The wire that goes from +coil to starter solenoid is connected to
a contact inside the solenoid. When the starter solenoid is closed
(ignition mode) the contact touches the center of the heavy copper
bar that supplies many amps to the starter motor. The idea behind
this is the following: when the engine is running normally the coil
is connected through the resistor cable to +12V. The resistance of
the cable means that only about 8V actually reaches the coil which
is apparantly what it was designed for (assuming the coil is
original.) To make the car start better, the wire to the solenoid
was added so that the coil receives a full 12V during ignition to
produce a hotter spark.
I had troubles in this same area and found several related problems
in my '77 MGB. The bundle of wires that goes down to the solenoid area
makes a sharp bend around a corner of the engine block. It is held
there by a loop of metal that is screwed to the block. On my car
the wires at this point had worn through and were shorting to
the block and to each other. This caused the small contact inside the
solenoid to melt which meant no more 12V during ignition. The way
the wires shorted in my car caused loss of 12V to the coil, but it
happened in such a way that the wires didn't melt (I'm not really
sure why---perhaps because the engine died when they shorted which
cut off the juice.) From your description it sounds like the resistor cable
is shorted to ground near the coil end. If the wire going to the
solenoid melted and the resistor cable is hot then the point
where they are commonly connected (the coil) is grounded. This
might be through the coil itself (check it's resistance) or through
one of the wires (as in the tight bend I described earlier).
Car wiring is difficult to trace using standard techniques such
as measuring resistance or voltages. Since the cables are so heavy
they can often carry the current of a short without melting and since
the power sources put out so much current, they can withstand a
short somewhere in the system without the voltage dropping. And
since everything is connected in parallel, resistance measurements
always show zero. Try to isolate the short (and be aware that
it might be multiple shorts occuring simultaneously) by disconnecting
wires at both ends and then looking for shorts to ground or +12 when
the wire is supposedly not in the circuit. It's a real good idea
to disconnect the battery and avoid wearing jewelry when working
on this stuff.
By coating the wires with silicon glue and then wrapping
them in a plastic tube at the bend (a hack, I know---it was easier
than opening the loom) I fixed the short and restored power to the
coil. I also took apart the solenoid and rebuilt the ignition contact
(I can give you some tips on doing this if you're interested.)
I've looked at that so called resistor cable fairly closely and
unless something is hidden in the cable wrapping it seems to be
just a an ordinary piece of wire; probably selected to be a certain
gauge so that it has a specific resistance such that the coil gets
8V when connected to 12V through it. I do not think it should get
hot or even warm. Even with it dropping 4V across it's length it
should stay cool since current is drawn through it in pulses (via the
distributor.)
Ron
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