Duncan Aviation tech reps have fielded a number of calls recently from Lear 35/55 operators needing assistance because the autopilot pitch command is inoperative in one direction. The squawk occurs when the A/P pitch command is initiated by either the pilot or copilot's trim switches and cannot be cleared by replacement of the autopilot computer or autopilot electrical box. After some investigation, we found the issue to be a shorted isolation diode in the associated MT-101( ) mach trim computer.
The mach trim computer fault can be verified by measuring the pitch command logic at the autopilot computer. In the failed direction, the autopilot computer will receive simultaneous command logic signals on both the pitch-up and pitch-down inputs. In the working direction, only one signal will be seen, either the pitch-up or pitch-down.
Another troubleshooting method is to disconnect the mach trim computer. If the A/P can be commanded in both directions, then the mach trim computer is faulty and needs to be repaired or replaced.
Contact Duncan Aviation Component Solutions to obtain a loaner unit while your mach trim computer is repaired.