I have more or less the same problem.
After several years copter and drone flying with my excellent performing APM 2.5 I finally switched to Pixhawk. Than I decided to rebuild an old Riding Lawnmower into a full autonomous vehicle using the good old APM 2.5.
For that on the mower I created Frontwheel steering using a windscreen wiper motor, Sabertooth Syren 25 and a pair of chain sprockets to the existing steering rod.
I started Mission Planner (latest), uploaded Rover Firmware 2.50, added APM 2.5 and 3DR GPS LEA6 and made al necessary connections.
After completing the Initial Setup and doing a number of tests everything seemed fine and I created a simple mission with 5 Waypoints.
Then I went outside and find out that the steering worked very well. The Mower moves perfectly in the Manual Mode.
However, after switching the vehicle into AUTO or RTL, it just moves forward in half circles and does’nt find or follow the waypoints. All tests for the presence of MODE’s and GPS signals are positive but the APM refuses to follow them.
Changing parameters, reading the wiki again and again, search the forums, studying the dataflash logs but nothing solves my problem. I’m desparate now (and suffering from my endless patience!)
I hope someone can help me?
Thanks in advance, Greetings from Holland
APM version 2.5 Steering on CH1, MODE CH8/Ardurover 2.50/Mission Planner (latest)
Spectrum DX6i Mode 2/Steering on right Stick (AIL)/AUTO on Gear Switch/No Throttle Input(Later on)
Sabertooth powered bij 12V Battery/ APM powered via Sabertooth 5V conn. and APM inletcircuit jumper placed
Dataflash logs attached