The main drive pulley rotates slow when ACU is on on 2001 Isuzu VehiCROSS

Why does the main pulley belt drive of my isuzo crosswind XT rotates intermittently slow when i turn ON my ACU?

Asked by for the 2001 Isuzu VehiCROSS
I guess you are talking about the crank shaft pulley. There is a layer of rubber between the inner and outer part of the pulley to damp vibration. The outer part (the belt drive) might be separated from the inside part and when there is more load on the pulley, it starts turning.