1. Remove wire 0 from the VR. Set your meter to ohms. Test continuity from wire 0 to the ground terminal in the control panel. It should read continuity. If not, the wire is broken.
Remove wire 160 from the VR. Remove wire 162 from the excitation breaker. Test the wire for continuity. If it is broken, fix or repair it.
2. Remove wire 16 from the start contactor relay. Remove wire 4 from the VR. Under the control box, disconnect connector C1 ( it has wires 0, 4, 162 etc). Set your meter to DCV. Connect the red lead to wire 4 at diode D1. Connect the black lead to a good ground.
Set the switch to START and note the voltage. It should have 12V.
3. Remove both wires from resistor R1. Set your meter to diode test. Place the red lead to the top terminal of the diode, and the black lead to the bottom terminal. It should read an open circuit. Now switch the leads around. It should read about 0.5V. If you do not get those readings, the diode is bad.
4. Measure resistance of the resistor. It should read about 25 ohms.