Carrier EM-17 Operation And Service page 18

Once the temperature approaches set point, the
thermostat will call for the system to unload. With the
thermostat calling for unloading, unloader valve UV1 is
energized. Energizing UV1 energizes one bank of
compressor cylinder unloading. UV1 may also be
energized if compressor suction pressure rises to the
closing point of unloader pressure switch UPS1.
If discharge pressure rises to the set point of UPS3 or
suction pressure falls to the set point of UPS2 unloader
valve UV2 is energized. Energizing UV2 energizes one
(or one additional) bank of compressor cylinder
05/09
unloading.
If either HPS or LPS open, relay FR is de--energized.
De--energizing FR opens a set of contacts to
de--energize the compressor clutch and closes a set of
contacts to energize A/C stop relays ACSR1 & ACSR2.
Energizing ACSR2 energizes the AC STOP light and
sets up a holding circuit while energizing ACSR1 opens
an additional set of contacts in the compressor clutch
power line. The holding circuit will not be broken until
power is removed by placing the ACMS or main vehicle
power switch in the OFF position.
2--3
T--336