[Ed. Note - my 66 MGB has a different kind of rheostat, I'm not sure what
year they switched over to this "printed ciruit" design. I opened mine
and cleaned the wire "loops" and "no resistance area" with a pink eraser
and had quite good results. You can easily bypass the rheostat entirely
by simply attaching all of the red wires to terminals on the
same side of the rheostat. -Skye]
Like so many other MG owners, I discovered that my panel dimmer switch wouldn't turn. Not on just one, but two of my MGB's. Since this item is about $70.00 I decided to see if I could fix it. According to my local British car parts house the Queen of England personally makes these for Lucas in her spare time. Since she is the Queen and can only make a few each month the price has to be high.
This is a factory sealed unit so at least one of the seals will have to be drilled out or filed off. I use a 5/32 drill bit to cut off the spread top of the stud and while still in the vise I use 3/32 bit to make a screw hole to reattach the cover when I'm finished. While looking for a small screwdriver it would be a good time to get the first beer.
If any of the traces of circuitry on either the cover or circuit disk are coming loose try to carefully push them back into position. Some of the traces can be torn or missing and the switch may still mostly work. Substantial amounts of traces missing and the switch will not function and the lights on the dash may not work.
Note 1: If this is the case, see if you can find another used unit and combine the parts from both. I was able to obtain a dozen from my local MG place that were in a junk pile for a couple bucks each.
Very carefully & slowly pry up the disk off the hex shaped spindle. The spindle is actually another disk with the hex part sticking up in the middle. Do not slip your screwdriver or other pry device very far under the circuit disk. You will crack the spindle. When the circuit disk is removed it is better to just leave the spindle disk alone. On one fourth of the units I have taken apart the spindle disk has shattered. (See Note 1 above.)
If all the parts seem in pretty good shape you are in luck. The thing will probably work when you put it back together. Often you will see something like the broken spindle shafts and loose traces shown in this picture.
Carefully clean any old grease, bits of metal from drilling, and trash out of the inside of the unit. Once again don't try to do anything to the electric traces. They are too fragile to mess with. If the five finger buttons on the connector block are dirty or discolored; you can lightly polish the tips. Do not use sandpaper (it will leave scratches that will rub off the electric traces).
Now is a good time for the next beer.
If you can still focus your eyes and hold a small screwdriver you can proceed with the next step.
Reassemble the thing. Slowly slide the cover back into place and put a small screw into the hole you made previously.