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MG MGB Technical - No brake lights

During a pleasant drive I discovered that my brake lights were not working. All other lights, turn signals, hazards, and electrics were working fine.

I searched the archives and found that it is probably the "brake light switch". My question is how difficult a fix is this. My auto electric knowledge is limited and could there be any other source of the problem.

1974 1/2 mgb roadster
J Mancone

If you have a hydraulic brake light switch this is located on the passengers side fender. Remove the wires and join them together if the lights are now on then its the switch. The switch is easy to replace. Just unscrew the original one and then screw the new one in after having put a couple of turns of PTFE tape on the threads. There will be no need to bleed the brakes after this operation. Current opinion is that the switches now available are of very poor quality and that you would be well advised to fit a relay to prevent the switch from carrying the full lamp current.
Iain MacKintosh

And the relay will need a diode wired across it to prevent even the relay welding the switch contacts!

A 74 1/2 American has the mechanical switch on the pedal box, but that doesn't change the warning about poor quality switches.

If bridging the switch doesn't cause the lights to glow, then it isn't the switch but a connection. You will need to use a meter or test-lamp on the switch terminals to work out where. If the test-lamp doesn't glow on the switch green, or it glows until you operate the pedal, then there is a bad connection back through the green to the fusebox.

There is a 2-way bullet connector in the switch green that comes off the screen washer motor. If the screen washer works then 12v must be getting at least that far, so it must be that 2-way or the spade on the switch.

If you have 12v both sides of the switch with the pedal operated then check the 2-way bullet connector in the green/purple by the switch, and another one where the main harness joins the rear harness in the mass of connectors by the fusebox.

If you have 12v both sides of those connectors with the pedal operated, then check the green/purple 4-way bullet connector by the right-hand rear light, the backs of all three bullets in that. If there then you could have two faults, which could be bulbs, the connections between bulbs and their holders, and the 2-way connector by the left-hand light.
PaulH Solihull

Thanks for the input gentlemen. I will begin to check those potential problems and get back to you.
J Mancone

J - For information on constructing and installing a Relay/arc suppression circuit, see the article, Brake Light Relay in the Other Tech Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

This thread was discussed between 20/09/2010 and 22/09/2010

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