Does the Commander rate the Motor Sergeant (91X)?

As stated by some others, no regulation dictates a rating chain. It depends on the organizational structure and what the supervisory chain is. The Motor Sergeant would be rated by either the Motor Officer, PL or by the XO as an additional duty if there is one. At different command levels, the Maintenance Sergeant would typically be rated by the Battalion Maintenance Officer usually a Maintenance Warrant Officer or, possibly, the Battalion Executive Officer. Like I said, it depends on structure, for instance at CSSB’s, there is a maintenance warrant in the SPO but the S-4 has a LOG officer and the maintenance NCO is in the S-4, so in this instance you could possibly have the log officer as the rater .

SFC (Join to see)

As others have stated, here is not a regulation that dictates a rating chain. I depends on the organizational structure of the unit and what the supervisory chain is.

Typically, at company level, the Motor Sergeant would be rated by either the Motor Officer (usually an additional duty for a Platoon Leader) or the Executive Officer, if there is one.

At battalion level, the Battalion Motor/Maintenance Sergeant would typically be rated by the Battalion Maintenance Officer (typically a Warrant Officer) or, possibly, the Battalion Executive Officer. The normal chain I have seen is Motor Sergeant --> Maintenance Officer --> Executive Officer.

Like I said, it is strictly up to whatever the rating chain is, as established by the commander.

For myself at the company level it was the company XO and at the BN level it was the BMT.