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MG MGB Technical - Hydraulic Brake Light Switch

Gents,

OK, jobs mounting up now, head gasket retorque tonight but also, this weekend I want to do some electrical upgrades to put the head lights onto relays and also sort out my back brakes which dont come on unless I put a fair bit of pedal power in!

I am pretty well armed for the headlights but for the brakes I want to replace the switch as the lights start dim then get brighter as I press the pedal harder - suggesting the contacts are burnt. I intend to fit a relay on the brake switch as well to protect it in future. My problem is I am having trouble from my desk at work to identify a correct replacement switch.

part number 081405 on this link http://www.autoelectricsupplies.co.uk/product/182

seems to be correct, can anyone confirm? It is a 1965 B roadster I have and really wanted to order the switch today so it arrives in time for the weekend.

If anyone knows of any other UK suppliers to check I would be greatful.

Regards

Chris
Chris

From the image it looks like Moss part C16062A (Lucar type) fitted to GHN3 34662 onwards. This is from Mar '64 onwards according to Clausager so fits your description.
Regards
Richard
Richard Coombs

Chris, be wary of these 'modern' switches. They are very unreliable, to the point that some of them only work when you stamp really hard on the pedal.

Try and use an old (secondhand!) Lucas one. They last forever!

Colin
Colin Parkinson

You might also contemplate adding a relay to limit the actual load seen by the switch, enabling it to last longer than your brake pedal.
David "gotta get around to it on mine, too" Lieb
David Lieb

Chris - The others are correct in saying that the new switch will probably fail after a short period of time. The only switches that I have seen recently that will hold up is either NOS, red box Lucas or the very expensive heavy duty switch from Ron Francis Wiring in the US http://www.ronfrancis.com/ Part number SW-32. The other alternative, as David L. states is to make and install a relay/arc suppression circuit when you install the new switch. Information on making and installing the relay/arc suppression circuit can be seen in the article, Brake Light Relay in the Other Tech Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

Thanks for the comments gents, I have sourced a replacement locally for £7 but before I changed it, I checked the switch.

As expected, slight pressure on the pedal the resistance across the switch drops to zero, then with an ammeter in series, slight pressure on the switch and only 1 amp drawn, stamp on the pedal and hey preto, full 3 amps - another case of resistance is fine but switch is screwed!

Anyway, the 1 amp is enough to drive a relay so I wired one up temporarily and hey presto, slight pressure on pedal and it all works fine. I found Davids web page very helpful to do the wiring for the relay, I will make it semi permanent tomorrow and then look to make a permanent wiring solution when I install more relays for the headlights.

So a bit of a result really, kept the original switch so no need to bleed brakes, and all works fine!

Thanks for the help.

Regards

Chris
CHris

In fact you can replace the hydraulic switch without having to bleed the brakes so long as you do the job quickly and make sure no-one goes near the pedal while you have the switch unscrewed.
Mike Howlett

That switch was used by most BMC & later Leyland cars in the 60s & into the 70s. My wife loved Mini's, so I've replaced quite a few hydr. break switches in the past. You might want to extend your search outside the MG box. FWIW. Barrie E
Barrie Egerton

Chris - Using an Ohmmeter to read the resistance across the switch is not going to give you a usable reading (0 on the ohmmeter can be anywhere between 0 and a couple of ohms. The current reading is much more accurate for telling you what is going on in the circuit. From what you read and experienced, the switch has excessive resistance across the contacts and really should be replaced before it gets any worse. The relay only draws about 140 milliamps, so it will operate with a lot more resistance in the circuit than the light bulbs will.

Mike is correct about not having to bleed the brakes after replacing the switch, but the only reason one needs to be relatively quick about screwing the new switch in is to reduce the amount of brake fluid that drains out of the union (be sure to put a folded paper towel under the union to catch any brake fluid that drains out. Since the brake master cylinder is mounted higher than the union, there is no chance that air can draw into the brake with the union open unless it is left open until the master cylinder is drained of all fluid. Cheers - Dave
David DuBois

Great gents, I dont want to replace the switch for antoher reason - the only one I managed to find locally doesnt have spade connectors, it has screw connectors (like an electrcial block connection) and I think looks a mess. Hopefully the existing switch will last with the lower current, if not, I will put the other one in.

Many thanks for the advice, back over to the low compression thread now!

Regards

Chris
Chris

Chris

I saw some where on a MG forum that a late model Harley Davidson brake light switch will work. Thay are the same threads, look like an OEM Lucas, and have Lucar terminals. For Mk1s a good replacement is the OEM for the Volvo 122 series. the OEM volvo switch is supposed to be much heavier duty.

Kent
DK McNeill

Chris, I had the problem some years ago where you had to press very hard on the brake to make the stop lights operate. By fitting a tiny micro switch on a bracket just up above the brake pedal and wiring it in parallel with the stop light switch, the problem was easily overcome. The first micro switch was rated at 5 amp and soon failed so I then fitted a 10 amp one and that is performing ok. The troble is that we are switching DC current which is always very hard on switch contacts.
Another benefit with the micro swich is that you can adjust it to operate just as you touch the brake pedal, seconds before the brakes come on which gives a little extra warning to the driver behind.


O. McNeill

I agree with David about the unreliability of ohmeter readings (ohmeter current being miniscule compared to 'real life' which on less than perfect contacts give different readings), but would advise you to go for voltage readings rather than current. This is because it avoids interrupting the circuit, as well as the fact that ammeters affect circuit conditions more than voltmeters. With a voltmeter connected to the green/purple (output) terminal, with the pedal released you should see 0v, and operated ypou should see battery volts. If you see less than battery volts there is resistance and volt-drop somewhere. If you do the same on the green (input) side, you should see battery volts all the time. If you do, then anything less than battery voltage on the output side is due to the switch itself, but any drop from battery voltage is due to bad connections back through the green, white and brown circuits to the battery, some of which will be inevitable.

Rubber bumper cars had pedal operated switches rather than pressure operated switches, but these are designed to turn the brake lights on almost as soon as you have hit the pedal, not just before the brakes start being applied. Also DC lamp loads aren't as hard on contacts as inductive DC loads, it is a factor of the very poor manufacure of modern replacement switches that they can't even cope with lamp loads. Even fitting a relay isn't enough on its own, as they are inductive and even though lower current they will still cause modern switches to fail unless you put quenching diodes across the relay winding. See http://www.mgb-stuff.org.uk/wn_electrics.htm and click on 'Schematics', 'Lighting' and 'Brake Lights'.
Paul Hunt

This thread was discussed between 24/06/2009 and 26/06/2009

MG MGB Technical index

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