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 - Brake light problem

I have had an ongoing problem with my brake lights. They do not work and it concerns the pressure sensitive switch. I know this because when I short the terminals and run the engine the brake lights illuminate - so it must be the switch.

I have replaced the switch twice to no avail - they work briefly and then dont, or dont work at all. Could it be a problem with the four way junction that the switch screws into?

For reference: I have recently had quite a lot of work done on the brake system of the car leading to an MOT last week that the car passed. I checked the lights when I got home from the MOT and they werent working then. The work included new master cyliner, some new hoses, a new caliper on one side and a new copper pipe going from the four-way junction to the front nearside wheel.

The upshot of the work is a very 'tight' breaking system that stops the car perfectly well but doesnt light the lights.

Apart from questioning the competence of the mechanic and the MOT tester, I have totally run out of ideas as to what the problem could be.

Any ideas anyone?

Andrew
1973 BGT
Andrew H

Hi Andrew.

You say that the brakes are 'tight'..

Is the pedal travel very short ?.. if so, it may be a matter of adjustment.

Try removing the switch and operating it manually.. if the lights operate, an adjustment should do the trick.

Don
Don

Does fluid seep out of the switch hole when the switch is removed? If not then the four-way junction could be the problem - a blocked hole. If it does then possibly both switches are faulty, not impossible if you got them from the same source. Doesn't matter how long or short the pedal is as far as the switch is concerned, it should operate reliably on quite low pressure, but how low I've never determined. There is no adjustment available apart from the MkI leg muscle. If the switch does operate with light manual pressure when removed from the four-way but not when in, then I suppose even though fluid might be seeping out of the hole there could be something inside that blocks the hole as soon as pressure is applied to the fluid. You could take precautions to stop the fluid getting on the paintwork and hit the pedal once to see what happens.
Paul Hunt

Andrew - You are probably suffering from the whimpy switch blues. The replacement switches available today are very prone to arch damage and burning due to the extremely light duty contacts (in fact there is no actual contact in the switches, just a dimpled piece of material that makes with a flat piece when the diaphragm is acted on by the brake fluid pressure). I finally got the switches in my MGB and TD to hold up by placing a relay and an arc suppression circuit in line with the brake light switch. Since doing this, I have not had any furthere problems. Information on doing this modification can be found at the following seb site: http://www.omgtr.ca/technical/brakelightrelay/brakelightrelay.htm
One caution, the modification should be done at the same time that a new switch is installed. If a switch is used for any period of time before installing the mod, it apparently is damaged by arcing sufficiently that it will fail regardless of the modification. I had that happen to me the first time I installed the mod and others have experienced that same situation. Good luck - Dave
David DuBois

Oops...

I forgot that older models had a hydraulic switch, and was thinking of the later system.

I can confirm (from my Morris 1000 days) that there have been probs with some new 'pattern' hydraulically operated switches.

Don
Don

Don - There are worse problems with the replacement mechanical switches that operate off of the pedals. I had one of those that lasted all of a week - they make the word "whimpy" inadequate. GIVE US BACK OUR OLD LUCAS SWITCHES! at least they held up for a decent period of time. Cheers - Cave
David DuBois

David: Why couldnt you switch to the later pedal box cover with the mechanical brake light switch?
Andrew Blackley

Andrew - I did just that in my 66 MGB, but the replacement switches for that set up are worse than the replacement hydraulic switches. The mechanical switch burned out in 1 week! I did finally get a couple of NOS Lucas switches from LBC, which are a much heavier switch and have backed them up with a relay and arc suppression circuit. I figure that this setup should last me well into the afterlife. Cheers - Dave
David DuBois

Hi all.

Switches are often designed with a toggle action, ie the contacts are forced by a spring mechanism to open and close rapidly, with a very brief 'part open' condition. Take a look at some fuel pump contacts for an elegant example.
This reduces the tendency to arc, which prolongs contact life significantly when switching larger currents.
I believe that most older brake light switches (NOS or not) do not have a toggle action.

I guess it would be possible to open up a failed switch and use the actuating mechanism to operate a 'microswitch', a common switch which requires very short actuator travel but has a built-in toggle action.

I wonder why CB points never aquired toggle action ?...

Don
Don

David: I replaced my (mechanical) switch on my 69 last year after the good old original gave it up after 33 years. I got it from Moss, via LBC, and no problems since. At least when it fails I wont have to deal with fluid loss and the need to bleed the brakes to replace it.
Andrew Blackley

Just a thought, I had a similar "apparant" problem with my brake lights when I rebuilt the master cylinder. I replaced the switch mounted on the cover and didn't change anything. It ended up being that I installed the spacers for the pedal linkages wrong and the brake pedal to brake switch contact point didn't line up leaving a sporatic brake lights. I found out by removing the brake switch and look at the brake pedal level arm through the threaded hold in the master cylinder cover to make sure it is lined up.

Eric
73MGB
Eric Willis

Andrew - I put the mecahanical switch on my 66 MGB about three or four years ago, so maybe in the intervening years, they have found a supplier that has decent switches, but the one I got was anything but decent.
Don - There are all sorts of ways to take care of the problem, I just find that a relay, diode and capacitor are the least expensive and easiest method of curing the problem. Cheers - Dave
David DuBois

All: thank you for your suggestions.

I had a 'play around' on Sunday and got the lights working. It is possible that the switch wasnt screwed in tight enough. What is the recommended tightness? I have overtightened in the past and severed one of the copper brake lines hence I am cautious about this now.

Andrew
1973 MGBGT

Andrew H

Tight enough not to leak. Screwing it in tighter or not will not have any effect on its operation as it works from hydraulic pressure, which will be the same (assuming no major leaks) whether it is screwed in ten turns or only five. However the physical act of screwing it in and out *may* have moved things internally so the contacts are now operating in a slightly different position. But did you short out the spades on the switch in your first post or only the wire connectors? If the latter then they could have been making a poor connection to the spades on the switch.
Paul Hunt

Paul,

The latter and I was suspicious of the quality of those connections.

My plan is to install a light on the dashboard that lights up when the brake lights do (quite an easy operation, tapping into the circuit at the switch and taking the wires through the firewall). Then at least I will know if the brake lights arent working because the dashboard light wont illuminate.

Thanks again for all your help,

Andrew
Andrew H

Andrew - A red LED wired into the brake light circuit does the job nicely and it can be tucked into the corner where the dash meets the side of the car. This puts the light in an unobtrusive spot and yet is quite visible tothe driver - I did the same thing because I became downright paranoid ofter having several brake light switches fail in quick succession. Good luck - Dave
David DuBois

Hi all.

My hazard warning switch failed earlier this year, and the replacement 'pattern' switch has just failed after it's first use.

Fortunately I found an '82 mini in my local breakers, and the hazard switch from it is now in my BGT and working perfectly.

I suggest that members stock up on used original parts from the few 70's and early 80's British cars that are left in the breakers.

Meanwhile, back at the thread, Morris Minors were fitted with a hydraulically operated brake light switch, and there are still a few 'moggies' being broken.

Don
Don

This thread was discussed between 01/11/2003 and 05/11/2003

MG MGB Technical index

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