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MG MGB Technical - brake light switch

I have owned my 77 MGB GT for about three years and over this time I have had to replace the brake light switch about 14 times! At first my local man was convinced that I was not fitting them properly and even fitted one for me (it lasted less than a day). Last week another ceased up and having had enough I have had my man (mg specialist Andy Rickets of Wroughton nr Swindon) fit a hydraulic switch and HIP HIP HOORAY IT STILL WORKS!!! But am still wondering just what has been causing this problem. Any ideas?
Graham Palmer

The problem is cheap switches. Some have put a relay in the circuit to take the lamp load off the switch. Others have custom fit switches from modern cars.
John H

And then there are those who fit a spring actuated switch under the dash connected to the brake pedal. Check the archives for more in this solution.


72 BGT
Larry Hallanger

Graham - Don't cheer yet, that hydraulic switch is only slightly more long lasting that the crap that is sold as a replacement for the mechanical switch on the pedal. For a permanent fix, either get a hydraulic switch from the Ron Francis Wiring Co. at: The part number for the switch is SW 32. I have not used one of these switches myself, but have heard good reports about them. The other alternative is to build and install a relay/arc suppression circuit that will protect the brake light switch I did this on both of our MGs (TD and MGB) about 5 years ago and have had not problem since then. Information for making and installing the relay/arc suppression circuit can be found at:
Cheers - Dave
David DuBois

A 77 does have a pedal operated switch originally. As David says many current stock switches of both the mechanical and hydraulic type are simply not up to the job. I was going to say that this is probably because modern cars do have relay actuated brake lights, and the switches for the MGB are manufactured to the same specification whereas they need to be a lot beefier for the higher current. But looking at my 2004 ZS I note that *doesn't* have a relay. So it is simply crap design and manufacture. Lets hope you have a NOS hydraulic switch, if not you will be replacing it again, so fit a relay first.
Paul Hunt 2

I went through 5 or so hydraulic switches for my 65 MGB. They only lasted days at best. I think the problem is inadequate contacts for the current.

Then I bought a NOS Lucas part from Little British Car Co. No more problems. Maybe they can help you find a NOS mechanical switch for your car.

The relay with a suppression diode will probably do the job also.
Robert McCoy

I've been using a Ron Francis switch for some time now and it's worked perfectly. Reproduction Lucas switches these days are garbage.
Steve Simmons

I can vouch for Dave Dubois' method of using a relay. My original 34 yr old brake light switch failed about 4 mos ago. Having heard the problems with new switches, and by searching through these archives, I found Dave's wiring diagram for installing a relay with capacitor and diode along with a new (likely crappy) switch, and have had no problems since. The amount of current through the new switch is minimal (just to keep the relay energized) which means it is less likely for its contacts to corrode/pit/otherwise degrade, and the brake lights are in fact brighter than before the original switch had failed. You may want to hook up a relay to your hydraulic switch as in Daves' diagram, and see how far that takes you.

BTW, thanks Dave for providing so many of us with that diagram.
Erick Vesterback

Dave D. will also sell a relay / arc supressor all wired up and ready to go, with instructions for a very modest price. I have one on the TD I'm building. Dave is a great asset to our community. Just contact him for info.

Dave Braun

This thread was discussed between 12/12/2007 and 15/12/2007

MG MGB Technical index

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