While we're on the topic of electrical gremlins in
our LBCs, it's worth discussing ignition primary
voltage drop through the ignition switch.
Colin Campbell, in his wonderful old book on
sports car engines, brings out a good point; The
primay ignition current in our cars passes through
the ignition switch. As these switches age and
corrosion builds up, the internal contacts become
resistive and lower the available voltage to the
ignition. This is often WORSE under load so a
bad static measurement (ignition on, engine NOT
running) will almost always be worse when running
and especially worse at high engine speeds.
If one measures voltage at the battery of my MG
Midget the meter reads about 12.4 volts (with a
full battery charge). Read that same voltage at
the 'SW' or '+' connector on the coil and it reads
about 11.2 volts! In this car the ballast wire has
been removed so the voltage drop is in the switch,
connectors and/or the associated wiring harness.
That drop doesn't seem too bad at first blush but
it is a 10% drop! ...and that is the static
measurement.
To address this problem, I am first going to clean
and/or replace all the connectors. We all know how
bad Lucar connectors can get. Once those are known
good, I will retest the drop and rework the ignition
switch, if required.
While still on the topic, another good test is to
measure the voltage across the points when the
ignition is on, points closed and engine not running.
For this reading one needs a digital VOM unless one
already owns an expensive analog VOM and is willing
to out it in peril. There should be VERY little
voltage read across closed points. Campbell argues
that properly aligned and clean 'contact breakers'
in a performance car should produce no more than
0.1 volts across the contacts. My meters are not
reliable at voltages that low and 'points' are not
manufactured today the way they were in Colin's day.
If I can get the alignment right and clean, my cars
typically read about 0.3 volts.
So that's it for Tuesday. Too many meetings and
too many other fun things to think about.
regards,
rick
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