My S-Tec autopilot when I bought the plane 8 months ago held a heading to within a degree.

Recently, whether in heading or nav mode it seems to track like a drunken sailor. It will start to bank but then seems to forget what it is doing until it is 15-20 degrees from its original heading then it meanders back only to fly past the desired heading/track on the opposite side. It does not seem

As it is flying way off course if I apply a little pressure to the yoke it seems to wake up and realize it has gone to far and stops the turn. I thought maybe the AP wasn't engaging at all but there is force on the yoke.

It goes through the motions of intercept, CAP SOFT, SOFT but no matter what mode it wanders though I believe the bank angles are proportionately smaller based on the mode.

It still holds altitude beautifully but you can't use altitude hold unless you have Nav or Hdg engaged.

I suspect this is an expensive trip to the avionics shop but wanted to see if anyone knows of a simple fix first.

I just got my Comanche out of the avionics shop due to problems with my STEC 60-2. My issue was that it wouldn't engage altitude or VS mode correctly - it would just descend aggressively. I initially sent the pitch computer to STEC thinking something was wrong there, and after $1,000 and "some adjustments and a new accelerometer" nothing was fixed when I test flew the bird.

I brought it to a good avionics shop and they did some good old fashioned troubleshooting. There were a couple loose ground connections. There were a couple bad pins in connectors. They hooked it up to the STEC "break out box" to check signals and things were good. The servo was requiring more voltage than it should so they made some adjustments there and fixed it. Overall, I spent another AMU with the shop and the 60-2 is working perfectly.

I have no idea if my problem will have any similarity to yours since yours is roll mode, but there are a lot of connections and pieces to the 60-2 and I would find a shop that can do some good diagnostic troubleshooting before you start replacing boxes.

Agreed. Finding a good avionics shop is a challenge. Most take the approach to just send whatever appears broke back to the factory. I had that happen with my HSI. Turned out it was just a broken wire from the slave unit. Only reason we discoved this was after they reinstalled the "overhauled" HSI it still didn't work. That and the fact the broken wire got caught in my control yoke jamming it prior to takeoff.

If you ever send you HSI back to the factory make sure you tell them ahead of time NO NEW KNOBS. Seems that every HSI has to have the little knobs replaced at $60 apiece! My old knobs were perfectly fine.

One of the issues the autopilots have is that the these are analog motors. When they are new, the motor will start when there is about 1 volt on the motor. If the brushes start getting carboned up, then it take more voltage to start the motor. The will cause over control which is what you may be experiencing. This can be tested by putting 1 volt to the servo motor Increase this up to 1.5 volts. If you do not hear the motor at all, ( it will be a small growl), then the brushes need to be changed.

Regards

jvb

Thanks for the information, John. I think that was part of the problem on mine, it wasn't responding at the right voltage. They "burnt in" the motor - whatever that means. It's working great now. It just needs a Ron & John's panel upgrade....

Every now and then my STEC 60-2 will start oscillating but goes away after a few up & downs. I can disengage the auto pilot and it will go away instantly then re engage and everything is normal. George

Thanks. I have tried engaging and disengaging. Mine wanders on heading. Altitude is rock solid.

When it starts to wander I apply a little pressure and it stops and then goes back the other way to get back on course. Unfortunately, it sort of just keeps going the other way. It is oscillating about 20-30 degrees each side of the desired course. Seems to be a static oscillation.

I had an older Century 1 in another aircraft. It had the exact issues. The servo motor would not move until the voltage was quite high - three volts. The servo was removed, cleaned, lubricated, and the brushes cleaned. Worked great after that.