All ignition and fuel injection functions are controlled by the Digital Motor Electronics (DME) control unit. Ignition timing is fully electronically controlled; there is no vacuum advance or manual adjustment. Ignition functions are calculated from internal maps and from the same sensors used for fuel injection. On vehicles with an automatic transmission, the control unit will retard ignition timing briefly when the transmission is about to shift up or down. For this reason, there is a data link between the DME control unit and the transmission control unit.
For complete testing of the Digital Motor Electronics (DME) system and sensors, refer to Emission Controls , Emission Controls. Descriptions and testing of the DME system will be covered under Electronic Engine Controls.
Variations of this ignition system are used on different engines. The M42 engine uses distributorless ignition with a coil pack mounted on the inner fender. The M50 engine uses distributorless ignition with a coil mounted above each spark plug. Most of the remaining engines use a distributor mounted on the front of the camshaft.
On all engines, the primary side of the ignition system is completely contained in the DME control unit. Only the secondary side including coils, cap and rotor, high tension wires and spark plugs may be serviced.
Never connect or disconnect any ignition system or control unit wiring with the ignition switch ON. Never use an ohmmeter with the ignition switch ON. Improper test methods can instantly and permanently damage electronic components.