brake lights

[biga1680]

My girlfriend has a 96 Mustang GT Convertable. Her tail light work when you turn on the lights. The problem is the brake lights don't work in the rear or on the roll bar. Every time you press the brakes the fuse blows. I check the brake switch, it harness wasn't plugged in all the way. I reattachted it, replaced the fuse and the brake lights worked about seven times. The the fuse started blowing again. I replaced the the switch and the wiring plug. The fuse is still blowing. I also checked the brake fluid wiring harness that plugs into the master cylinder, because that caused my brake lights to blow the fuse, that is fine. I'm assuming that i have a bad ground but, I'm not sure were to look for it at.


Adam

[DiggerStang]

Does the fuse still blow immediately, or only after several applications?

My first thought is that you have a wire shorting into the chassis (ground, that is) somewhere after the switch. Disconnect the bulbs and see if the fuse still blows. If it does, you're going to have to scrutinize your wiring.

[DiggerStang]

This is generalising, because I'm not familiar with the wiring of a '96, but I bet they're all more or less the same. Each brake light probably has 3 connections. One is ground. One has 12V (appx) when you turn the headlights on. One has 12V when you step on the brake.

Disconnect the Negative battery terminal.
Access the plug that fits on one of the brake lights.
The ground wire is probably black. Using an ohmmeter, measure from the ground wire to a good, bare ground on the body (it has to be metal, of course!). There should be almost 0 ohms of resistance. Make sure your meter is calibrated, though.
Now, if either of the other wires reads 0 ohms to ground, you definitely have a short on that wire.
Note that it might not read 0 ohms; 1 or 2 ohms might still blow the fuse. Also note that your ground may not read 0 ohms, either; it just depends how good your contact is with your probe.

Frankly, I've never had to do this test on my '88, so I hope things are the same on your car, otherwise it won't turn out the way I explained.

BTW, if you have to buy a meter, I prefer an old-fashioned analog meter for these simple tests.

[GTStang]

Definetly sounds like a short. My first suspected culprit would be the wires for the light on the aftermarket styling bar.

[biga1680]

the fuse blows as sonn as you push in the brake pedal

[DiggerStang]

Your best bet is to do as GTStang said and check the wiring for your roll bar. Pay particular attention to where the wiring for the roll bar is spliced into the circuit. You could try bypassing the roll bar wiring by returning the wiring to stock condition. Then see if the fuse blows.
Its probably a simple problem. The trouble is, the wiring is not always easy to access. The most straightforward way to troubleshoot these problems is isolate the short by disconnecting parts of the circuit and checking if the short is gone. On a circuit board, this is easy to do. But in a car, where the wire is hidden for cosmetic reasons, it can be a little difficult to disconnect part of the circuit. It sometimes means cutting wires. These wires then have to be reconnected and PROPERLY insulated afterward.
Who installed the roll bar? How long ago? Are there any wiring mods you may have forgot to mention?