Western star speedo problem

So only when your in OD it does this?

 

What code is it throwing? If no code, it could be the voltage to the ECM. Could it be starving for fuel?

 

Most likely code ring is moving to far from the speed sensor, as it's possible the output shaft is moving around enough to cause issues. High split is direct and removing the thrust load could let output move far enough if the tranny is wore out. Put it in high split and see how how much play you have at the yoke.

 

Yeah my theories were only valid if it was happening from low to high, not direct to OD

 

Steve, something is moving fore and aft when the splitter shifts. Either the case or the shaft. The VSS is losing the signal from the shaft. The derate is to prevent tampering with the speed governor, of course.