Vacuum Advance Correction: Operation

When oil temperature is below 59°F (15°C), the thermal switch is closed. The solenoid valve closes, cutting off vacuum to the distributor advance mechanism. The switching relay connects the secondary sending unit (in distributor) to the ignition control unit. The secondary sending unit is offset 6° (flywheel) from the primary sending unit.

When oil temperature rises past the thermal switch operating point, the switch opens. This switches vacuum to the advance mechanism and causes the switching relay to reconnect the primary sending unit to the ignition control unit. Ignition timing is now controlled by vacuum advance.

On air conditioned models, a second vacuum solenoid valve is provided. When the air conditioning compressor is "ON", the solenoid allows manifold vacuum to reach the vacuum advance mechanism. The additional advance raises engine speed to compensate for the compressor load.