The purpose of the solenoid is to provide a massive amount of current to the starter.  The solenoid requires much less current than the starter, but still enough that if any of the four or five connections between the ignition switch and the solenoid are corroded there could be enough resistance to prevent the amount of current required for the solenoid to operate.

A relatively simple fix for this is to install a 'hard start' or auxillary relay between the ignition switch and the solenoid.

This is a schematic, over simplified, of how the solenoid and starter are connected to the battery through the igniton switch.  There are a number of connectors and fuses in the line that can increase in resistance, therefore reducing the current to a level that will not activate the solenoid.

The auxillary relay has one of its contacts connected directly to the heavy wire going to the battery.  No current limiation here.  The coil requires much less current than is required by the solenoid.  The rules of electricity (E=IR for you electrical scolars) imply that less current draw means less voltage drop and everything works.

For more information or to order an auxillary relay email order@dyno-metrics.com or call (813) 833-3918.