KAP 200 Autopilot Disconnects when turning on Instrument Panel Light Rheostat

[Lima Whiskey]

I thought I would post a recent problem we had with our 1980 M20J.  

It started out with the NAV flag on our HSI starting to randomly popping in and out.  However, the autopilot continued to fly the proper guidance from our GNS530W.  Everything appeared to be working properly from the self-test that the 530W runs when it boots up.  

Then things got even crazier.  

On a night flight home from visiting our grandson in Dayton, Ohio, at 9,000 feet, in and out of IMC, the autopilot dropped both ALT and NAV but the power to the unit stayed engaged when I turned on the Instrument Panel Light Rheostat.  It didn't happen immediately but about 15 seconds after I turned on the Rheostat.  I turned the Autopilot off and on, reengaged ALT and NAV, it stayed up for a couple seconds and then dropped.  At the time, I didn't realize that it had anything to do with the Rheostat, so I ultimately ended up hand flying the last 1.5 hours of the flight, in the dark, in and out of IMC (an aside - I know some will argue the wisdom of flying in IMC at night in a single engine plane - I get it - it wasn't fun - and if the big fan at the front of the plane stopped spinning it would have been less fun - but that isn't what this post is about).

The other thing that I need to add to this little mystery is that since we purchased the M20J back in 2018 the Instrument Panel Rheostat was either off or on, there was no dimming.  The panel lights didn't turn on until the rheostat was turned all the way up.

After a visit to the avionics shop, a flight with the technician to show him what was happened (where some times the NAV and ALT dropped in the autopilot and sometimes the entire autopilot turned off), and 8 hours of shop time trying to diagnose the problem, the problems were solved.

First, the avionics tech discovered that there was no ground to the Trim Power Switch.  Adding a ground stopped the autopilot from shutting off when the Instrument Panel Light Rheostat was turned on.  It also stopped the NAV flag from randomly popping in and out.  I don't know the why behind it all, but the ground wire fixed it.

Second, when the avionics tech installed a new rheostat, the instant he turned it on, he "smoked" the rheostat.  Apparently there was too much power coming from somewhere.  He ultimately discovered that the power lead to the transistor which controls the dimming system had become disconnected.  This transistor was located on the right side of the panel behind the circuit breaker panel.  Once he connected the wire, the new rheostat dimmed everything properly.

Hopefully this post is helpful to someone who experiences the same problem and they will be save 8 hours of troubleshooting and about $1,000.  I am extremely thankful to the guys at our avionics shop, it took some time but they didn't give up.