Laura,
When the car will not start... do you hear any "CLICK"
sounds, or is a matter of just the starter turning very
slowly?
TWO ITEMS:
FIRST:
The solenoid needs a good inrush of current to pull
back the internal plunger. This plunger has two functions.
One is to engage the starter gear to the flywheel, second
is to close the large contacts on the solenoid, providing
power to the starter.
If the solenoid gets too hot, the resistance of the coil
will increase. Less current flowing through the solenoid,
less ability to pull in the internal plunger.
Quite often with age the harness and starter switch will
have higher resistance, leading to a voltage drop. The
voltage drop in the harness and switch contacts mean less
voltage to the starter solenoid. This will also drop the
current of the starter solenoid.
Some times just cleaning up all the spade connectors
with a little emery paper, and cleaning everything will
solve the problem.
SIMPLE FIX: Add a relay to the starter circuit. The relay
will be closed by the ignition switches "start" function.
It takes a lot less current to close the relay than operate
the solenoid. The power source for the relay then comes
directly from the battery through a fusible link. Other
side of the contact goes to the starter solenoid spade connector.
VIOLA: A good solid 12 volts direct to the starter solenoid.
Use a GOOD QUALITY RELAY: Bosch, Hella, Siemens all come to
mind (USA & GERMAN built). You should get a 30A relay.
SECOND:
Starter is getting overheated. A simple shield on the
header to block radiated heat will help. The shield can
be made from an old cookie sheet, and attached to the
header by a couple of hose clamps.
Also do not over look the routing of the starter cable,
battery connections, starter connection. All should be
clean and in good condition.
A quick ROAD TRIP solution is to hose down the starter
with water. Yep, just use running water to run over the
starter to cool it down. I had to do this with an older
V8 Chevy Van that would "cook" the starter on road trips.
That or go for an hours lunch. ;-)
Cheers,
Tom Walter
Austin, TX
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