A common squawk heard with the Collins Autopilot AP-106 / 107 systems is “Autopilot will not engage.” Both of these systems have a computer, 913K-1 or 913K-1A, which are the Flight Director (FD) computer and Autopilot computer all in one. If the computer flag in the ADI pulls out of view and the FD functions still appear to work but the autopilot itself will not engage, you most likely have a defective servo causing the problem.
Four servos, each with its own monitor output are diode isolated and put into one pin on the autopilot computer. If any one of them "fail," it will keep the autopilot from engaging. Since the autopilot monitor does not care if the servo is plugged in or not with this system, unplug the servos one at a time to isolate which one is bad. The four servos are the aileron, elevator, rudder and trim.
If the computer flag is in view and the FD does not work, there is either a computer failure or input power missing. It would be a good thing to check the connectors on the computer. It is not uncommon for the contacts to lose some of their spring tension and end up with intermittent contact. A good cleaning and visual inspection can reveal the condition of contacts. As with most autopilot systems, this one also requires a valid attitude gyro to engage.