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MG MGB Technical - brake lights

my brake lights are not working. not the bulbs, wiring at back looks ok as the side lights and winkers'work. has anyone had' a similar problem please?thanks
les howard

les. The brake lights are independent of the side marker lights and turn signals.

Look for power at the brake light switch, power through the switch when the brake pedal is depressed, then, follow the wire rearwards until you find the problem.

It would help to know what year your car is as the brake switch changed over the years.

Les Bengtson


You did not list what year you have. If you have a B with the brake light switch on the brake/clutch cylinder cover, it may be out of adjustment.
BEC Cunha

Bruce. Please note the difference between "Les" and "les".

I have noted, in my post, that the different years had different brake light switches. Thus, "les" needs to let us know what model year his car is.

But, the basic information I have provided should be sufficient to check out the system.

If not, he need to take a copy of the factory wiring diagram and trace the power into the switch, through the switch, and back to the stop lights.


Les Bengtson

Ground wire in trunk?
Dan Robinson

A misadjusted brake light switch on the pedal box cover can result in the brake light being on continuously, but will not keep them from coming on. Check the ground to the brake lights, depending on the year of your car, it should be under one of the mounting bolts for the registration plate bracket. Also check that you are getting power to the brake light switch on the green wire. If power and ground are good, try placing a jumper wire from one terminal of the switch to the other while the ignition is on. If the lights come on when this is done, the switch is bad. Be aware that replacement switchs are very poorly made and tend to burn out in a very short time. To get around this problem, make and install a relay/arc suppression circuit and install it at the same time you install the new switch. For information on making and installing this circuit, see my article at:
Cheers - Dave
David DuBois


If your car isa CB one it has a pressure switch on the inner wing on the drivers side. This is a VERY common failure and almost certainly your problem if you have one fitted. Takes at least five minutes to unsrcrew and put a new one on! I now keep a spare in the garage as I have gone through one a year! Don't take the old one off until you have the new one in hand as it may seep brake fluid. When removing protect paint with a paper towel or similar and then unscrew and imediately pop the new one on. If the old one is difficult to remove you may need to grip the union where it is fitted. No need to bleed the brakes. Reconnect the wire and check the lights and you are good to go.

67 BGT

PS To test the switch just take the two wire off the switch and connect them. If the lights come on it is your switch that is faulty.
I D Cameron

Thanks for the tremendous response on my problem I am sure I will be able to sort it now. For info the care is a 1964 roadster. I am a new boy to MG's so having to learn fast.
No doubt i will be caling on your help again.
Happy motoring
Les Howard
Les Howard

les howard-

"... wiring at back looks ok ..."

You can't check wiring alone by looks. You need a test light or a multimeter to determine if the electrons are actually traveling through the wiring.

Below is a link to upgrade the brake light circuit to help extend the life of replacement switches.
Many people have complained about new replacement switches failing within days or months of installation.

This thread was discussed between 19/06/2007 and 20/06/2007

MG MGB Technical index

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