2--4
©
2012 Mobile Climate Control
T-348 Rev. 07/2012
2. Compressor Unloader UV2 Relay. When suction
pressure decreases below 23 psig, unloader UV2 is
energized unloading the second compressor cyl
inder bank; this output will remain energized until
the pressure increases to above 31 psig.
c. Discharge Pressure
Discharge pressure is also controlled by the
unloaders:
1 Compressor Unloader UV1 Relay. When the dis
charge pressure increases above set point A (see
chart below), unloader UV1 is energized; this un
loader will remain energized until the pressure de
creases below set point B (see chart below).
Table 2-1. Unloader UV1 Relay
HPS Switch
(PSIG)
Set Point A
(PSIG)
Set Point B
(PSIG)
300
275
220
2 Compressor Unloader UV2 Relay. On R-134a
systems when the discharge pressure increases
above set point A (see chart below), unloader
UV2 is energized; this unloader will remain ener
gized until the pressure decreases below set point
B (see chart below).
Table 2-2. Unloader UV2 Relay
HPS Switch
(PSIG)
Set Point A
(PSIG)
Set Point B
(PSIG)
300
285
225
2.3.7 Evaporator Fan Speed Selection
Temperature control is the primary method of
determining the fan speed selection. Table 2-3
indicates relay operational status for the various fan
motor states while Figure 2-1 provides Logic Board
speed selections at various deviations from set point..
Table 2-3 Evaporator Fan Speed Relay Operation
STAT
E
HIGH
SPEED
RELAYS
EVAP FAN
RELAY
Off
Off
Off
Low
Off
On
High
On
On
2.3.8 Condenser Fan Control
The condenser fans start in low speed when the
compressor clutch output is energized. The fans will
switch to high speed when the discharge pressure
reaches 190 psig (R-134a) and will remain energized
in high speed for a minimum period of 5 minutes.
The fans will switch to low speed when discharge
pressure decreases below 135 PSIG. High speed will
also remain activated if a high pressure alarm has
been activated and operation has not been locked
out (refer to Table 3-2).
2.3.9 Compressor Clutch Control
A belt driven electric clutch is employed to transmit
engine power to the air conditioning compressor.
De-energizing the clutch electric coil disengages the
clutch and removes power from the compressor.
The clutch will be engaged when in cooling and
disengaged when the system is off, in heating or
during high and low pressure conditions.
The clutch coil will be de-energized if the discharge
pressure rises to the cutout setting of the compressor
mounted high pressure switch. An alarm will be
triggered if this condition exists for more than a 0.5
second. The clutch coil will energize when the
discharge pressure falls to the reset point of the high
pressure switch.
The clutch coil will be de-energized whenever the
suction pressure decreases below 10 PSIG, an alarm
will be triggered if this condition exists for more than
10 seconds. The clutch coil will energize when the
suction pressure rises to the reset point. If the alarm
is triggered 3 times in a 30 minute time period the
system will be locked out (See 3.2.1 Alarm Codes).
The clutch coil is prevented from engagement when
the ambient temperature is below ambient lockout
setpoint.
2.3.10 Liquid Line Solenoid Control
The liquid line solenoid is energized (open) when the
compressor clutch is energized and de-energized
(closed) when the clutch is not.
2.3.11 Alarm Description
Alarm descriptions and troubleshooting procedures
are provided in section 3.
2.3.12 Hour Meters
Hour meter readings are available in the parameter
code list of the Micromate. The hour meters record
the compressor run time and the total time the
evaporator fans are on. The maximum hours are
999,999. Refer to paragraph 2.4.2 for instructions on
reading parameter codes.
2.4 MICROPROCESSOR DIAGNOSTICS
The Micromate control panel allows the user to
interface with the microprocessor based control.
This allows system parameters, alarms and settings to
be viewed and modified.
