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MG MGB Technical - Brake Lights & Ignition Switch

Today I lost both brake lights and power to the radio on my 1972B. They worked fine yesterday. I checked for current across the wires that feed the brake switch, and no power with the key on. No power to the fuse on the radio either. Both the radio and brake lights are switched off the key, I have had problems in starting the car, difficult to diagnose since 3 out of 4 tries the car starts fine. I have suspected the ignition switch, could the switch have failed to the point the car will still start, but I loose power to the brake lights and radio? I don't believe the brake light power is fused anywhere, correct? Whoever installed my radio placed a fuse at the unit, it is not blown and there is no power there either. No fuse problems in the box under the hood.
Joe

I may be way off, but have you checked your flasher switch. I know this switch controls a lot of things and when it goes bad it messes everything up.
Robert Browning

I have noted the flashers are also acting strange, they start normal for 2-3 flashes, then slow way down. Where is the switch located, is it under the console, or under the dash?
Joe

On my car, a 1973, it is behind the center console.

How old is the actual flasher switch? From what I have read in this Forum, I think the switch can cause problems in itself besides the flasher unit.

I suggest you search the Archives - plus drop Paul Hunt a separate email. He is very good with these type probelms.

Paul Hunt
West Midlands
United Kingdom
paul.hunt1@blueyonder.co.uk_http://www.mgb-stuff.org.uk/
Robert Browning

Update: I learned the bulb was burnt in my test light, I tested with a multimeter and have power to the brake switch and radio, have ordered a new switch because the lights work when I jump across. Just fooled because the switch was installed late last summer, failed with less than 500 miles on it. Not sure about why the radio suddenly doesn't work. Picked up a flasher at the local auto parts, just a 2 terminal round can, will try to see if directional lights flash correctly.
Joe

FWIW problems with the hazard switch only affect the turn signals, nothing else.

Unless your new brake light switch is new old-stock prepare for almost instant failure! There has been a lot of problems with these in the recent past and people have had to fit relays as some switches just aren't up to the job of powering two 21 watt bulbs!! It's not up to the job of powering an inductive load like the relay either, so you have to either get a relay with integral quenching diodes, or add one to the relay yourself. You must be careful to get the diode and wiring the correct way round as getting it the wrong way round will also blow the switch!!! The diode must be connected so that it opposes current flow from the wiring. Some quenched relays only have one diode, so you have to be careful which way round you power these as well or you blow the diode as well as the switch!!!! Others have two diodes and the second prevents reverse connection damaging the quenching diode or the switch, it also prevents the relay operating at all.
Paul Hunt

Joe - Re. Paul Hunts comments about brake lights switches (Paul very politely restated what my comments about replacement brake light switches - they are crap!). You can either go the relay route - see the article, Brake Light Relay in the Other Tech Articles on my web site at: http://homepages.donobi.net/sufuelpumps/ when you replace the switch or get the heavy duty switch (SW-32) carried by Ron Francis Wiring http://www.ronfrancis.com/ Cheers - Dave
David DuBois

This thread was discussed between 19/04/2009 and 21/04/2009

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