Mechanical Inspection: Compression

Check engine mechanical condition with a compression gauge, vacuum gauge, or an engine analyzer. See engine analyzer manual for specific instructions. Ensure battery is fully charged, engine is at normal operating temperature and throttle valve is fully open when checking the compression.

On vehicles with coil-on-plug ignition, remove Auto Shutdown (ASD) relay to prevent engine from starting when checking the compression. ASD relay is located behind driver's side strut tower at rear corner of engine compartment and contains a Gray 9-pin connector. See Fig 1 . Once compression test is complete, reinstall ASD relay and use scan tool to clear Diagnostic Trouble Code (DTC) from Powertrain Control Module (PCM). See appropriate SELF-DIAGNOSTICS article for clearing of DTC.

On vehicles with Distributorless Ignition System (DIS), ensure electrical connector is disconnected from ignition coil to prevent engine from starting when performing compression test.

Fig 1: Locating ASD & Fuel Pump Relays

Courtesy of CHRYSLER CORP.