The GM solenoid has two windings within it, a pull and a hold winding. The pull winding does the initial job of pulling in the Bendix drive, but to do so draws about 40A from the purple wire/ignition switch. Over time, these connections may be loose or corroded which adds resistance to that circuit, reducing the current available to the pull winding. Compounding the problem is as the solenoid gets hot, it's resistance goes up too, further limiting the current it can draw, thereby limiting the force with which to pull in the Bendix.
Using the external Ford-type relay, (it's not really classified as a solenoid), the path for the GM solenoid pull current is very low resistance, coming thru the large cables, not having to come from the bat thru the firewall, thru the ign switch, thru a neutral start switch, thru the purple wire to the solenoid.
The Ford relay only needs an amp or two to trigger it, which the original solenoid circuit has no problem delivering.
There is the additional benefit that the bat cable to the starter is normally electrically dead, only live when you start. Considering it has to wind around the exhaust manifold, etc. it's an added safety.
George
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"...out to my ol'55, I pulled away slowly, feeling so holy, god knows i was feeling alive"....written by Tom Wait from the Eagles' Live From The Forum
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