Possible pitch autotrim failure?

About 6 months ago while getting an IFR certification done I went to depart and noted increased friction on motion of the elevator during preflight prior to even turning on the master. We checked for anything involving the linkage to the elevator and it all looked fine. Removing the autopilot pushrod from the elevator bell crank resolved the friction as well. The owner of the avionics shop took a quick look at it and thought the servo motor was likely going bad. The friction seemed very “ratcheting” and slightly worse when power was turned on to the auto pilot, but would persist with all power off for several days; but once, when not flying for more than a week I did note it had resolved completely but still returned after power was turned on.

I was able to find an unused old servo and initially it seemed fine but after a few flights showed the same friction as the original servo. I also noted that just moving the old servo out of the airplane felt smooth but if I connected it back up to the autopilot cable it immediately developed the ratcheting friction again. I then also noticed immediately after removing the cable from the new installed servo it immediately felt normal, then re-attaching the cable caused the friction to return. It seemed like there was some voltage bleeding into the servo causing at least partial activation of the stepper motor.

Finally I removed the autotrim module from the circuit and plugged the cable coming from the autopilot control head to the servo directly and it remained smooth on elevator motion and the autopilot functioned normally in flight ( but obviously without autotrim control).

It seemed to me the autotrim is the issue and I was wondering if anyone else has had a similar experience or know of further tests to confirm that. Also is the autotrim unit repairable or do I just need to replace it? The unit is a TruTrak Sorcerer on an RV-10.

Ted lockard
RV-10 flying since 2009

I have the TruTrac Vizion 385 control head with an autotrim module installed and have not experienced any issues. Good luck in getting this resolved.

The ratcheting you feel is because this servo uses stepper motors. A very minor ratcheting feel is normal when not flying, but usually not noticeable in flight. Now, the auto trim should be connected to the trim motor, not the autopilot servo. I’d suggest looking very carefully at all the autopilot wiring, seeing if somehow the auto trim is crossed with a servo wire.
PS. As you may know, TruTrak was sold to Bendix-King. Mid continent can do service on some units.

I have had the autotrim for several years and the ratcheting feeling only developed in the last several months. Usually moving the elevator control is silky smooth with no detectable ratcheting. I may be mixing my terms on servo versus stepper motor. There is a cable coming from the control head. Originally it attached to the electronics on the stepper motor. I was using servo to mean the entire device moving the elevator bell crank, including the motor. After flying a few years without auto trim and listening to the autopilot bark at me for up and down trim I had Stein wire up the harness for the autotrim. He was the one that did the original wiring as well. The control head bundle attaches to a bundle going into the auto trim module and that then attaches to the servo. When I go back to the original wiring without autotrim everything is nice and smooth again.

Ted Lockard

Possible autotrim failure

Never did get a satisfactory explanation for the original problem as described above. Never received a reply from any of the avionics repair places contacted. I did correspond with Andrew who thought the autotrim could be the problem but didn’t sound like it was a common symptom. I therefore resorted to the “ throw money at it” method of diagnosis and purchased a new autotrim module. This was advertised as a “plug and play” unit. The first thing I noticed was it did not have the black speed adjustment knob on it even though it was pictured in the advertisement for it, and there is a hole in the case for it but doesn’t appear that the mechanism is inside.

Unfortunately after installation I tried to ground test it and there is no down trim response at all. The panel control head will indicate down trim needed when nose up pressure is applied to the elevator. The unit just continues to apply nose up trim. When nose down elevator pressure is applied it also applies nose up trim.

Does anyone know if there is a different pin out for this unit or is it just defective?It is a Bendix King XCruze 8000-028 part number.

Ted Lockard