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 TD TF 1500 - Brake light need Help

Hi Friends, I need your help.
Today I worked around my '53 TD, cleaning and inspection. Nothing removed or installed but now the Brake light doesn’t work.
I inspect all the cable from switch to rear lamps, but nothing was fond.
There is a Relay next to the brake swish but if I short cut the pressure switch with a screwdriver I hear the “clack-clack” but no light at the rear.
Any method to check the Pressure swish or the relay without removal?
I have to go to the technical inspection by tomorrow, therefor I’m under pressure. It is 2 O’clock here in Germany, time is running.
Any proposal to check something?
Thank you
GK Guenter

I had a similar problem. I was using Silicone Brake fluid and it migrated to the inside of the relay from a minor leak in the brake master cylinder. Replacing the relay and everything worked fine. I have relocated the relay away from the master cylinder and fixed the leak.

Guenter, did you have the ignition switch turned on? Bud
Bud Krueger

@ Rick, thank you I will check. Yes I use DOT5 as well.
@ But, Yes

I believe it is the Pressure switch did somebody the correct part number of this Item, I do not mean the Moss number but the standard number?
GK Guenter

Get somebody to press the brake pedal you should hear the relay "clack". if not then check that the brake switch connection is OK. You should have a live supply from the input side of the brake switch to the switching side of the relay.
If that is OK then the brake switch is faulty. If so the switch is a standard part on British cars right up to the original Mini.
You can quickly change them over without letting any air into the system. I have never had to bleed after renewal
Ray TF 2884
Ray Lee

Sorry Guenter,

It is Lucas brake light switch SMB 423
You should be able to reference it OK
Ray Lee


Try to short out the two terminals on the switch with a piece of wire. If the lights work then the problem is the switch, if they don't then the relay circuit is at fault. If it is the relay circuit then just for your test tomorrow, remove the relay circuit and connect the switch directly into the light circuit.
You can then spend time working on the problem afterwards.


J Scragg

You can check with a test light. I'm guessing that your wiring is like mine and Dave DuBois' as seen at A test light must show 12 volts at the green wire coming to the stop light switch. If you jumper that green light to the green/purple light at the relay the brake lights should come on. If that doesn't turn on the brake lights you have a wiring problem. You say that you can hear the relay clicking. The lights should be coming on. If not, your "wiring problem" is in the relay. The same thing happened to me last year. Bud
Bud Krueger

I presume you have the original set up where the brake light becomes the indicator.
Do your indicators work?, if not then check your bulbs.
Ray Lee

Guenter - If the relay is clicking when power is applied to it, then the contacts in the relay are not conducting current to the brake lights. You should be able to remove the outer case of the relay and spray the interior with something like BrakeCleen or other such cleaner. After the cleaner has dried, place a thin bead of RTV (silicon sealer) around the bottom of the relay, such that when the cover is replaced, the RTV will seal the relay.

If there is DOT 5 (silicon brake fluid) in your brake hydraulic system, I would check around the brake light for signs of brake fluid leaking, which will be the culprit.

"There is a Relay next to the brake swish but if I short cut the pressure switch with a screwdriver I hear the “clack-clack” but no light at the rear."

What about when you press down on the brake pedal? If the brake lights don't come on, press harder on the brake pedal and see if the brake lights come on. If they don't, or if they only come on with hard pressure, then the brake light switch is leaking internally (and is probably the source of the brake fluid getting into the relay). Cheers - Dave
DW DuBois

Thank you all, the Trouble maker was the hydraulic brake switch. I replace it by a new one and the system is running.
Hanky you and Cheers,

GK Guenter

This thread was discussed on 02/08/2017

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.