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MG MGB Technical - Brake light switch

Hi - have a problem with brake lights. They wont work. Have by-passed the switch and they work just fine so wiring and earth ok. Have put continuity tested on switch and each time pedal depressed a circuit is made. Put the two together and nothing. Is this a common fault with the swich which must be at least 25 years old the time I have had the car. Any thoughts appreciated.
Tim

This is indeed a common problem, the replacement switches available today could last from 2 weeks to 2 years. I had replaced 5 in the past 8 years, finally installed a relay prepared by Dave Dubois that will serve to protect the switch. It was a well built device, installs easily and I am anticipating a much longer life for the brake switch.
Joe

It's likely the switch contacts are burned. A continuity meter may be happy as it draws little current but the switch resistance is so high you can't get the correct current and voltage to the bulbs.

Replacement switches often fail rapidly as they have very little current capacity compared to the originals. I know that is true of the hydraulic type used on Mk I's.

Check the archives. Dave DuBois has written a paper on this and designed a relay and diode circuit to take care of the problem. I don't know his web site but you should be able to find it on the archives.

Regards,
Bob McCoy
Robert McCoy

Tim - Both Joe and Bob have given you the cause of your problem (it doesn't take much resistance in the switch contacts to dim the brake lights so much that they can't be seen). You don't say which switch your car uses, the hydraulic switch or the pedal operated mechanical switch, but with the hydraulic switch, one can usually get the lights to come on by pressing very hard on the brake pedal (much harder than normal braking would use). You can get instructions on making and installing a relay/arc suppression circuit on my web site at: http://homepages.donobi.net/sufuelpumps/ click on Other Tech Articles, then on Brake Light Relay. Cheers - Dave
David DuBois

Forgot to add, if your car uses a hydraulic brake light switch a heavy duty switch, that will hold up for many years is available from Ron Francis Wiring http://www.ronfrancis.com/ under part number SW-32. Cheers - Dave
David DuBois

...it doesn't take much resistance in the switch contacts to dim the brake lights so much that they can't be seen...

I had exactly this happen on a hydraulic switch. Made worse since I was measuring with a digital multimeter. It draws very little current.

So the meter indicated the switch was fine but as soon as you hooked up the bulbs it wouldn't work.

Simon
Simon Jansen

Dave's solution worked a treat for me: 10 mintues with a soldering iron and a relay and you're good to go - hopefully for years!

And it's a much cheaper option for those of use outside the US who don't have easy access to order Ron Francis's heavy duty switch.
Curtis Walker

I rebuilt the switch on my '77B, which I believe to have a design flaw. My account is in the archives.
Fred Doyen

This thread was discussed between 31/05/2009 and 05/06/2009

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