Apparatus for controlling and diagnosing a prime mover of a vehicle traction system includes an alternator having a rotor and coupled to a crankshaft of the prime mover, a source of electrical energy, a position sensor supplying a position signal representative of an angular position of the rotor, and a control computer. The computer supplies energy to the alternator to crank the prime mover at a controlled rate. In a diagnosis mode, the computer generates a condition signal indicative of a malfunction of the prime mover, from a compression signal derived from an acceleration signal which is derived from the position signal. In a barring-over mode, the computer also supplies energy to the alternator and monitors the position signal to place the crankshaft in a desired angular position in an operation separate from diagnosis mode.