XJ 1100 Brake Light

[schoolguy]

I have a brake light question. My 1100 Maxim has 3 volts going to the brake light bulbs when the brake light switches are not on. When the brakes are applied the bulbs get the proper voltage. Then when the brakes are released it drops back to 3 volts again. The 3 volts are present even when the switches are disconnected. Tail lights are okay. Any ideas?

[Jerry]

Hmmm...

The rear lights on an XJ are dual-filament 1157s. When the ignition switch is turned on the running light should be on, meaning there should be 12 volts to the lights. When you apply the brakes the 2nd filament lights up, these are the brake light filaments. They should have voltage only when the brakes are applied.

So, the question is: do your tail lights (running lights) come on when you turn on the ignition? They should. If they don't, the problem is not enough voltage. This can be caused by bad ground connections, a common problem. The XJ grounds the rear turn lights on two places: A wire connected to a ring under the nut that hold the turn light stem in place, and if you remove the lens, there is a metal strap inside the turn light that makes contact with the bulb for the ground. Both locations can corrode, giving either high resistance or no ground connection at all.

The tail light holders have the same internal grounding strap, which should be checked and cleaned if there is any corrosion present. I don't have the schematic handy and cannot remember where the tail light ground is located, but follow the wires - one should be a ground - and make sure the wire to the frame connection is clean.

You might also check the bullet connectors under the seat. The wiring harness for the tail and rear turn lights plugs into the main harness using bullet connectors. Not a bad idea to unplug and clean each of these connections as well. I have found a dremmel with a sanding drum and spray contact cleaner do a pretty good job of cleaning up any corroded connections. 400 grit sandpaper also works.

Once the powered line connections, those under the seat, and the grounds are in good shape, your lights should work OK with 12v to the running lights any time the ignition is turned on and 12 volts applied to the brake light wiring when the brakes are applied.

If you have voltage to those lines with the ignition turned off, more trouble shooting will be needed. Start by checking for voltage at the connections under the seat, and see if you can isolate which line is powered. From there you will need to trace the line backward into the main harness. You might have a wire rubbing someplace making contact with another wire, or possilbly a bad relay.

Also check the bulbs. I had a strage experience once where one of the bulb filaments had broken and fallen across the other, created some very strange problems as the broken filament would vibrate against the other one, causing intermittent problems. Kicker was that both filaments would light when the brakes came on, as the broken filament still had a path to ground, so the bulb appeared to be good. Go with "sport" or "heavy duty" 1157s if you have to replace the bulbs.

[psycoreefer]

I've got the same thing on my taillight on my 79 special just a little current, not enough to light the bulb, I think it has something to do with how the light checker unit works.....

[schoolguy]

brake light

Thanks for the replies. The running lights come on with the key but the brake light circuit also has 3 volts as soon as the key is turned on. I unplugged the brake switches but the wires still had 3 volts. I never thought about the diagnostic computer. I'll put in the heavy duty 1157s and see what happens.

[Ivan]

FWIW the oil pressure light is also the burned out bulb indicator. IIRC that uses a small current through the fillaments to check the bulb. If there is an open it lights the oil light. I'd bet that's whats going on.

[schoolguy]

Thanks everyone. The 3 volts was from the diagnostic test device. I found a corroded terminal, and put in long life bulbs. Everything is working fine now.If it would just stop snowing now I could try it out!