MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - '67 BGT no brake lights


The brake lights don't come on in my '67 BGT. Faulty brake light switch? Ideas? Tail lights and indicators work properly.

Rgds,
Rick
Rick

If it has the hydraulic switch, that is almost certainly the cause. Test the switch with a multimeter while a friend presses the pedal. New ones can be bought, but are poorer quality than the originals. Fitting a relay takes the load off the switch, which will prolong its life. The switch can be changed without having to bleed the hydraulics so long as you are quick and no-one presses the pedal while its out!

Mike
Mike Howlett

I would strongly recommend fitting a relay and arc suppression circuit at the same time that you replace the switch (switch is probably bad if the brake sights have just failed). If a relay and arc supression circuit are not installed with the new switch, the switch contacts will start burning from the first application of the brakes and will build up enough resistance that a relay aded later will not energize properly. For information on parts needed and installation of the relay and arc suppression circuit, go to the following web site:
http://www.omgtr.ca/technical/brakelightrelay/brakelightrelay.htm
Good luck - Dave
David DuBois

One day 2 electrical niggles. We went to the Wallingford show with the MGB. I was surprised to see it needed fuel but called in to fill up. When it only took 3 gallons it was a hint that the gauge was in trouble. leaving the filling station it showed empty so I stopped and checked the fuses. Both fine. On the way there and back it showed a variety of readings which was a bit of a clue. When I got underneath it with a DVM and croc lead I got no further than pulling the lead from the gauge off, it was full of road dirt. I cleaned it up and did the same for the earth tag while I was there and normal service is resumed. The reason I'm reviving this thread is that after the parade round the town, great fun you beat up a cramped medieaval town centre and get cheered on not arrested like when we were kids, the driver of the Mini who followed us gave the bad news, no brake lights. I carry a cheap and cheerful multimeter and 5 minutes at the fairground with some help from my glamorous assistant proved it was the switch at fault. Shame, it was original. I played around some more just now, the DVM shows it as 0.5 ohms, but if you load it with the brake lights it will not drive them. Does anyone know, are the replacemnts still as bad as in 2003 when this thread was live? I can relay it of course, I would make up a bracket to pick up on the 4 way junction mounting bolt, but would rather avoid it if the present crop are up to the job.
Stan Best

AFAIK there's been no improvement in quality.
Dave O'Neill2

Hi Rick,

You could bypass the original switch with a modern microswitch, as I described here.

http://www.mgb-stuff.org.uk/saga19.htm

Herb
Herb Adler

I found a microswitch when I was looking for a relay earlier and the thought had crossed my mind Herb! I may use a smaller relay though,not an automotive one they are so big and ugly.
Stan Best

Converting to LED tailights will probably increase the longevity of the switch. Sort of a kludge solution but also a worthwhile improvement at the same time.
Steven Rechter

You can also change the brake cover over to a newer B (68 to 74)that has the electrical brake light switch in the brake cover. (Moss part 181-980). That is what they did for my 67.
Bruce Cunha

Thanks for the suggestions, nice to see I have other ways to go. I dont think +ve earth LED lights exist. I would use them if they did. I do realise my car will be the last one with that polarity extant. If anyone is interested in the event some pics are here,
http://s982.photobucket.com/albums/ae305/S_MG_B/RGS_BugleHorn_Walllingford_12/
plus the RGS show the week before. Included is one Jane took of the Mini following us at a respectful distance in the parade.
Stan Best

After all that I just fitted a new switch. I went over to Summit Maidenhead and Mike and Garry were both there. They had them in stock and I asked about durabilty. They have no complaints about quality, and they noted that about 3 years ago the switch insert changed from black to white plastic. Both the MGOC and Moss parts are the same now. At £7 a pop I thought I would try one as a research project for the board.
Stan Best

Fitted an MGOC switch (hydraulically operated type) last summer and it has been no problem. I put a tell-tale in the cabin so I could check it is working because of the bad reputation these have had for some time so I'm continually reminded it is OK.
PaulH Solihull

I had considered an LED tell tale Paul but in the end thought I would just continue my practice of making sure the brakes work before exiting onto my sloping drive and adding a brake light check as I do it. I did partly chicken yasterday and made up a plug on arrangment with spade connectors that adds a 100nF spark quench cap across the switch. I agree that this is a safety item and is important.
Stan Best

A relay needs a spark quench as it is inductive, but lights aren't so shouldn't. The switches are failing (when they fail) because they aren't up to the current taken by 2 x 21w bulbs, maybe because they are designed to be the same as modern brake light switches which *do* operate a relay.
PaulH Solihull

Interesting comment Paul, if the switches couldnt take the steady state current on a safety item that verges on criminal negligence. With 3A flowing there must be some sparking when you open the switch, straight wires have inductance, a lot less than a realy coil of course. There will also be an inrush current whenthe bulbs are cold, and the AC impedance of the cap will pass a slug of current, but I would need to scope the waveform and do some sums before saying 100nF will help
Stan Best

