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MG MGB Technical - Brake Light Switch

A couple of weeks ago, I was giving the car a once-over before a run the following day.

Checked brakes and added some ceramic grease to the backs of the front pads, as they've been squealing annoyingly. Adjusted rear brakes, too. Greased kingpins and took it for a quick road test, all good.

When I got home, I was putting the car away for the night when I realised the brake lights were stuck on. I fitted a relay a few years ago, in order to give my 50+ year old switch a chance of surviving. Unfortunately, it decided to fail the opposite way, sticking on rather than burning out.

I had a look at the switch this afternoon. Tested it with a multimeter first and it was showing 30Ω. I then tried it with my test lamp and it was glowing, but not very bright. My test lamp is from a fruit machine and I believe the capless bulb is 2.2w from memory. Nothing printed on it.

I then decided to try my brake light switch tester, which uses a single 21w lamp. With no pressure on the switch, it didn't light up . When applying air pressure, it switched on at about 25psi and off just above 0. I cycled it about half a dozen times.

After that, I checked the switch again with the multimeter. It was now measuring 2.9KΩ.

I think if I had left well alone - i.e. not fitting a relay - I wouldn't have had this problem.





Dave O'Neill 2

Dave

Do you have a back EMF diode across your relay? Although I'm not saying this caused your problem, good electronics practice is to put a diode across an inductive load.

Bob
R.A Davis

Bob

It was a while ago now, but ISTR putting a diode inside the relay casing.

I will have to check.
Dave O'Neill 2

It should have 0 resistance at rest--for all intents and purposes the switch is dead--The diaphram in the switch has probably allowed some fluid to bypass into the contacts side of the switch giving the 2.9K reading you're getting--------it's the beginning of the end for the switch.
William Revit

The switch should have infinite resistance with the brake pedal released and ideally 0 ohms when pressed.

30 ohms when released is probably enough to hold the relay operated even if not enough to operate it when turning on the ignition.

A diode across either the relay coil or the switch contacts should have prevented back EMF damage to the switch which usually causes the switch contacts to burn and go high resistance which - without the relay - makes the bulbs increasingly dimmer over time. However you have to get the diode the right way round for the polarity of your car or it will blow a new switch instantly, which is why I use a varistor in the ones I supply.

With a relay even modern switches _should_ last for many years. I say 'should', as we know modern stuff is frequently rubbish these days and with your switch failing to fully open when released is I suspect a switch quality problem and nothing to do with adding a relay. Without a relay they will probably start going high resistance in the first year.

Initially I was supplying them for hydraulic switches but recently I have been asked to supply them for mechanical as well so presumably they are going the same way. Original switches of both types lasted for many decades, my roadster hydraulic lasted getting on for 40 years and the V8 mechanical was still there 50 years later when I sold it. I've now converted the roadster to mechanical to get the benefit of the lights coming on as soon as I move the pedal instead of waiting for hydraulic pressure.
paulh4

The date on the switch is ‘1/67’, so it’s done well.

I needed it back on the road early the following morning, so I fitted a spare, used switch that I had.
Dave O'Neill 2

I am pleased to report that I did indeed fit a diode, externally rather than internally.

Dave O'Neill 2

Looks like you might have has a bug in the relay ...
paulh4

'had'
paulh4

I thought I'd better check my cooling fan relay. No diode, ooops!

It now has one.
Dave O'Neill 2

FWIW whilst the V8 had a relay for the factory cooling fans the 4-cylinder didn't, but diodes weren't needed in either case.

After-market fans should come with whatever is needed.
paulh4

The fan on my car was fitted during its time at SU. It was operated via a toggle switch where the O/D switch would normally be.

When I installed an O/D gearbox, I decided to have a boss fitted to the radiator for a modern switch. I also added a relay, as well as a manual override switch.
Dave O'Neill 2

This thread was discussed between 16/09/2025 and 18/09/2025

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