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MG MGB Technical - Brake Light Switch

I have a hydraulic brake light switch on my '70 MGB, and brake lights that don't work. I have 12 volts coming in from the green wire (Fuse is fine). With the Green/Purple wire leading back the the rear brake lights disconnected, I get 12 volts on the terminal when the brake pedal is pressed. However, as soon as the wire is reconnected the contact is broken, even though the pedal is still pressed. I have checked the continuity of the green purple wire all the way back to the lamps, which is fine. Is the Brake light switch knackered?
Iain Bubb

Looks like there is a high resistance joint in the switch. Take both wires off the switch and join by inserting a piece into them, turn on the ignition and the brake lights should then be on permanently. If this is the case switch is fault so change it.
Iain MacKintosh

you might want to do some searches on this BBS on the subject...the switches are pretty sensitive and and can get dodgy. If you have problem with it burning out again quickly there is some info around regarding a re-wiring solution so that less current runs through the switch itself. Can't remember if solution was by David Dubois, Paul Hunt or Chris at Octarine Services or someone else. That said, I replaced mine and it has been fine for couple years.
J.T. Bamford

The symptom of 12v being registered on a meter when connected to the switch output with the green/purple disconnected but not with it connected is a prime example of why you should always measure voltages on circuits with them connected up. The typical voltmeter draws so little current that the circuit it is testing can be practically a full open circuit before the meter will start to show a problem.

New hydraulic switches are notorious for not being able to carry the current of the brake lights and failing very soon after fitting. There is much comment in the archives about using the switch to operate a relay that operates the lights, originally from David Dubois I believe. You should also use a diode across the relay winding to protect the switch from the back emf of the relay as well. Note that getting the diode the wrong way round will blitz the switch instantly!
Paul Hunt

Iain - You have the classic brake light switch failure. The contacts in the switch have developed a high resistance over the years and all of the voltage is being dropped across that resistance, with none left over for the lights. As Paul said, replacement switches (at leas the ones we can get on this side of the pond are so whimpy that I had one fail after only two weeks. I have found that it is absolutely to install a relay and arc suppression circuit at the same time you install a new switch if you have any hope of the replacement switch lasting. Directions for installing a relay and the arc suppression circuit can be found at: http://www.omgtr.ca/technical/brakelightrelay/brakelightrelay.htm
Good luck - Dave
David DuBois

The archives have lots of good information about relays and suppression diodes. They are absolutely correct that the replacement hydraulic switches don't last at all. In no time at all the contacts start to fail and excessively high pedal pressure is required to make them work.

I solved my problem by buying a LUCAS original part. That's right the "Prince of Darkness". It has been flawless for years. I got mine from the Little British Car Co. They may not be available anymore but ask.

If it is not available go the relay and suppressor route.
Robert McCoy

I went with a NOS Lucas switch also (the only thing wrong with Lucas electrics is what DPOs have done to them), but considering the cost of the relay, diode and capacitor, I consider it is cheap insurance ans should make the Lucas switch outlast me. Cheers - Dave
David DuBois

"the only thing wrong with Lucas electrics is what DPOs have done to them"

Hear hear.
Paul Hunt

Iain,

I get through brake light switches about every six months heavy braking seems to knacker them really quickly (i hillclimb and sprint my B)
If you press really hard i bet your brake lights will work therefore knackered switch.
TJ
T J Nicolson

TJ - You are another candidate for the relay and arc suppression circuit. Once done, you won't have to fuss with the switch any more. Cheers - Dave
David DuBois

This thread was discussed between 21/05/2004 and 25/05/2004

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