Fig. Fig. 1 Dual diaphragm distributor operation
This system is used on 1975 and later models.
When the engine is idling or operating at low speeds under light load or deceleration, the exhaust gas temperature is low, resulting in incomplete combustion of the air/fuel mixture. To prevent this, ignition timing is retarded under these conditions to maintain high exhaust gas temperature.
The units in the Ignition Timing Control system are:
Dual-Diaphragm Distributor-This distributor has both retard and advance mechanisms operated by vacuum.
Thermo Valve-This valve is used to protect the engine from overheating. When coolant temperature reaches 203°F (95°C), the advance unit is allowed to operate, causing an increase in engine speed and a decrease in coolant temperature.
Single diaphragm distributor-This distributor has a single diaphragm vacuum advance unit, which advances the ignition timing as engine vacuum dictates. The single diaphragm distributor must not be interchanged with the dual diaphragm distributor. The distributor operating curves are different and would cause increased emissions. A thermo valve is not used with this type of distributor.
Distributor maintenance is at tune-up intervals.