The full load switch would be part of the throttle switch. Not a lot of fun to replace. As the wiring is easy to get to and a switch can only do a couple things wrong, I would make it look open and see if it is the problem. Since both open and closed are acceptable positions the fault would have to be from a closed full throttlesignal while the idle switch is also closed.

You can try this with a new switch but it sure would be easier to do at the easy to access three wire connector just forward of the air horn.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician