hello , I have a PC-78 MR-6 that has some sort of electrical issues. The first problems that we noticed were with the solenoid that controls the swing parking brake. prior to this point all of the functions worked low and high speed the fuel gauge and temperature worked to include warning buzzer. we replaced the solenoid and while waiting for that new solenoid we cleaned and lubricated the old one to see if we could get by. you could start the machine shake the boom a little bit and then turn it off and turn it back on and you could rotate but if you jiggled too hard or were on uneven ground it would trip and lock up still. my current symptoms are fuel gauge is not moving the hour meter is not moving the temperature gauge is not moving I am stuck in slow speed and the replaced solenoid is not acting like it should it still locks up if you take it off and put it back on everything is fine but then it will lock up again eventually it seems to maybe be a heating up issue with it?
My guess at this point is that there is a bad ground somewhere. You will need a service manual with a wiring diagram and the locator pages to be able to find the places where the wires ground to the frame.