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MG MGB Technical - Brake Light Switch

Any advice on brake light switches on a "B". Replaced my switch in July when I bought the car, it worked well for 3 months and now it is working sporadically. I took it out, inspected all contacts which are OK, when I work the switch by hand it will work 4 out of 6 times, seems like the internal spring mechanism hangs up, you can't take it apart so I have ordered another from a different supplier. Any ideas, or do others experience short life spans for these switches?
Joe

I'm sure I made this comment before, but adding a relay in the circuit seems to help vastly. I guess the switch can't handle the current of the brake lights very well (which is kind of surprising because it isn't very much current, probably around 5 amps at the most for both -- they're probably 30 watt bulbs or something like that). The other benefit of fitting a relay is the lights get somewhat brighter. Relays being cheap and those switches not usually being cheap, you might as well give it a shot.

I guess the other thing you could do is use those led bulbs for the brake lights, which I believe consume very, very little power.
Harry

Joe - I am with Harry, put a relay in the brake light circuit. Harry is correct about the relative low current through the brake light switch, but if you ever take one apart and look at the contact arrangement in them, you would feel lucky to have gotten 3 months of service out of them. I also suspect that the lights of today have a higher inductance in them that gives a greater back voltage when the switch opens, thus causing the points to burn in a short time. I have gone so far as to put a diode across the coil of the relay and a capacitor across the switch contacts. If you are unsure how to wire up a relay in the brake light circuit, send me an e-mail and I will send you a diagram of how to wire it up. Good luck - Dave
David DuBois

I too have had a lot of trouble with replacement swithes. Seems the copper rivets that connect the wires to the switch tend to get lose after a while. Tightening them has worked for me so far but i carry a spare too just in case.
James Skinner

James - You must be refering to the original Lucas switched (which are about 1000 times better than the replacement switchs sold today). I, too have noticed the loose rivits. I have been trying to solder the rivits to the wire connections on the outside of the switch, but have not had a great deal of success so far. When I get the time I am going to try again with some better flux. Good luck - Dave
David DuBois

Could anyone explain what type of relay and where to wire it into. I am installing my new brake switch next week.
Berady

Berady - Send me your e-mail address and I'll provide you with information on wiring up the relay. Dave
David DuBois

Having just replaced mine and having a chat to the supplier, he told me a story about these hydraulic switches not reacting well with silicon brake fluid. Apparently he was supplying replacement units for a Jensen owner about every three months and a call to the supplier to complain started the investigation.
Lin Ison

Lin - I am not sure where these stories about the brake light switches vs silicon fluid come from, but I suspect it is from some old wife somewhere. I have been using silicone fluid in my TD for some 20 years and have not had the original switch fail yet. I have however had the switch on my MGB fail and it is a mechanical switch mounted on the pedal box and actuated by the brake pedal - completely isolated from the brake fluid. Since silicone fluid, like synthetic motor oil will leak through the smallest hole that there is, it is possible that some people have had hydraulically actuated switches that have had a leak in the diaphragm that was small enough that normal brake fluid would not pass through it, but when they switched to silicon fluid it leaked past the diaphragm and caused a problem. The problem with the replacement brake light switches today is that in an effort to make a bigger profit (or keep prices down if you are less cynical), the manufactures are making them as cheaply as possible and as a result, they are unable to handle the 5 - 6 amps that is required of them. I even had a switch give out after installing a relay (with a draw of only 0.25 amps)! This doesn't say much for the switches that are available. I finally installed a diode across the relay coil and a capacitor across the switch contacts and so far everything has worked for the past year. I have become so paranoid about my brake lights that I installed a LED indicator at the corner of my dash that lights us with the brake lights so I can be sure that everything is still working when I step on the brake. Good luck - Dave

David DuBois

I've had recent problems with my OEM Lucas brake switch, too.

Tested on an ohm meter, the resistance across the leads would vary from 50½ - 1.2½
...enough to prevent the brake lights from working. A thorough cleaning brought the
resistance down to 0.2½ or so (enough to get the brakes to work) - but this would only
last a week or two before the resistance went back up and I would find myself back to
square one.

Upon much closer examination I had noticed that both copper (tab) contacts were
bent upwards very slightly (almost impersceptible) and that they were only marginally
touching the contact washer along it's very outer edge.

Very gently tapping both tabs down slightly with a drift and small hammer made things
so that the tab contacts now touch the contact washer well within it's body. Re-tested,
the resistance across the leads = 0.0½. That was 2 weeks ago and so far, so good.

