap one with another one of the same current rating to see if that gets the circuit working. Those are used in safety systems where you want the circuit to turn on again if the short is intermittent and has gone away for now. That mainly applies to head lights and wipers.
Was this
answer
helpful?
Yes
No
Sunday, January 26th, 2014 AT 1:33 PM
Tiny
THOMAS WAGNER
  • MEMBER
Its very obvious you know your stuff and its a very cool system to have such a personalized answer within a short time. I finally had someone around to look at the tail lights while I was pushing the break pedals. If the "running and head light" switch are in the "off" positon nad the break pedal is depressed then the break lights come on. If the switch is in the on position than not even the break signals work.
There was an old trailer tow package setup rigged in the truck and there are multiple wires that don't seem factory or orginal so sounds impossible to get clear diagnosis.
But having said that there "ARE" break lights working when the switch is in the OFF position, does it change the posiblility of what maybe wrong? I just wish I knew exactly what to look at.

Anyway; any other suggestiions, let me know. Thanks a lot!
Was this
answer
helpful?
Yes
No
Sunday, January 26th, 2014 AT 3:20 PM
Tiny
CARADIODOC
  • EXPERT
That adds a lot. My guess is the license lamps don't work either. The brake / signal filament and tail light filament are in the same bulb, and they share a ground wire. When the ground wire is broken current can't get through either filament when there isn't a complete path back to ground. It looks for an alternate path, and that is though the tail light filament, then through the front running lights. If you look closely, you are likely to see the front running lights glowing dimly when the brake pedal is applied. The brake lights won't be full brightness either.

With the headlights / running lights turned on, voltage appears on both of the rear filaments. With the same voltage, (electrical pressure), on both sides, no current flows so the brake lights no longer work.

Start by inspecting the wires in the rear. To verify a ground wire is broken, use a voltmeter or test light to test the voltages on one of the rear sockets. I can't remember if you have the standard round brass base bulbs with two terminals plus the socket that is grounded or the newer four-terminal rectangular socket. If you have the bulbs with the round base, turn on either the brake lights, tail lights, or both, then measure the voltage on the side of that bulb. There should not be any voltage there. If there is, the ground wire is broken. If you're using a test light, you'll see it glow dimly, and the brake light bulb will be dim too.

If you have the plastic base bulbs, remove the bulb from the socket, then see which terminal has voltage. The one across from it should not, although it may get voltage from the bulb on the other side. If you remove the right bulb, then check again on the left socket, only one terminal will have voltage with the brakes applied, and another one will have voltage with the tail lights on. When you reinstall the bulb, the other two terminals should not have voltage, but they will if the ground wire is broken.
Was this
answer
helpful?
Yes
No
Sunday, January 26th, 2014 AT 6:11 PM

Please login or register to post a reply.

Recommended Guides