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MG MGB Technical - brake, side and parking lights on when braking

I have a 1977 MGB and noticed yesterday that the parking lights (front + rear) and all the side lamps come on when the brake is applied. The brake lights come on as well. I'm not sure how long this has been going on as I'm always in the driver's seat when the brakes are applied. There hasn't been any electrical work done on the car in quite awhile. It was painted and all the lights removed about a year ago but it's hard to believe that the problem could have existed that long and not be noticed sooner. I've checked the archive but didn't see any relevant threads. Any suggestions would be appreciated.
RL Lucus

Hi,

The first thing I would check is that all lights have a good ground connection. What you describe is typical of bad grounds.

When it was painted, did you clean back to metal for the ground connections?

Herb
Herb Adler

I didn't and I dont know if the painter did but that sounds like a good place to start. Thanks.
RL Lucus

As Herb says, it sounds like bad ground connections.

Personally, I would prefer to add ground wires to the lamps than remove paint and hope that the lamp bodies make good contact.
Dave O'Neill 2

Rear lights are grounded by black wires attached to the license plate mounting bolts in the trunk. If this is not good, the brake lights ground through the side/park bulbs. They all light but not well.

FRM
FR Millmore

Also
check to make sure someone hasn't fitted a single element/contact globe in a stop/tail light hole
This will join the two circuits together
I know the locating pegs on the globes are different but I have seen them forced in there and the result is what you have there
cheers Willy
William Revit

Thanks to all of you for the advice. I'm a little embarrassed to admit how many times I looked at the bulbs before I finally noticed that the double and single element bulbs on the passenger side were indeed switched. And yes it had to have been forced in because it was a bit stubborn to remove. I guess it's been that way since the painter reassembled the car. Thanks again. Ralph
RL Lucus

"Rear lights are grounded by black wires attached to the license plate mounting bolts in the trunk"

Not in any MGB or schematic I have seen. Only the reversing lights and later number-plate lights have earthing wires as described, the rear light clusters were always earthed via their mountings to the wing. Ditto chrome bumper front clusters, rubber bumper indicators had earthing wires.
PaulH Solihull

Paul -
Got me! Because on all the cars I've been working on for years, I fit earth wires to the lamps and run them to the existing black circuits, since the earths are unreliable. The US cars have side markers which have B wires for earth, so there is one in the area. I do the same with front lamps.
This greatly reduces the trouble factor, especially flashing rate problems, but of course not when people fit the bulbs wonky.

FRM
FR Millmore

Ah yes, side markers as well ...

I've not had a problem with the rears but at the front on the chrome bumper I put a nut on the studs first which goes against the front of the panel, as well as the securing nut on the back, which can be tightened up pretty hard. That was primarily to hold the lamp unit away from the panel, as the chrome bezel would have cut into the paint otherwise. That and Waxoyl, plus a couple of extra foam backing pieces, has been fine for over 20 years so far in all weathers.
PaulH Solihull

Paul -
When cars get painted with catalyzed paint, or rusty, or just 17 coats of normal paint, bolt-to-body earth is questionable at best. The goofy tin-with-teeth nuts work fine by cutting through the paint, but then you get rust, and those nuts are hard to find if broken or lost. Body shops are very good at loosing them! So I put earth leads on the studs if the lamps are good. If the lamps (esp, fronts) are questionable, as was very common when cars were in regular winter use, the multiple dissimilar metal path to earth from socket to body panel is extremely unreliable. I prefer to solder earth leads direct to the socket, but it is admittedly a difficult solder job! Then there are the miserable plastic sockets used by Triumph...

FRM
FR Millmore

When restoring mine I couldn't get the number plate lights (chromed thingies on the overriders) to work until I added earth wires ... a problem the factory never had :o)
PaulH Solihull

This thread was discussed between 13/05/2011 and 17/05/2011

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