Actually giving it some thought the cap will be charged with the switch open, and it dischrges through the switch when it closes. I suppose this is why you sometimes see a series RC across switches. Gary at Summit did say when the present ones fail they will still light the bulbs, it just needs a shove that would stand the car on it's nose. This sounds like tarnished contacts to me. The OE one measured 0.5 ohms with the DVM across it after failure, but when you put the load back on the entire 12v appeared across it, so it was non linear. I did wonder if a transistor controlling a relay would have worked, but that would heve been naughty.
Stan Best

Hi Stan,
I would remove that capacitor, as it will destroy your switch very quickly.
Capacitors across switches or other contacts destroy the contacts rapidly.
Consider that it is charged when you apply the brakes, then, electrically the switch discharges it very rapidly, by shorting across it. Massive current. Add this to the turn on current of cold bulbs, which is several times that of the hot operating current, and you have a recipe for rapid failure.
You could insert a series resistor, which reduces the peak current, but still adds extra to the cold inrush current of the bulbs.

My advice is to use a relay, with a diode across it, to quench the turn off spike.

Herb

Herb Adler

I partially agree with Herb and partially agree with Stan.

Stan, you must include a series resistor with the capacitor to limit the current through the circuit when the switch is closed.

This R/C circuit placed in parallel to the switch will definitely reduce contact arcing when the brakes are applied.
Steven Rechter

Nothing is simple is it? If I add 100 ohms 1/8 watt (the joules in that cap are nothing, ) Then the turn on surge is fine. It will only take a current of 100mA or so when the switch opens though.This may still be enough to damp the small inductance in the circuit. I make it about 12uS to charge fully.
Stan Best

It's not so much steady state current as the inrush current that destroys under-specced switches, incandescent bulbs have a much lower resistance when cold to when they are hot. As I say there is no inductance (to speak of) in the brake light circuit. Whilst capacitors were used (without resistors) as spark quenches in the highly inductive fuel pump circuit they still resulted in some points burning, which is why first a diode, then a transient voltage suppressor, replaced it.
PaulH Solihull

I used David DuBois' circuitry (relay/diode/capacitor) on my 65 with the hydraulic switch. The switch was just a standard hydraulic one from NAPA. The setup has worked well for several years and is relatively simple. So far nice low operating pressure has been maintained. Without the circuitry you can expect short life sometimes measured in weeks.

Alternately or in combo with the relay circuitry order a heavy duty switch from Ron Francis. www.ronfancis.com Order switch SW-32. Reports of this on the net are good. He touts longer life and lower pressure operation. It's 3 times the cost of a standard switch but likely worth it.

At one time LBCARCO sold a Lucas hydraulic switch with a different number than the Moss item. You could try asking them. I used one of these years ago and it lasted probably 4-5 years with moderate driving.

There also has been complaints about the replacement mechanical switches sold by Moss (the ones for dual circuit MC's).

Bob



Robert McCoy

"mechanical switches sold by Moss (the ones for dual circuit MC's)"

FWIW the switches on the pedal box are the same for both early dual (unboosted) circuit and UK single circuit. A similar switch was used with the boosted dual circuit master (all markets) but was in the cabin operated by a different part of the pedal.
PaulH Solihull

Another approach is to buy a replacement hydraulic switch that is built to carry a higher current. Ron Francis Wiring sells one (link below). It costs about three times as much as the normal switch, but it might end up being cheaper and less trouble than adding a relay. So far the one I installed in my '67 GT has lasted twice as long as the previous stock type switch. It has been in service for almost two years and 18,000 miles. -G.

http://www.ronfrancis.com/prodinfo.asp?number=SW-32
Glenn G

Another option, for mechanical or hydraulic switches is this pull-on switch which incorporates a pedal return spring. Does need a bit of engineering to attach it to the underside of the shelf in the passenger compartment
http://www.electricalcarservices.com/brake-light-switch-push-or-pull-ecs-d057951-p-1538.html
PaulH Solihull

Well reviving this thread again the replacement white insert switch failed today when we went to MG Era day ar Brooklands, maybe to 10nF killed it maybe not.
Anyway I'm underwhelmed with the new improved white insert switch so will fit another and relay it and put a spark quench diode across the coil.
Stan Best

Why not get aftermarket LED stop/tail light bulbs? They are a direct replacement, draw hardly any current so the brake light switch lasts, & you cant tell the difference from original through the lenses (except the brake lights come on faster when you hit the pedal). I put them in my 72 B a couple of years back & they've been great.
G Britnell

As noted in the thread above, I have kept the car +ve earth so I cant use led bulbs. A shame as I think they are great.
Stan Best

This thread was discussed between 28/03/2004 and 10/04/2013

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.