Ignition timing is a measurement, in degrees of crankshaft rotation, of the point at which each cylinder's spark plug fires. It is normally listed as a specification in degrees before or after Top Dead Center (TDC) of the compression stroke.
On the 4.2L engine, the timing can be adjusted by turning the distributor in the engine. A graduated timing degree scale located on the timing case cover is used for reference when timing the ignition system. A milled index notch in the vibration damper is used to align the No. 1 cylinder ignition position of the crankshaft with the correct timing degree mark on the graduated scale.
On the 2.5L and 4.0L engines, the Electronic Control Unit (ECU) or the Engine Controller, as referred to on later models, determines the amount of spark advance and electronically changes timing as operating conditions vary. For further information, please refer to Electronic Engine Controls in Emission Controls .