Your problem is burned contacts in the switch. The burned condition
causes a high resistance and when current is drawn by the light bulbs,
all of the voltage is dropped across the switch. This is why you see a
voltage in a no load condition and no voltage when the bulbs are hooked
up. The contacts are sealed behind a diaphragm, so there is nothing
you can do to clean them. I am not sure if the Echlin part number that
Bud quotes is for the T series cars (grub screw connections) or for the
later MGs with the spade lug connections, just ask for a switch for a 54
or 55 TF at NAPA and they will find it listed (but not for the Much
older TD). I have to disagree with Ted, the fitting should be 1/8"
British Standard Pipe thread and not tapered (all of the pipe threads on
the T series cars are straight rather than tapered pipe. Some previous
owner may have put a switch on Ted's car with tapered threads and
distorted the fitting. The Echlin switches may have an American thread,
which is 1 thread per inch different from British, and while they will
thread together ok, there may be enough interference to cause a leak. I
always wrap the threads with teflon pipe tape to insure a good seal.
I would suggest adding a relay to the brake light circuit to remove the
heavy current from the brake light switch. The replacement switches sold
today are extremely light duty and don't hold up to the 3 amps drawn by
the brake lights for very long before the contacts burn again (I have
had them burn in just two weeks). This is something you can do after
your event this weekend and then you will not have to worry about the
switch in the future. I have a write up and drawing for makeing this
modification if you are interested.
Good luck,
Dave
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