On a related matter - I was shopping around at a local electronics parts store a few months
ago and I had noticed that they stock a heavy-duty NC momentary contact switch ($9).
It's built robustly and it emits a quiet but reassuring "click" whenever the contacts open/close.
The only real drawback is that installing it would require drilling-out and tapping the boss
on the brake pedal cover as it is slightly bigger, overall, than the Lucas switch. In any case,
I am considering swapping to this when the OEM Lucas switch finally gives up.
Daniel Wong

Daniel - Let us know the part number on the switch and how it works if you make the change. It certainly sounds like an easier fix (at least for the later MGBs with the switch on the pedal), than all the fussing around that I have gone through. Cheers - Dave
David DuBois

If you have the early B brake switch, you could replace it with a pedal lever acutated switch without a great deal of hassle. I did this on my MGA and have not had any problems with that switch (from a GM application) and it was relatively easy to install. Just another option, I suppose.
Bob Muenchausen

Knew I should have kept the part number... Anyway, there is a Dodge switch with a threaded shank and bullet type connectors on the end of a round body. It'd be in the Buyer's Guide at your local parts place. Considerably longer than the OEM switch, but reliable, and about 5 bucks.

I suggest you check the travel on that snap-action switch. It could need too much travel to reset.
Jim Blackwood

Mechanical or hydraulically operated? There have been comments over many years that the hydraulic type last no time with silicone fluid. Also that new mechanical types, like many re-manufactured items, don't last anywhere near as long as the originals.

Paul Hunt

Just another thought if you have a pedal box mounted switch. If the end of the plunger gets worn away, and no longer moves sufficiently far enough to light the brakes, you can extend the life of one of these OE Lucas switches fairly easily by either grinding or cutting off about 3/32" from the plunger end of the threaded housing on the switch. This reveals more plunger and, Viola!, the thing is usable again for many more miles.

Also, you can drill a tiny hole in the plastic end of the Lucas switch and flush the contacts with a good contact cleaner and then seal it back up with a trace of silicone seal. FYI
Bob Muenchausen

Mechanical. I haven't had one apart, but it hasn't failed me yet. Only problem I ever had with that switch on american cars was from bad adjustment.

I just looked at it with an OEM switch for comparison and it's a 10 x 1.25 thread. I guess I ran a tap through the boss.
Jim Blackwood

Replacing the switch with a sort of upgrade has been a side-project with me and I haven't quite
sorted everything out.

For some reason, I didn't think about maybe swapping a switch from another type car.
A switch from an early 70's Chevy Nova or Camaro would seem to be a good choice as there
are plenty of them laying about the wreckers and I've never heard complaints of them failing at
an unusual rate. Fabricating a bracket to place the switch inside of the legwell is an appealing
route because it wouldn't involve drilling and tapping the pedal box cover (which I loathe to do)
and it opens up the range of switches in which to choose from.

In anycase, here are the switch options that I mentioned in my earlier post:

Manufacturer, Selectra; part # SS229-BG; SPST On/Off (mon); rated at 15A-125V
cost approx = $8.70

Manufacturer, GCE; part # 31851; SPST On/Off (mon); rated at 6A-125V
cost approx = $13.25 . This switch has the "click" action, but this may become annoying
to listen to after a while. It is also lower rated than the switch (above), but is OK for this application.

Both of these switches have larger threaded shanks than the Lucas OEM switch and will
require a bit of modification to the pedal box in order to mount it there. Relocating the switch
elsewhere inside of the legwell with a fabricated bracket would seem to be an easier route
to accomplish this.
Daniel Wong

I have a problem with by brake lights too. They work but you have to press the pedal quite hard to make them come on. Would adding a relay to the circuit cure this problem?? Any help would be greatly appreciated.

Thank you.
Anthony McGrath

I assume you are talking about a CB with the hydraulic switch. Unlikely. The brakes would probably require just as much pressure to operate the relay. Could be that the switch is acting as a variable resistor and is only passing enough current to illuminate the lamps at high pressure, but that is just a bad switch and it is probably on its way to total failure anyway.

PaulH.
Paul Hunt

You're right, a CB with a hydraulic switch. I replacedthe switch a couple of months ago as the other one failed completely the only thing that I could think of is that maybe there was air in the switch, I replaced the calipers and had to drain/bleed the whole system not long ago.

Anthony.
Anthony McGrath

My hydraulic switch only works if you stand on the brakes (but hitting it sometimes makes it work for half and hour or so). I have a new switch but can't shift the old one to change it. Any ideas?

Tim.
Tim Jenner

This thread was discussed between 07/10/2001 and 12/10/2001

MG MGB Technical index

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