These are the A/C controls for a '65 Riviera. The vacuum modulator is
circled in the picture. Engine vacuum is applied to the large orifice; a
hose goes from the small orifice directly to the Suction Throttle Valve.
To see if the modulator works, apply 11-15" of vacuum to the large
orifice. From the 1964 Chassis Service Manual, section 11-15 (g):
The Modulator should regulate manifold vacuum from 6 to 7 inches when the
temperature control lever is moved from the Low position to 0 inches when
the temperature control and Fan switch levers are respectively in the cooler
and high position.
You can also test the modulator as follows:
- Set the temperature lever all the way to COOLER. The back end of
the lever should push on the armature on the modulator.
- Put a hose on the large orifice and suck on it. The modulator
should hold vacuum.
- While maintaining vacuum, move the temperature level to LOW. You
should hear and feel the vacuum release.
