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MG MGB Technical - mgb brake light failure

1965 Roadster
Is the brake pressure switch the most likely source for failure of the brake lights? One of the switch terminals seems slighty loose when removing the green/purple wire. I have checked wiring, fuse and bulbs. A test lamp shows power at the green wire connection. Has anyone had problems with the switch on the early chrome bumper cars?
Thanks
R Stevenson
Ayrshire
R Stevenson

Fit a Relay on the switch, check this site out !!

all your needs...

http://www.mgb-stuff.org.uk/electricstext2.htm
K Harris

If you short across the terminals of the switch and the lights come on then the switch is suspect. AFAIK the early cars OEM switch is a highly reliable unit.
Stan Best

In my old 70GT I had a faulty switch. It failed such that it went high resistance when current went through it though rather than out and out failure.

So plug in the lights and it wouldn't work, disconnect the leads and connect a (digital) meter and it would say the switch was fine. The meter wasn't pulling much power though it, the bulbs would so it would fail.

Took me an embarassingly long time to work out what was going on I hate to admit.
Simon Jansen

As Stan states, the early OEM hydraulic switches are very reliable, but they do finally die of old age. When they fail, it could be a completely open circuit, but more often it becomes a high resistance connection and if the pedal is pressed hard enough, the lights will come on (however, with the pedal pressed that hard, would put you into a wild slide). Unfortunately, the replacement switches are absolute JUNK! The only good replacement switches that I know of are available from Ron Francis Wiring in the US http://www.ronfrancis.com/ at a rather high price. the other alternative is to get one of the junk switches and install a relay arc suppression circuit with it. For instructions on making and installing such a circuit, see the article, Brake Light Relay in the Other Tech Articles on my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

Simon - that's why one should always check circuits under real-world conditions i.e. looking for volt-drops when (in this case) the switch is powering its lights. An ohmmeter can give very misleading results where bad connections are concerned, or when testing switches that haven't been used for a long time. Measuring the voltage with respect to earth on each switch contact in turn while the brake pedal is pressed will tell you whether it is the switch that is faulty (12v one side something less the other), the supply (something less than 12v both sides), or something else closer to the lights (12v both sides). As Stan says simply linking the switch contacts together will tell you if it is the switch that is the problem (the lights will work) but you will need a voltmeter if it doesn't.

Little point fitting a relay to the original switch, if that switch isn't working! But it would certainly be advised with a replacement non-OEM, unless you want to buy another one really soon. Personally if good quality switches *are* available I would rather buy one than a cheapo and have to add a relay.
PaulH Solihull

"Personally if good quality switches *are* available I would rather buy one than a cheapo and have to add a relay."

Paul makes a very good point here. Personally, I would go with a new old stock 'red box' Lucas switch if one could be found (the newer green box Lucas switches are not much better than what can be found in the discount auto parts stores here in the US. Cheers - Dave
David DuBois

Each time I get into my car I step on the brake pedal and watch the needle on the amp gauge to make sure that the brake switch is functioning.When I see it swing into discharge, I know that the switch is working. Twice the needle didn't move. Both times it was because of a poor connection of the spade terminal on the switch. This is a car that gets used once a month, maybe twice, so poor connections are something to watch out for, especially in a damp climate. RAY
rjm RAY

I had to change the switch on my 1971 car. As Stan says, connect the two terminals together. If the lights come on the switch is faulty. It can be changed without having to bleed the hydraulics providing you are reasonably quick and don't touch the pedal while the switch is unscrewed.
Mike Howlett


Ford used a similar switch in the early sixties (and earlier). Perhaps it would work...

http://www.rockauto.com/catalog/x,carcode,1333910,parttype,10474

Wayne
Wayne Pearson

"Ford used a similar switch in the early sixties (and earlier). Perhaps it would work..."

But are they *currently* any better than the items sold for MGs?
PaulH Solihull

I had to replace 8 of the pressure switches under warrenty bought from Moss in the space of 2 years. 4 years ago - Some only lasted a few days. I now buy Bosch ones.
Garth
Garth Bagnall

Part number?
PaulH Solihull

Can't comment on the reliability/quality of the Ford switches - only mentioned as additional manufacturers/sources of similar switches.

Just looking at the Standard Motors Products web page, http://www.standardbrand.com/Online%20Catalogs-eCatalog/Content.aspx , search for SLS-34 and you can see what that switch also fits (besides 65-67 MGB).

Wayne
Wayne Pearson

For some reason my link doesn't work. Try:

http://www.standardbrand.com/Online+Catalogs-eCatalog/Content.aspx
and do a part number search for SLS-34 (if you care, of course).

Wayne
Wayne Pearson

It's the embedded space (%20) that stops clicking on the link working, but copying and pasting does. OK, you can find out what other cars use the same switch. But then you have to search for and find some other parts listings for that car that contains brake light switches, which may or may not be Bosch. Much easier if Garth would tell us the part number ...
PaulH Solihull

Went to the site and checked the switch. the first thing that I saw is that the switch is from Intermotor, which gives me pause - my experience with Intermotor products have been less than stellar (read, I will not use their products again). The next thing that caught my attention was the following - "Warranty Covered for 90 Days Or 3,000 Miles (Whichever Occurs First)"

I think that I'll stick with the Echlin switches from NAPA and keep the relay/arc suppression circuit in place. Cheers - Dave
David DuBois

I didn't think Wayne *was* recommending the switches from that source, if he had been there would have been no point in looking for other models that used the same switch, just "Buy SLS-34 from ..."
PaulH Solihull

Thanks for the various comments / suggestions. I ordered a new switch and have fitted it and the brake lights are working perfectly again. The switch supplied was an Intermotor ( I note some of the comments on this). Easy to fit and now to see how long it will last. I will report any problems.
Thanks
Ron Stevenson, Ayrshire, Scotland
R Stevenson

This thread was discussed between 20/07/2010 and 28/07/2010

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