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MG MGB Technical - Brake lights not woking

I have a 1975 MGB on which the brake lights are not working. I have checked the fuse identified in th eowner's manual, and it is good, hot on both ends when the ignition switch is on. When I removed the two wire connector from the brake light pressure switch under the master cylinder, I see two little brass terminals. They are maybe 1/16" in diameter and 1/16" long. They look rather short. Should they be longer, to stick into the switch farther? This is where I started this evening. I THINK that if I short across these terminals the brake lights should come on. They don't. Obvioulsy if the lights should come on, then I have another problem. A test light at the bulb's location does not light either, so it's not just a double bulb failure. Darn.

Does anyone have any suggestions as to where I should look next?
Chuck Asbury

Chuck - I think you are confusing the switch that lights a light when there is an imbalance in the dual brake lines, in other words when one side of the dual braking system is bad. The brake light switch should be protruding through the pedal cover in line with the brake pedal. If you short the two terminals on that switch, the brake lights should come on. If they do, then you have a bad switch, and will need to replace it. Unfortunately, the replacement switches are not very good and don't last more than a short time. To make a real fix of the problem, see my write up on installing a relay in the brake light circuit to protect the brake light switch. The write up can be found at the following link: http://www.omgtr.ca/technical/brakelightrelay/brakelightrelay.htm
Good luck - Dave
David DuBois

Yes, I thought it was the brake light switch, just like the pressure switch on my MGA. I have printed off your "ultimate fix" and will look at the situation again tomorrow morning.

Thanks very much

Chuck
Chuck Asbury

Chuck, it could also be that the "pin" in the center of the brake light switch, which rides against the pedal arm inside the cover, is either worn or the switch housing is out of adjustment and not allowing the pin to travel far enough to make the contact. As David says, the new ones are generally not as long lived as the originals and his use of a relay will help to make the contacts last a long time.

However, if the pin is worn down to where too little of it sticking out past the threaded shoulder to travel far enough to make contact, then that is probably your problem. A trick I have used to make an old switch last a while longer is to take a rotary tool, such as the Dremel MotoTool, with a cutoff wheel on it, and carefully use the cutoff wheel to cut back the threaded brass shoulder of the switch about 1/4" but without cutting the pin. This seems to be enough re-exposure of the pin for good contact to be made when you reinsert the switch and readjust it. FWIW.
Bob Muenchausen

With the mechanical switch in the pedal cover if the operating pin has worn down or the adjustment is out such that it doesn't reach the pedal lever the brake lights would be on all the time. The switch is a 'normally on' type which switches off when the pedal arm operates it.

If shorting out the switch contacts still doesn't work the lights (ignition on) then check the voltage on the contacts while they are shorted. If you see 12v on both then the problem is towards the back of the car, which includes the green/purple single connector in the rear loom near the fusebox. If you don't see 12v on both with the contacts shorted you have a bad connection forwards to the fusebox, which includes two double connectors along the way.
Paul Hunt

We adjusted the correct switch per the owner's nanual this morning. The brake lights are now workibg properly. Thanks to alll who replied.
Chuck Asbury

Paul is once again quite correct about the switch being the "always on sort". I forget this as the lights are only energized when I press the pedal, giving the illusion that the switch is closed as the pedal is pressed in. The truth is, the pin is released once the static pedal lever lets go of it as you press the pedal and the pedal arm contact area allows the pin and the contact to resume its "normally on" position.
Bob Muenchausen

This thread was discussed between 15/08/2003 and 18/08/2003

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