Michigan L90B problem

i have a Michigan l90b will start fine but then shutdown. I tested the solenoid its good. 24v to ground at the solenoid. but at the plug power wire to the ground wire its 20v slowly drops to 16v and the hold side of the solenoid lets go. if i probe the ground wire at the solenoid with a test lead and touch to ground it will run fine. 24v is present at all times , seems that the ground is switched on and off with the ignition switch.

Clean and check your grounds and go from there.