Ported or manifold vac for overdrive bypass vac switch?
Many different ways to control most things and Trailbeast's wiring is different from yours in some ways. The OP switches ground the terminal through the plumbing to the trans case and are independent of each other. You could ground either solenoid and cause lockup or overdrive to engage at any speed and gear, since this bypasses the governor pressure in third gear that activates the OP switches. I'll draw a schematic.
Many different ways to control most things and Trailbeast's wiring is different from yours in some ways. The OP switches ground the terminal through the plumbing to the trans case and are independent of each other. You could ground either solenoid and cause lockup or overdrive to engage at any speed and gear, since this bypasses the governor pressure in third gear that activates the OP switches. I'll draw a schematic.