This picture shows how I test the contacts inside of the solenoid. I have one lead, in this case the black lead on the side of the solenoid that is always hot. The red lead, in this picture is barely visible but is hooked to the switched side of the solenoid. The picture was taken while the engine is cranking and as you can see there is only a fraction of a volt drop through the solenoid. When the starter is not turning, the voltage would reflect full battery voltage. This picture only shows if there is voltage loss through the solenoid while it is pulled in
If it shows full battery voltage either the solenoid is not getting power on the small control wire or the solenoid is not functioning. Anything more than 2 V or so the contacts in the solenoid are pitted or corroded and the solenoid needs to be replaced
If it shows full battery voltage either the solenoid is not getting power on the small control wire or the solenoid is not functioning. Anything more than 2 V or so the contacts in the solenoid are pitted or corroded and the solenoid needs to be replaced