Engine will only start with AUX Battery Start

j-d wrote:
The Linked Wiring Diagram
suggests to me that the AUX connects the House Battery to the Chassis Battery terminal of the Starter Relay (perhaps called Starter Solenoid, more on that in a minute). This suggests a problem with the Chassis Battery Connections or Cables. As noted, check GROUND as well as the Hot Side.

This vehicle has TWO Solenoids. The one in the fender liner sends power down to another solenoid mounted on the Starter Motor. I'm calling the one one on the Fender Liner the "Starter Relay" and it looks like The copper terminals are to Battery and Starter, the small ones are to the wiring that energizes the Relay/Solenoid. It's probably what you're hearing CLICK. Then it sends the power down to the Solenoid on the Starter Motor, shown on top of Starter