Speaking of Sherlock Holmes and battery cut-out switches, I was lead to
install such a switch by the following mysterious experience:
Upon pulling into the garage and turning off the ignition of my '63 B
following a very normal commute home, I heard a faint "klink" sound. Given
all the other noises the car and the wind had been making, the sound would
have passed unnoticed by anyone less familiar with the car, but after
driving it over 200,000 miles, I thought I knew its every nuiance and
klinking wasn't one of them.
I had one foot out the door when, wondering what the sound was, I glanced
back at the dash. I had installed an ammeter 20 years earlier after the
batteries had been discharged by a slipping fan belt without lighting the
ignition idiot light. The ammeter needle was pegged on discharge. The
"klink" was obviously from the needle hitting the inside of the meter
housing with some force, so presumably the discharge was much greater than
the indicated 30 amps. I assumed there was a dead short somewhere that
could easily be pulling more than 100 amps; an imminent fire was then a
real possibility.
For lack of any better ideas, I restarted the engine. The ammeter,
mercifully, returned to normal. With the engine running, I disconnected
the battery then shut off the ignition. Apparently the contacts in the
cut-out relay (in the voltage regulator) had stuck closed, allowing the
battery to discharge through the generator once the generator stopped
turning and was no longer producing an opposing voltage. (When stationary,
the generator winding looks like a long, *heavy* wire to ground - the
cut-out relay opens this circuit when the ignition is turned off.) The
resulting high current had welded the relay contacts together beyond
repair.
Had I not installed the ammeter two decades earlier, the first indication
of the problem would probably have been the smell of smoke several minutes
later. This scared the H out of me. I promptly ordered a new voltage
regulator and a battery cut-out switch. Installed behind the drivers seat
on the vertical wall of the battery enclosure, the switch is very
accessable and its use is now as automatic to me as setting the parking
brake. I use it every time I shut off the engine. I recommend it. Later
cars with alternators do not have cut-out relays and so couldn't have this
exact mode of failure, but they are dependent upon an isolation diode to
provide this protection. Diodes are not infallible either.
Get a battery switch and carry a fire extinguisher!
Russ Wilson
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