Brake light switch

Technical MGB discussion
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David Witham
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Brake light switch

Post by David Witham »

My car is a February 1974 MGB that was originally sold in California USA. When I converted it to RHD I retained the type of brake light switch used on such cars which is operated by the brake pedal. I think of these as the mechanical brake light switch as opposed to the hydraulic switch used on the UK spec cars at that time.

I have been more than disappointed by the durability of the mechanical switches. Since I bought the car in 1992 it has done no more than 40k miles and has got through about 4 switches. Last time I introduced a relay in the brake light circuit to reduce the load on the switch.

But this morning when I set out to the NEC at the first set of traffic lights the driver behind told me the brake lights were not working. I pulled into a side turning, got my multi meter out and checked the circuit. Surprise surprise, I discovered the switch had failed again. So it was back home to get another car.

So I am considering converting to the hydraulic type of switch. The plumbing changes will be straight forwards. Before I do so I am interested in other people's experience with the hydraulic switches. Are they reliable?
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Paul Scott
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Re: Brake light switch

Post by Paul Scott »

Hi Dave,
I'm unsure if the hydraulic switches are any better.

1975B switch on peddle box.
When my un relayed switch failed I found the fourway female connector the switch attaches to was getting hot. My wife was in the car foot on pedal and I had fingers in the wiring, lucky to find it.
Further investigation under the black rubber the connector was cracked and broken.

When I replaced the switch I used a relay with diodes and capacitor fitted as per https://www.mgexp.com/article/installin ... rience.net

So far no issues, apparently the capacitor helps save the switch.

Unsure if it will help.

StaySafe
Regards
Paul
1975 MGB Roadster
Webguru for MGB Register
David Witham
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Re: Brake light switch

Post by David Witham »

Paul, the link is interesting. My relay includes the diode but I don't have the capacitor on my switch.

Clearly, an option is to replace the mechanical switch with another, add a capacitor and carry a spare! After all, I carry a spare dizzy with points in case my electronic ignition gives up.
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Paul Scott
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Re: Brake light switch

Post by Paul Scott »

I have my Capaitor located on the relay accross the terminals 30 & 86 we use them in Kenwood Chefs at work.

The one I used is similar to this one https://www.ebay.co.uk/itm/0-047uF-275v ... usEALw_wcB

.047MFD x2

Regards
Paul
1975 MGB Roadster
Webguru for MGB Register
David Witham
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Re: Brake light switch

Post by David Witham »

Paul, that is a great help.
Ian Fozzard
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Re: Brake light switch

Post by Ian Fozzard »

My 72 BGT has the hydraulic switch fitted into the hydraulic lines on the RHS inner wing adjacent to the fuses.
I have owned the car since 1978. Never had any issues with the switch which I am sure is the original component. I guess that is a recommendation.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
Dave Linkson
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Re: Brake light switch

Post by Dave Linkson »

The hydraulic switch in my 1972 BGT failed about 7 yeas ago, and by the look of it was original, so had lasted well, but I understood that the modern day switch a replacement pattern part were of poor quality, so I acquired a relay specially made for the job which included the cable and connectors complete from Paul Hunt the MGB guru who has a website called hammer and spanner (mgbstuff) where he comments on said switches and problems encountered and where the relay part can be purchased.
Bear in mind it it was some years ago, so maybe there is an update on the site regarding the issue. BTW the brake lights are still working well.
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Michael Barclay
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Re: Brake light switch

Post by Michael Barclay »

Hi Dave
Owned my B for 43 years and still on it original hydraulic switch, so I guess you can say they are reliable.
If you do go down the relay route can I suggest fitting a transil rather than a diode or capacitor. This is a transient voltage suppressor and will do a better job by limiting any Back EMF to a max of 30 volts.
They are available from RS components Part No 486-4202, price 71p including VAT. However you have to buy a pack of 5. They are also perfect for fitting to petrol pump points as a replacement for the capacitor.
On the subject of parts quality, I was having this discussion with Rimmers at the classic car show on Friday. There take is that customers won’t pay the price for quality!!
Best regards
Mike Barclay
David Witham
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Re: Brake light switch

Post by David Witham »

Thank you everyone for your comments.

Well this afternoon I removed the switch and opened it up to look inside.

The mechanical switch body is in 2 parts. A black plastic part and a pressed steel part that overlaps it and is crimped where they join.

I cleaned up the inside and reassembled the switch. It now works again. So I will stay with the mechanical switch for the time being.

As a precaution I plan to buy a replacement switch to carry in the car so I can do a quick swap if the problem arises again.

Also to deal with the safety issues, I need to get in the habit of testing the brake lights against the inside of garage door before I open it to reverse out.
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