what should I do?

I currently have two identical trucks, except the drivetrains.
One is a 3126 Cat with a 6 speed manual.
This one is Mercedes engine and Allison automatic.
Just fixing what is there.

I leave truck in park, drop PTO in gear with switch,
engage throttle increase switch to 1,500 RPM to operate.

Manual truck differs as it is air that drops PTO into mesh with trans. gears.

I believe the switch energizes a solenoid to engage PTO on this one.
That whole electrical engagement is possibly suspect,
maybe solenoid is weak??? IDK?

(Cabover Freightshaker) --- I had a chance to buy a running one for 2,000 bucks,
w/a wet-line, and 40 ft alum. dump for another $3,500, needed everything.
Shoulda, Coulda, Woulda.
But that's another story for another day.