Relative Compression Test

The relative compression test determines the relative compression per cylinder by monitoring changes in crankshaft speed during the compression stroke of each cylinder. The relative compression test is carried out by the PCM during a 10 second cranking event. Greater compression adds resistance resulting in lower crankshaft speed. The crankshaft speed data is calculated by the PCM software. The result is a difference in RPM per cylinder. This is displayed on the scan tool in bar graph form indicating each cylinder contribution.