"The switch", hereafter called TS, has three terminals with three wires
going to it. The two heavy terminals are connected between the battery
& chassis ground when TS is on. The third terminal connects the point
side of the distributor to ground when TS is off & opens the point
ground circuit when TS is on. This grounds the points when TS is turned
off. A ground anywhere along the point grounding wire grounds the points
& the engine doesn't run. Fortunately there are usually only two switch
positions & one or the other will usually make the car run. When the
switch fails, which is frequently, It's anybodys guess about what
position may or may not work. If TS fails internally, there is no
battery ground or there can still be a battery ground connection but the
point grounding wire can be grounded at the same time. Or With less than
total failure, the contact resistance gets high & there is not enough
cranking voltage to start the engine.
A note about "the switch" - It has very small contacts with very little
contact pressure. The moving contact is less than 1/16" thick. Way
under-designed for the 200 or more cranking amps that it is required to
carry. To make things worse, the spring which holds the contacts
together is a very small spiral wound compression spring. If there is
any resistance in the contacts, due to dirt or corrosion or low spring
pressure, the contacts will heat which heats & anneals the spring.
Less spring = more heating = more annealing = more contact resistance =
failure. Even more than normal starting current can cause contact
heating & start the failure chain. To top it off, the whole movable
assembly with it's spring, is held together by a miniature C clip which
can wear into the bakelite housing & reduce spring tension or fall off
altogether. When this happens, no switch at all.
If it sounds like I don't like TS, it is only because I think it is the
worst thing that Lucas ever made. Guess I already said that, but it
bears repeating.
Dave Russell
BN2
pdeturck@rochester.rr.com wrote:
> This where my confusion begins. If the switch fails internally, what POWER
> does it provide to the car? Or if the wire grounds anywhere along its length,
> isn't that the same as running correctly through the cutoff switch to ground?
> It seems to me that a short to ground in the black/white wire would allow for
> the coil to work properly and still provide a spark. What am I missing?
>
> I could see an intermittent short to ground causing eratic conditions which
> would
> likely prevent proper ignition, but a continuous short to ground anywhere
>would
> still enable the coil.
> -pd-
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