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MG MGB Technical - Brake Lights

Having a problem with intermittent brake lights on my 1973 MGB. Just got the car back from paint and body. The brake lights work on occasion, all other lights work fine. When the brake lights work, sometimes they are normally bright, sometimes they are a bit dim. This car does not have the side marker lights (PO removed them). Bad ground????

The car sat for two years while in the shop for paint and body...
John Staub

I don't remember where these are grounded. Make sure there isn't paint or rust at the ground point and that the connection is sound.

It could also be your switch is faulty. Sometimes when switches sit unused for a period of time they develop a film on the contacts that has to be burned off via repeated use. (The SU fuel pump has this trouble on occasion.)

Another problem is the brake light switches do wear out. You can test it by jumpering across the switch leads and see if the stop lights work correctly.

Also, check the bulb and sockets for corrosion.
Robert McCoy

Bob - when you refer to jumping across the leads, would I do this at the switch across the two leads?

Also, I pulled the switch. When I activate it manually, the switch becomes rather warm quite quickly....
John Staub

Bob - I jumpered across the two leads at the switch and the lights worked...Guess it's time to visit Moss motors.

Thanks for the assist!!
John Staub

A poor ground, at the tail lamp assembly, can cause the problem as well as the overheating switch. It can also be a poor connection at the terminals, of the switch, or the contacts inside the switch. Remove the wires, from the switch, and connect them together. If the lights work normally, you have found your culprit. If they heat up, you have a poor ground, or possibly a short circuit, in the brake wiring system. RAY
rjm RAY

The 73 MGB uses a switch in the pedal box and while the pigtails can be disconnected, it is more difficult that on the hydraulic switches. The best way to check the switch is to use a piece of stiff wire (paper clip works) and touch the ends to the two wire contacts at the switch and see if the lights come on. If they come on bright, you have a switch problem. If the lights don't come on, or come on dim, the problem is the ground for the lights. The light assemblies get their ground via the mounting screws, which could be the problem since you just had the car painted. Remove one of the nuts holding the light assembly and scrape the metal under the nut with a knife until it is bright and shiny. Do this on both light assemblies, then retest the brake lights - if they work properly, you have cured the problem. That said, I doubt this is your problem, particularly after you said that the switch get warm when operating. The switch getting warm indicates that there is a high resistance across the contacts that is dropping a lot of voltage and generating heat. The switch will not get hot as a result of a bad ground (sorry Ray). You are most likely going to have to replace the switch, which brings up another problem - replacement switches are junk and will fail in a very short period of time (I had one last all of two weeks). The solution to this is to install a relay/arc suppression circuit at the same time you replace the switch. For instruction on making (or purchasing) a relay/arc suppression circuit, see the article, Brake Light Relay in the Other Tech Articles section of my web site at: Cheers - Dave
David DuBois

The rear light clusters are earthed through their mountings to the wing in all MGBs. While a bad earth will affect the stop lights more than the parking lights, there is usually a clear indication (ho ho) something is wrong when they or the indicators are used as all sorts of lights will flash and the indicators may only glow and not flash. Also the earthing points, bulbs and holders being separate for each side any problem there will only affect one side and not the other ... unless there is a fault both sides!

Both hydraulically and mechanically operated brake light switches can be tested in the same way i.e. by bridging the exposed parts of the terminals on the switch. If that makes the lights work normally when pushing the pedal doesn't, then the switch is the likely problem. However if it is intermittent when using the pedal then it could still be something in the wiring and you have just happened to have done it in a working period. If the switch gets worm when operated then it is indeed faulty.

The brake light switch is different to the SU fuel pump issue after a long layup, the brake light switch contacts are normally open and the first operation of the switch wipes the contacts together which should clean off any such film and the lamp current will burn through it. On the pump the contacts are closed when it is idle, oxidisation can work between the two faces to insulate them, and it is not until you manually rub the contacts against each other or bridge them to tickle the pump into action that they will clean themselves. The lamp current burning through any film is also why you should always test them in a real-world situation i.e. operating the lamps and not with an ohmeter. I know of someone who regularly finds he has to change reversing light switches when testing them with an ohmeter after overhauling a gearbox, but how often do they fail in practice?
PaulH Solihull

Dave - The brake lights are bright when I jumper across the two contact points. I ordered a new switch from Moss, and will have a go at your relay/arc suppression circuit.

I sincerely appreciate all of the help and assistance.

John Staub

Paul - thanks again for your detailed explanation. It's been a long time since we traded correspondence, and your detailed response really helps. All of the other lights work normally when turned on/activated, with the exception of the brake lights.
John Staub

John, sounds like you've found the problem. Be careful how you adjust the new switch. If you run it in too far it will start to press on the brake pedal lever and possibly block the master cylinder re;ief port.

You want it to close just as you slightly move the pedal.

Paul, thanks for the enlightment on the difference between how the brake and fuel pump contacts work. My fuel pump is a German aftermarket Hardi pump which has never had a problem in over 10 years. Non serviceable however.

I used David DuBois' instructions for my hydraulic switch relay and arc suppressor. So far so good. I think it's good insurance for your mechanical switch also.

Have fun!
Robert McCoy

Good point on the mechanical switch, on my pedal-box mounted switch there is only 1.5 turns between the light just going off at one extreme, and starting to reduce the free-play at the pedal at the other. Not that easy to check on this type, slightly easier with the later switch in the cabin where you have the dual master with integral servo.
PaulH Solihull

This thread was discussed between 30/05/2011 and 04/06/2011

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