Problem with tail lights / brake lights

[Aimless]

Have an electrical issue going on with tail and break lights. There are 4 break lights, two on the left and two on the right. The small round ones on the bottom are on but when we break the left one goes out and the right one gets brighter. The series of 3 light on the left, the middle break light doesn’t work at all. The socket is extremely rusty and my guess is need to be replaced. Can’t figure out why the bottom left one goes out when breaks are applied though. I tried wiring the bottom pair of lights to the top pair wires and this gave the same result, leading me to think all four lights are the same.. help! Thanks! 🙂

[Rivetboy]

Originally Posted by Aimless

Have an electrical issue going on with tail and break lights. There are 4 break lights, two on the left and two on the right. The small round ones on the bottom are on but when we break the left one goes out and the right one gets brighter. The series of 3 light on the left, the middle break light doesn’t work at all. The socket is extremely rusty and my guess is need to be replaced. Can’t figure out why the bottom left one goes out when breaks are applied though. I tried wiring the bottom pair of lights to the top pair wires and this gave the same result, leading me to think all four lights are the same.. help! Thanks! 🙂

It sounds like you have ground issues.

[Danjo]

You probably have a short to ground. Are the sockets rusty? Carefully remove the bulbs, use some 250 or 320 grit sandpaper to clean them up. You might need to replace a socket, but start there.

[ewo1]

Originally Posted by Danjo

You probably have a short to ground. Are the sockets rusty? Carefully remove the bulbs, use some 250 or 320 grit sandpaper to clean them up. You might need to replace a socket, but start there.

A short to ground would cause a fuse to blow.
Positive 12v to ground = POW - blown fuse.

More than likely you have an open ground or open wire somewhere.

First thing I would do is get a hold of a volt-ohm meter.

IF your bulbs are two wire then the process is easy.
If your bulbs are three wire , still easy.

First thing to do is determine which wire is the ground wire, set your meter to "Ohms", place one lead on the ground terminal of the socket and the other lead to any part of the unpainted chassis/bus body. The meter should either beep or at least show zero (infinity) ohms. if it reading any kind of resistance, say 1 ohm or more, then I would check / clean and repair the ground contact.

IF that checks out turn on your running lights, set the meter to DC volts.
Black lead goes to chassis ground and the red lead on the other wire.
You are looking for a steady 12 volts dc.

Do this on both sides of the bus.
If running light voltage is ok then turn on one of the blinkers and recheck for 12 volts. It won't be steady voltage, the meter will go from 0 volts to 12 volts, it will alternate. If found on both sides, your doing good.

On a two wire system it is simple, one wire is ground and the other wire is 12 volts.

On 3 wire system, you will still have one wire to ground but the other two wires, one will be a steady 12 volts and one should me alternating.

Steady voltage for running lights and alternating voltage for blinkers.

If you have a helper you can do the same check for brake lights. When you step on the pedal you will find a steady 12 volts.

With an open ground the bulbs will try to find a way to ground them selves thru the other positive 12 volt wire and that is why they are acting crazy.

Also, before you begin it is a very good idea to make sure you bulbs are good. If not sure just buy new ones and start fresh.

Oh yeah, if your sockets are really rusty, that is an open invitation for what is called a high resistive open. In short, current can't make it path to ground as it should thus creating an open circuit not a short.

If this is the case, best to just replace the socket. Heck I would even go as far to replace the bulbs with LED stop\turn signals.

Good luck!

[Rivetboy]

Originally Posted by Danjo

You probably have a short to ground. Are the sockets rusty? Carefully remove the bulbs, use some 250 or 320 grit sandpaper to clean them up. You might need to replace a socket, but start there.

A short to ground will trip the breaker / blow the fuse. An open ground with the brake lights and tail lights sharing the open ground will cause problems. When 12 volts goes through the filament of the brake light, and since there is no ground, it backfeeds through the filament of the tail ights and can cause all kinds of interesting problems as that 12 volt finds ground. My advice is to make bright and shiny all of the grounds for each individual light and clean or replace the pigtail in the socket and replace all of the lamps. More time than money. A new gasket in the lamp housing will help keep moisture out of the fixture which is the cause of the problem. Upon recognition of this problem in my bus it was dealt with in a reasonable manner. R and R you know...