Nuts and Bolts with Buffalo Jim

Buffalo Jim,

When the lights are on and I hit the brakes in my 1994 Nissan Altima, my right taillight goes out. But when I let off the brakes, it comes back on. I'm assuming it's a short, but I'm not an electrician. I've researched the manual, and the guy at AutoZone said to check the grounds. Unfortunately, I have no idea where to find the brake-light grounds. He also suggested the terminals could be wet or the bulbs are out. So, of course, I started with the easiest job and replaced the bulbs, but the problem remains. Please help in any way you can.

--Demetrius Robbins

To be honest, bro, I don't usually give people do-it-yourself advice on electrical systems. It's not because they're especially complicated but because electrical systems require wiring diagrams and a whole bunch of specialty tools to diagnose and repair. To do the job right on a brake light, for example, you have to begin at the battery and test every part of the circuit back to the ground. This can be a time-consuming process that should only be undertaken by someone who can explain the difference between a volt and an amp.

In this case, though, I'll make an exception, because it really does sound like you have a bad wire running from the light housing to a ground somewhere on the body. You might be able to trace it back by following the wire that's attached to your light housing, but in your Altima, the wires are probably hidden beneath some kind of plastic cover. It's not very hard to remove, but you need to know what you are looking for. Once you find the ground, there could be one of three problems. Either the wire is bad, the ground connection is loose or the connection is so corroded that it's acting as a barrier to ground. No matter what it is, the circuit won't be completed until you or a qualified mechanic fix the problem.

I actually saw a similar problem in a Honda CRX recently, but in this case the brake light grounded into the fuel pump relay. The relay shorted out, and every time the guy hit the brakes, the car died. It was a tricky problem that almost had me stumped. Incidentally, you were absolutely right to start with the bulbs. When you're doing work at home, always start with the cheapest, easiest solution and work your way back.

Tip of the week: Master cylinders

Let's make this real simple: Braking systems transmit the force of your foot to the brakes by means of an incompressible liquid called brake fluid. This process is facilitated by the master cylinder, which uses a dual piston system to compress fluid into the front and rear brakes.

All of this is important only because it will help you diagnose problems with the master cylinder. The easiest way to detect such issues is to monitor your brake fluid level and the height of your brake pedal. Does the pedal slowly sink as you wait at a stoplight? If it does, one of the pistons in your master cylinder is not sealing properly and some of the brake fluid is escaping into the reservoir. If it gets really bad, the pedal will eventually start falling to the floor and you will have to pick up your foot to regain your braking power. Needless to say, this is a very bad thing--especially for that poor car that is stopped in front of you.

Buffalo Jim is the owner of Allstate Auto Marine. He recently returned from a cruise to Catalina, which he characterized as one of the worst vacation decisions he has ever made. He didn't throw up, but his daughter, Little Buffalo, did. Both of them have resolved to more actively appreciate dry land in the new year. E-mail him you car questions at buffalojim@lasvegasmercury.com.