Does anyone know how to test the speed sensor on one of these machines ?
They can be tested with a breakout harness but I seldom bother. I either watch for erratic motor rpms for that side on the display or just swap it out depending on the symptoms, I always had a spare on the truck. Most of the time you will get 1 track locking up and a mistrack code for the opposite side.
the machine will turned hard to the right so is it a bad left or a bad right sensor
Most likely the left side sensor. If a speed sensor fails the TCU may see 0rpms on that side, it will then try to stop the opposite track to match it. Check your fault codes. Does your machine have a clear window over the TCU so you can see the red digital display. Should be a black box by your left elbow when seated.
On the monitor, select Menu, then Codes, then choose Active or Stored codes and scroll through by hitting the Next button. Are there any codes that begin with 907.X/908.X? 907 is for the left sensor, 908 is for the right sensor. After the machine starts to pull, do you see the check service light on the monitor? I agree with Deeretracks, if its pulling to the right the left sensor may be dropping out. Check the wiring on the left and unplug the sensor to check for dirt.
You might also be able to check Menu>Diagnostics>Live Values>Speeds and hit Next to scroll through and look at the motor speeds. Might not be available depending on how old your machine is though. Good luck