Functional Test
- Set parking brake. Shift automatic transmission to Park or manual transmission to Neutral. Start engine and attain normal operating temperature. Ensure A/C is off, if equipped. Stop engine.
- On 4.2L, disconnect 3-wire connector from 4 in. and 10 in. Hg vacuum switch assemblies.
- On all engines, disconnect and plug vacuum hose connected to distributor vacuum advance mechanism. Connect hand vacuum pump to vacuum advance. Connect timing light at No. 1 spark plug. Connect tachometer to coil negative terminal.
- Start engine. Increase RPM and apply 18 in. Hg vacuum. Observe ignition timing degree scale and index with timing light. The ignition timing should advance smoothly.NOTE: On 4.2L, a defective MCU or ignition system control unit can alter ignition timing.
- Remove timing light and tachometer. Ensure all connections are correct.