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MG MGA - Brake Light Switch

I have just replaced my brake light switch as I was having to put full pressure on the brakes to get the lights to illuminate. New switch is working fine.

I decided to break the old one apart to see the problem. To be honest, it looked fine. The rubber diaphram was intact, no fluid had seeped past; the metal contact disc was clean, no corrosion; and the terminal posts were also clean.

Weird, I wonder why it was not working correctly.

Steve
Steve Gyles

Steve - Did you look closely at the surface of the contacts? That is where the problem occurs. The switch from our TD, that I cut open showed heat discoloration on the spring of the movable contact, which would have been the result of the contacts having developed a high resistance due to corrosion or filming of the contacts. The build of of corrosion or film is what causes the symptoms you were experiencing - this is a normal failure mode of the switches (as opposed to exposure to silicone brake fluid as the manufactures would like for us to believe). The old switches, originally installed in the cars, just like the old, original points sets in the SU fuel pumps, were made from a better material that was not as prone to corrosion or filming than what is on the market today (even the green box Lucas switches are not up to par). If (when) you start having problems with the new switch make up a relay/are suppression circuit per my instructions in the article, Brake Light Relay in the Other Tech Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ and install it at the same time as you replace the switch. Cheers - Dave
David DuBois

Steve, I have been told that later brake light switches are designed to operate relays rather than convey full current. Dave Dubois' comment appesr to bear this out. I have replaced them in both of my MGA's but the current (sorry!) ones have lasted quite well so far (touch wood, I bet I will regret saying that!)
Barry Bahnisch

David and Barry

Thanks for the info. I put the disc and contacts under a magnifying glass and it would seem to support what you say. The terminals were a bit tarnished and the disc pitted (see attached image). For those looking at the image, its actual size is 15mm diameter, so the pitting is not visible without magnification.

The previous switch lasted 6 years and 3 months in the car. At 5 ($7.50) a time it's probably just about acceptable, bearing in mind how often the contacts make and break. It was only a 5 minute job to change the switch, so I will probably stay with the same set-up.

Cheers

Steve

Steve Gyles

"The previous switch lasted 6 years and 3 months in the car. At 5 ($7.50) a time it's probably just about acceptable, bearing in mind how often the contacts make and break. It was only a 5 minute job to change the switch, so I will probably stay with the same set-up."

6 years and 3 months sound like a reasonable amount of time between failure until one compares that to the original switches used, with 30 or 40 years on them before they failed. Don't be surprised if the replacement you install fails in 6 months - about the average for replacement switches I installed before finally making up the relay/arc suppression circuit.
Cheers - Dave
David DuBois

This thread was discussed between 06/02/2009 and 07/02/2009

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