I don't know this equipment but i'm assuming its a dc drive type? Your wondering if the tacho is giving an incorrect error feedback? One way to test this is either swap tacho's, get someone to drive the lift on inspection and test the tacho output and then test this against another lift. Also you can test the volts/amps at the machine and then verify this with another lift. Most dc high speed lifts overspeed purely because of an over current situation either directly via the dc drive or because they are told to by the encoder/tacho or a friction issue.
