Vacuum Checking Relay
Engine vacuum is supplied to port #5 (see illustration) of the vacuum relay through the ATC vacuum harness. Port #2 connects to the vacuum regulator in the compensator, and port #1 connects to the vacuum motor in the programmer.
A small hole in the rubber diaphragm inside the relay allows engine vacuum to pass into the chamber that feeds ports #1 and #2. A portion of the vacuum from port #2 is constantly bleeding to the atmosphere to control the amount of vacuum applied to the vacuum motor through port #1. This bleeding of vacuum is necessary to position the vacuum motor which positions the temperature door via the programmer output shaft.
The vacuum checking relay also has another function. Whenever engine vacuum drops to within 1 1/2" or less of control vacuum, the spring loaded rubber diaphragm is pushed against the base of ports #1 and #2. In this manner, port #1 is sealed and the vacuum motor is locked in position. Air outlet temperature will remain constant until engine vacuum is again restored. When engine vacuum is increased, the vacuum relay opens and system control is continued. This feature prevents the programmer from going to full cold operation whenever low engine vacuum conditions are present.