r0ckst4r

So I just took a another 3 hour 30 minute flight from CT to SC. I only used the defrost for heat. The auto pilot did not get hot and functioned normally. I tried to feel around for air leaks after switching to the floor heat vent but could not feel any but holy hell do those pipes/hoses get hot. It may have just been the ambient heat back there for the long flight (the autopilot is the lowest in the stack and thus closest to the pipes) but it's never happened before at least not that I've noticed and I've had the plane 6 years now with plenty of winter flying. I'll still try and do some more inspection now that it's in the hangar but still a little bit of a mystery

I believe I do have to optional defrost fan. When I pull the defrost another fan comes on and the air is diverted up to the defrost vents and away from the floor vent. When I have some more time to work on this I'll put up some pictures of where the hoses are going. I can give a description that there is a hose that comes from the firewall to a metal box which then has a hose that goes down to the foot vent and another up to the defrost vent. the diverter and fan I assume are in that metal box. The avionics fan is buried behind everything in a less than accessible location as many of you know. Fortunately for me it currently works.

So the blower works fine. I tried to follow the ducts but it's kind of a jumble back there. However, the floor heater duct runs directly under my autopilot. I tried to check the integrity but its hard to feel an air leak until the blower is on. I will check this next flight but it seems to be the culprit. It would explain why diverting hot air to the defroster ducts which are no where near the autopilot significant cooled it.

The avionics fan has its own circuit breaker? Is it activated on the Master or Radio Master switch?

1995 M20R with KFC150 autopilot Took a long, almost 4 hour flight today. About 3 hours in I noticed some ups and downs with the pitch. I went to touch the autopilot and it was very hot. Much hotter than it should be. Now it was between -5 and 0 C at altitude for most of the flight so the heat was on the whole time and coming from the lower vent. I disengaged autopilot and hand flew for a while but it did not seem to significantly cool the unit. I then pulled the defrost lever which changes the heat from the lower foot vent to the dashboard vent aimed at the windshield. This does not heat the cabin as well but it also significantly reduced the temperature of the autopilot unit to its normal operating temp (by feel). I re-activated it and it seemed to function normally for the rest of the flight. My questions is this. Is it possible that I have some sort of leak in the ductwork blasting heat against my radio stacks? I have a DME and radio above the KFC-150 that were also hot to the touch. The KFC-150 autopilot is on the bottom of the stack. It would maybe explain why switch the vent cooled the electronics. Or..... I've read about the cooling fan for the electronics failing and overheating the stacks. Maybe it was just the fact that switching the vent didn't heat the cabin as well and it cooled off just because the ambient air was cooler? Is there a simple way to test these hypotheses?

So here is the voltmeter result. Sure enough, it gets about 24V at the socket. (engine off)

I just happened to have a scanned copy of my POH and I'll be damned, it's 28V at the cigar lighter. They even specifically warn you. I'm still going to test this out when I get to the hangar.

I was actually thinking about that myself as well as there are a few things on the plane the for some reason do convert the voltage to 12v. I'll give it a test when I head to the hangar again and report unless someone chimes in first with an answer.

I was looking at a USB charger for the "cigarette lighter" socket in the M20R. I know its a 28v system and commonly available usb adapters say they are 24v compatible. I'm just wondering the importance of those extra 4 volts. Am I over thinking this or am I setting myself up for an electrical fire?

Just to post the resolution of this for future searches I sent the unit out to @Jake@BevanAviation and as anticipated there was significant capacitor leak that damaged the board. The unit was able to be repaired and is now back in action functioning normally. Thanks again for all the help.

Thank you. I'm assuming the plane can still be hand flown with the unit out and the circuit breaker pulled. Does this hose need to be capped or plugged?

Does it just pull out with an Allen wrench like the other radios? Any special procedure?

PM sent. You will be getting that unit as soon as I can send it.

I would be headed there ASAP but Witchita is pretty far from me in coastal NC. Going to have to find someplace closer. That sounds like a absolute possibility since it seems to perform normally cold but fails after time.

Attitude indicator was the first suspect before the weird flickering lights. I can't imagine how a bad attitude indicator input to the autopilot would cause it to spaz out like that. I am fairly certain it is option one or option three.