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 - electrical problem--brake lights

I am nearly ready to put my '79 B back on the road after several years of tinkering and rebuilding. I had major electrical problems but have everything for the most part working now but found that the right rear brake light does not come on. I have tried different bulbs, no change. Tried a different taillight mounting, no change. When the lights are on and I hit the brake pedal, the tail light goes out and no brake light. The left side of the car works fine. I do not have the trunk light wired up because there is a short somewhere that I have not found. The purple wire is just hanging back there. Under the dash, I have purple wires with a white tracer just hanging and I believe that is for the door/courtesy light which are not hooked up yet. I have one purple wire with green tracer just hanging and I have no idea what it is for. I am heading back to the garage in a few minutes to start fighting it again. Any Ideas? Do the brake lights only work when the ignition switch is on? My brake lights (one light!)will only work when the ignition is on and the fuel pump ticking away. With the ignition off and I hit the brake pedal, I get no brake lights.

Larry Youngblood

Am I correct in assuming that you're trying to figure this out without the correct wiring diagram? I wouldn't try that. I'm trying to find something on the net to help you, but so far no good.

Derek Nicholson

This might help:-

Derek Nicholson

This might help as well:

Sound to me like you've got a ground problem. Try running a ground jumper from the taillight housing to a good ground and see if that makes a difference...

Rob Edwards

Here's a pretty coloured one:-

Geez, Rob. That's fantastic. Duely saved to my hard-drive. That's exactly what he needs. Especially if he has a printer that'll handle 11" x 17" paper and laminating machine handy. Or is Kinkos open today?

Derek Nicholson

Thanks! Found the link to it on the MGB Experience BBS. Dan does nice work.....
Rob Edwards

Looks like you need pages 25 & 32, Larry. Not sure what the differences are, other than being from Bentley and Haynes, respectively. But it's nice to have 2 available. While trying to figure out the wiring on another LBC, I did come across an error in the wiring diagram. I think it was a Triumph. (Let the jokes begin!)

Derek Nicholson

Or for the KISS principle:-

Isn't the 'net wonderful! In days of yore, you'd have to conjure up an electician wizard.


Derek Nicholson

Larry - Now that you know where to get a good wiring diagram (while it is ultimately a good idea to get it printed at teh full 11 X 17 size, it can be printed in a smaller version on your home printer), Rob has pretty well nailed the problem, you are not getting a ground to the light socket. I have run across this problem on a MGA several years ago and found it to be the crimp that holds the socket to the mounting plate. It seems that corrosion builds up in the crimp joint and the only real fix is a new assembly. Good luck - Dave
David DuBois

Success, Success!!! Brake lights now working as they should. Turns out when I put a ground jumper on the socket as Rob suggested, I had light. Checked all black gound wires and fittings, could see no problems. Thought about soldering a wire to the socket and attaching the other end to a body fitting but decided to check out a friend a few miles away and he had another taillight housing. Switched the housings and everything is like new, except for the chrome finish on the working unit. Oh well, brake lights are more important than chrome shine! Thanks to all who responded and yes, I have a Haynes diagram that has been getting me through all this. I had seen some of the schematics that some of you have forwarded but had forgotten what sites they were at so I have printed out all of them today. Thanks again, everyone...

Larry Y.
Larry Youngblood

This was a symptom of a bad ground, which can occur in three places:

There is no wired ground connection to the rear light cluster and so it relies on the mechanical fixings, and you can easily get corrosion on the surface of the wings. The symptom of this is casting of the light cluster being at greater than 0v with respect to ground when the brake-lights are on.

Secondly it could simply have been a corroded builb holder, which would result in greater than 0v wrt ground on the body of the bulb, but 0v on the plated bulb holder.

Thirdly I have come across one other case where the above two were fine, and it was a bad connection between the plated holder and the casting.

If the chrome on your 'bad' light cluster is better then the replacement I would certainly be investigating which of the above three was the problem and fixing it.
Paul Hunt 2

This thread was discussed between 07/05/2006 and 08/05/2006

MG MGB Technical index

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