A stubborn autopilot trim issue on a Cessna 172 that a simple resistor check fixed

The plane kept pulling left, and after tracing wires for an hour, I checked the trim servo feedback resistor with a multimeter. It read 2.1k ohms instead of 1k, and swapping it out solved the whole thing. Anyone have a good source for those specific Dale resistors besides the usual suppliers?

2 comments

2 Comments

the_james1mo agoMost Upvoted

Wish more people would check those feedback resistors before diving into the autopilot computer or servo motor itself. Had a similar pull issue on a 172 and it turned out to be a cold solder joint on the resistor leg, not even a bad value. Mouser usually has the Dale RN series in stock but check the wattage rating on the original, some of the replacements run hot and drift faster.

eric_ramirez671mo ago

...or you could just buy a whole new trim servo and be done in 10 minutes instead of hunting down a 50 cent resistor that might not even be the real problem. I've seen guys spend two hours tracing wires and soldering in a new resistor only to find out the servo motor itself was dragging.