MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey 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 line pressure failure indicator

The other day I told a friend that I thought I had all electrical items on the car working properly. Of course the next day I discovered I'd been overlooking something!

Turns out I'd forgotten all about the brake line pressure indicator light/test switch. It does nothing when pressed. As such I'll need to track down this last(???) problem.

I know I'll have to check for a bad or loose bulb on the switch (located on the left side of the dash) first, but beyond that where do I go? Just where is the sensor located? And are there any other issues involved other then a sensor and light/test switch on the dash?

Thanks,
BH
BH Davis

There is a plug on top of the brake light sensor that needs to be cleaned now and then.


Dan Robinson

Oh.......1973 MBG Roadster........sorry.

What do you mean by "brake light sensor". I know of the brake light switch on the pedal box, but a "sensor" is new to me.

Thanks,
BH
BH Davis

BH,

there is a brass fitting mounted on the LH inner wing where four brake lines an an electrical plug are connected. If you pull the plug and bridge its points, the warning lamp in the switch should light up. If not it might have been removed, as it is a joke to make this unnesessary OE working and it does not really help when something becomes weak with the breaking system.
I have this fitting removed from the two B's in my garage that had been fitted with this installation and wired the switch as an oil warning light that has helped to save one engine when a oilpipe started leaking.

If you want to keep the installation as it is, you will need to read about it's setting in the workshop manual, as you have to centralise the plunger inside the fitting every time the brakes have been bleeded.

Ralph
Ralph

Ralph,

Thank you !!!.............just the info I needed. Much appreciated.

BH
BH Davis

BH, Usually the when the shuttle seals leak the bridge contact between the two terminals in the plastic switch on the failure valve corrodes enough to create an open circuit between the two terminals. When the test switch is depressed for testing the circuit ground goes through the dash switch through a B/W wire to the plastic switch on the brake failure valve, through the connector strap, then through the other B/W wire to the test lamp. By-pass the plastic switch at the two wires, easy way is to remove the plug and connect the terminal pins together. If the light now works the plastic switch is bad. I have never needed to centralize the shuttle when bleedling the brakes. I think most shuttles will seize in a few years and may not work, but unless the shuttle seals are leaking the test function will probably work.

Clifton
Clifton Gordon

Clifton,

Thanks. I pulled off the plastic switch connector and the pins are indeed badly corroded. I'll clean them up and see what happens.

Thanks,
BH
BH Davis

BH, You should be able to see the strap/link connecting the two plugs pins, the link is what failed on mine, it was open.

Clifton
Clifton Gordon

Early brake pressure switches (67 only) only had one wire and no link, and the test switch tested the 12v supply and the bulb only. After that the switches had two wires, conencted by a link as mentioned, and the test switch send a gound up one wire and back down the other to prove the circuit through all the wiring and the switch body, Could still have a faulty switch, though.
Paul Hunt 2

This thread was discussed between 21/08/2007 and 22/08/2007

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now