This is a for a cars about the break lamps. They have a subdivision of the green circuit with its own in-line 35 amp fuse supplied by the white/brown (ignition relay) circuit feeding things like heated rear window, indicators, heater fan and tach, which leaves the original green circuit fuse (2nd one up in the four-way fuse block) feeding things like reverse lights, stop lights, washers, wipers, and circuits associated with the seat belt warning lamp and time delay buzzer. This is the figure of the diagram circuit.
The old switch diced and sliced. I had cut off the ring at the top
of the 'nut' which is what looked like was holding the plastic bit in, but it
still wouldn't come out. Only later could I see that I would have had to cut a
ring around the nut level with the point where each flat joins to get below the
'peened over' part. Surprising number of bits - rubber diaphragm in the bottom,
then a metal disc with a pip on each side but a tin cover on the diaphragm
side, then the plastic bit with the two contacts, a light spring holding the
moving contact away from the fixed. Pressure on the diaphragm pushes the metal
disc up against the moving contact, compressing the spring, and bringing the
moving contact to the fixed, so completing the circuit. The design of this is
such that as the moving contact is pressed against the fixed by the pip on the
disc, it will flex, which imparts a rubbing action between the two contact
surfaces, helping to keep them clean. Compare this with the simple 'bridge'
connection on modern switches.
Burnt and pitted
contacts, hardly surprising it had failed, surprising it lasted so long! As far as fluid contamination goes although the
rubber diaphragm is probably squeezed pretty tightly between the body of the
switch and the plastic part forming a seal, maybe silicone can squeeze through
that, and maybe modern switches don't compress the diaphragm as tightly anyway,
and maybe the contact material is just poorer. But these (a very old switch
much before my time 21 years ago, and quite possibly original) look pretty
flimsy anyway.
