2--3
T--30404/08
2.3.8 Evaporator Fan Speed Selection
Evaporator fan speed(s) selection is one method of
controlling the cooling and heating throughout the bus
passenger compartment. The thermostat control is the
other.
2.3.9 Compressor Clutch Control
(A--6, TM--16, TM--21)
A belt driven electric clutch is employed to transmit
engine power to the air conditioning compressor.
De-energizing the clutch’s electric coil disengages the
clutch and removes power from the compressor. The
clutch will be engaged when in cooling and disengaged
whenthe system is off, in heatingor duringhigh andlow
pressure conditions.
The clutch coil will be de-energized if the discharge
pressure rises to the 365 ±10 psig (19.42 bar) cutout
setting of the compressor mounted high pressure
switch. The clutchcoil willenergize (AutomaticRe--Set)
when the discharge pressure falls to 280 ± 10 psig
(11.41 bar).
The clutch coil will be de-energized (open) if the suction
pressure (LP) decreases below 6 ± 3 psig (0.45 bar).
The clutch coil will energize (Automatic Re--Set) when
suction pressure rises (close) to 25 ± 3 psig (1.7 bar).
2.4 SEQUENCE OF OPERATION (280P / 282P)
2.4.1 Electronic Thermostat
With a signal from the Hydraulic BrakeModule (orother
12 VDC source) the A/C Power Relay is grounded,
sending 12 VDC to the K1A relay. K1A relay energizes
sending battery power (Line B+) to the Dash Control
switches (S1A & S2A).
Begining with the Fan Speed Switch (S1A) in the low
speed position (vent 1) the following actions take place:
a. Power flows from the Fan Speed Switch (S1A)
through relay K4A normally closed contacts. Line U2
energizes evaporator fan motor low speed relays
(K1M, K4M, K7M & K10M). Closing these relays al-
lows power to flow from the battery (line B+) through
the fan motors with two motors in series, operating
the motors at low speed.
b. The ON/OFF switch (S2A) is then placed in the ON
position. Power flows from the switch to energize the
Thermostat. With the Thermostat calling for cooling,
power also flows from the cooling switch:
1. Thorugh the Humidity Control Sensor and the Ambi-
ent Sensor Switches located in the return air and
condenser respectively. Ifbothof theseswitchesare
in the closed position the following sequence will
take place.
2. Power will flow to the Clutch Lockout Relays (CLR1
& CLR2) allowing power to energize the Clutch Re-
lays (CR1 & CR2). Relays CR1 & CR2 are powere-
don terminal 30 from an OEM breaker , (A/C Low
Voltage Breaker). Energizing these relays will send
power through the high and low pressure switches
and to both compressor clutches energizing the
clutch coils and starting the compressor.
3. To the liquid line solenoids (Y1A & Y2A) to start the
flow of refrigerant.
4. Through line U3, to energize the condenser fan re-
lays (K14M & K15M). Energizing these relays will
send B+ power to start the condenser fan motors.
The unit is now in low speed cooling.
5. To bring the evaporator fans to high speed the fan
speed switch (S1A) is placed in the HIGH (Vent 2)
position.PowerflowsfromtheswitchthroughlineU5
to energize the high speed relay (K13M). The nor-
mally closed low speed relay (K4A) is de--energized
opening the low speed circuit. Power flows from the
high speed relay (K13M) to energize the high speed
fan relays (K2M, K3M, K5M, K6M, K8M, K9M,
K11M,&K12M). Energizingtheserelaysindividually
grounds each evaporator fan motor separately plac-
ing them in high speed operation.
c. With the thermostat calling for heating, power flows
from the heat switch:
1. Through line UH to energize the heat relay (K3A).
With the heat realy energized, power flows from the
battery (line B+) to start the water pump and open
theheat valve.Theunit isnow inthe heatmode.Fan
speeds can be adjusted the same as in the cooling
mode.
d. With the Thermostat calling for Heating and the need
for Dehumidification is required:
1. The override switch, located in the return air inlet is
switched to the ON position.
2. Power will flow through the ambient and humidity
controlswitch,ifclosed,andenergizethecoolingcir-
cuits at the same time as heating.
This will put the system in a Reheat Mode of operation.
The thermostat will only cycle the heat valve and pump.
The cooling circuit will stay energized as long as the
override switch is in the ON position and both sensor
switches are closed.
CONTROL ACTION -- CYCLING CLUTCH
RISING
TEMPERATURE
F ALLING
TEMPERATURE
SETPOI NT
SETPOI NT
FULL-
COOL
VENT
HEAT
+1.8° F ABOVE
SETPOI NT
-- 1. 8° F ABOVE
SETPOI NT
Figure 2.2 280P / 282P Thermostat
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