Autopilot trim
 

The Cessna 400A Autopilot uses a trim servo, mounted in the STB boom, as this a/p is a 2 AXIS system with ALT HOLD is does require an elevator trim system.
However the original option when the a/c was made did not specify electric trim (thumb wheel switch).
To add electric trim you will require a voltage regulator which keeps the trim speed in limits and a series of relays to change the trim from electric to autopilot.
There was a kit avialable from Cessna at one time.
The voltage regulators were a little expensive.

Hope this is ok.

Peter Somers