Welcome to our resource for MG Car Information.



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 lights

Hi all, I have just purchased my forst mgb roadster and am loving it! one problem I have is the brake lights do not operate. I am thinking matbe its a switch issue but cannot find where the darn thing is! I have traced the wiring Green & green/purple to a small round device but this has no means of connection or pedal operation. 2 Questions, Where is the stop light switch and what is this other thing that I have found? Its in the footwell above the pedals.

Many thanks for any assistance
John Valentine


there are different positions used for the switch, depending pon modell and year af the car.
If it is a late RB car with dual line sevrvo system, the switch is positioned behind the pedal box, pointing to the driver.
Fi it is a early 70's car, there is a small switch atop the cover of the pedal mechanism, pointing to the front.
If it is a 60's car, it should have a hydraulic switch positioned on the inner fenders on a 4-way brass distributer fitting.

Hope this helps


Much as Ralph says but the dates are a bit different for UK cars. You have found the correct colours, and the switch should either be part of a four-way splitter for the brake hydraulics on chrome bumper cars on the right-hand inner wing in the engine compartment, on the pedal box for rubber bumper cars up to May 77 (remote servo), after that on a bracket in front of the pedal (combined servo and master cylinder).

Sound like you have one of the latter type i.e. 77 or later car. With the brake pedal released the switch is operated to turn the lights *off*. As you push the pedal the switch is released to turn them *on*. If the switch is malajusted the usual problem is that they stay on all the time. If yours aren't coming on at all it is an electrical problem. With the ignition on bridge the green and green/purple connections at that switch, and if the brake lights come on the switch is faulty. There is some evidence that unles you can get an OE switch the alternatives are pretty poor and cannot handle the load of the lamps, the switch failing again in a short time. In that case you will have to replace the switch again fit a diode and relay.

If bridging the switch doesn't light them, then check with a meter or test lamp that you do have 12v on the green with the ignition on. If not there is a 4-way bullet connector somewhere that takes a green from the fusebox and feeds it to the electric screen washer motor and reverse light switch as well as the brake light switch. If they work power must be getting to that connector at least, if not the problem is probably at the fusebox.

If you have 12v on the green at the switch then chock the pedal so the switch is released (i.e. on). The green purple should come out of the main harness by the fusebox and join to the same coloured wire in the rear harness which goes down in front of the drivers toe-board and under the car. Check that 2-way connector for voltage both sides. If that's OK the green/purple surfaces again in the boot by the right-hand rear light cluster at a 4-way bullet connector, with one wire going to that light cluster and another going across the car to the other.

Of course, both bulbs could be blown!
Paul Hunt 2

Wow, thanks Paul, great info. I will investigte in the morning. Any idea what the other little widget might be? its around an inch and a half in diameter and approximately one and a bit inches long. It just has the Green and Green/purple wires connected to it and has been mounted just above the pedals inside the vehicle. I am a trifle suspicious of this as it is held onto a metal tab simply by a cable tie. Could it be some kind of relay? It is completely sealed and has no markings that I can see. Curioser and curioser said the MG owner!

As you can see from my technical jargon I am not an engineer!

Thanks again.

John Valentine

Do you have pictures?

On the one hand I'd say that is almost certainly going to be the switch, but it shouldn't be anywhere near that diameter, that sounds like the switch for the hydaulically actuated system! I'm beginning to think that the PO was sold a hydraulic switch for the mechanical system, couldn't see how to mount it (the mechanical switch has a threaded shank that goes through the hole in the mounting bracket with a lock-nut to adjust its position), so just hung it up there with a cable tie! The hydraulic switch *does* need pressure to operate it, unlike the mechanical switch which works the other way round, which if it *is* the hydraulic switch explains why the lights aren't on all the time.

I'm attaching an drawing of the two types to help you identify. Item 12 is the hydraulic, Items 31 and 32 are the mechanical (the ones you need).

Paul Hunt 2

Found it and solved the problem! it was indeed the switch which has now been repaced and all works fine - thanks for all your help.

John Valentine

This thread was discussed between 13/08/2007 and 17/08/2007

MG MGB Technical index

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