Brushed DC servo motor speed measurements issues

[shamooooot]

I have a servo motor with 4 brushes on the commutator. The motor is connected to a controller that enables the user to control the speed and at the same time it has an analog RPM meter which most probably measures the speed of the motor through the two additional brushes on its commutator.

I had to give the motor a push to make it spin before which drove me to do some maintenance for the motor (checking the brushes, greasing the bearings, cleaning the commutator), but the RPM meter then became very inaccurate like reading about half of what the speed should be. I have tried and tried, but matter what I do I can't calibrate it back to how it was.

I was wondering what is the mechanism for this speed measurement to see if I can find another way to measure the speed continuously through these brushes. Or what may cause this issue.

[m k]

Datasheet says it's a tacho and recommends a filter.
https://docs.rs-online.com/479b/0900766b806bd72a.pdf