A drive line (A) for testing a component (D) of a drive train of a vehicle comprises a torque sensor. The torque sensor measures AC variations in torque due to the operation of the component (D). The component (D) is coupled to the drive line by an adapter having a coupling for coupling the drive line to the axle. The torque sensor may output both AC and DC components of measured torque variation. A servo controller controls the drive line so that the drive line is driven into abutment with the axle and then backed off by a small preset distance.