Abstract: The compression in the cylinders of an internal combustion engine is tested by analyzing the battery voltage waveform when the engine is cranked by the starter motor with ignition or fuel inhibited. The detected voltage waveform is amplified in a manner to emphasize the a-c component having cycles between positive voltage peaks, each cycle having a negative voltage peak due to a compression stroke. A missing negative voltage peak indicates a very low compression in one cylinder. The voltage cycles are integrated and the integrals are compared to detect compression imbalance. The base level of each integration is made equal to the level of an adjacent positive voltage peak.