Distributor Vacuum Control: Description and Operation

System Schematic (California Models):

Sub-diaphragm Vacuum Advance:

The vacuum advance ignition timing control system alters timing in response to changes in manifold vacuum.

The distributor has 2 vacuum advance diaphragms. The sub-diaphragm is used to improve fuel economy at idle. During cruise conditions, the sub-diaphragm does not operate. Vacuum switch #2, located in the sub-diaphragm vacuum line, does not influence diaphragm operation. It senses and relays vacuum information to the ECU, which utilizes that input to shut off the fuel cut solenoid during deceleration, or periods when engine vacuum is below 14.5 in Hg.

The sub-diaphragm receives a vacuum signal from the fuel cut port. The main diaphragm receives its vacuum signal from ported manifold vacuum.

The main diaphragm functions as a traditional vacuum advance diaphragm. As RPM increases, the throttle plates open allowing vacuum to pull the diaphragm.