MGB: brake lamp problems

I have my share of Brake light problems. This time again a passing motorist shouts "hey your brake lights are not working" just what ya dont wanna hear when the weather is just turning perfect. Anyway from experience I order a new switch from moss . It was received this evening but before changing it out I tried just jumping the two switch connectors assuming this would yield some valuable info Nothing! I connected and installed the new switch outside the box still nothing. The brake bulbs look fine on inspection. The running lamps work well as do the other lamps.

I am thinking that when I jump the connector that the lights should be on and that this simulates what happens when the brake pedal is depressed with all working well.

anyone have an idea for me??

I believe if the wires are connected together the lights should come on. If they don't check out fuse #3. There's also a wire connection between the fuse and the switch that may have been popped loose. If you have a multimeter or a test light I'd recommend checking for voltage. If you have voltage at the switch then check the bulb socket, it's possible the ground is bad or even the split connection is disconnected after the switch. Make sure to print out your copy of the schematics. They're a lifesaver.


http://www.advanceautowire.com/mgb.pdf

Dumb questions?...
Did you:
1/ Pull out the bulbs and use a VOM to check the continuity across the filament?
2/ Did you test the sockets with a 12V light probe?
3/ Did you disconnect the bullets where the brake lights connect and test to see if there is current there [to ground] with your light probe when the brake is depressed?
4/ Did you use a VOM across the brake light switch connections to check continuity?

These tests will isolate where the problem is... and I suspect it is dirty/corroded sockets.

Trouble-shooting electrical faults is best accomplished with a logical end-point start and work 'up' circuit.

Good Luck.

Mark,

Did you turn the ignition key to the on position or have the car running to do your test?

I am not sure if all Bs are wired the same way or not, but my Bs are wired so that the brake lights only work when the ignition is in the on position. Power to the switch is otherwise not delivered to the switch.

Cheers,

Rob

Thanks to Ned, Peter and Rob

Yes the ignition was on and the 74 B was running. The main fuses are all ok, the bulbs are intact with circuit across filaments. I will seek out the other more proximal fuse and continue the test outlined in Peter's suggestions for narrowing it all down.

Thanks AGAIN

is that Lucas laughing ?

The problem was NOT the switch in fact the new one purchased from Moss did not work ?

Anyway, the problem was in corrsive build up in the contacts all along the brake light electrical path. Cleaning up the connectors with emery cloth, tightening the connections even at the fuse relay took care of the problem.

"The problem was NOT the switch in fact the new one purchased from Moss did not work?"

Must be a new and improved switch - By being non functional out of the box, it saves you the trouble of having to install it and then wait fore someone to roll down their window at a signal and tell you that your brake lights are not working after a few weeks ;)
Cheers,