Service Bulletin

Volvo Trucks North America, Inc.

Greensboro, NC USA Trucks
Date Group No. Page
This Service Bulletin replaces Service Bulletin, “Cab
Climate Unit, Design and Function” (11.2002), publication
4.2004 874 21 1(25)
number PV776–TSP174409.
Cab Climate Unit
Specifications, Design and Function
CU-BAS, CU-MCC, CU-ECC
VN, VHD VERSION2

Cab Climate Unit, Specifications, Design and

Function
This service bulletin describes the specifications and design and function of the
climate unit.

Contents:
• “Climate Unit” page 2

• “Specifications” page 3

• “Pressure monitor switch points” page 3

• “General” page 4

• “Air Distribution” page 5

• “Structure” page 6

• “Controls” page 14

• “Heater” page 15

• “A/C System Components” page 17

PV776-20 007445 USA15196

Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 2(25)

Climate Unit
There are three versions of the climate unit, two of which
are air conditioning units (that is, combined hot and cold
air units):

• Heat “CU-BAS”

• Manual air conditioning “CU-MCC”

• Automatic air conditioning “CU-ECC”

The units are combined in this manual since the

differences between the units are only the components
that affect air conditioning.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 3(25)

Climate Control System

Specifications
Refrigerant, type .......................... R134a (Tetrafluoroethane)
Air conditioning, volume ..................... 1400 g (3.09 lb.) - VNM, VNL, VHD
1600 g (3.53 lb.) - VN 430, 630
1700 g (3.75 lb.) - VN 670, 780
Compressor, marking . . . . . . . . . . . . . . . . . . . . . . . SD 7H15
No. of cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Cylinder volume . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 cm³ (9.46 in.³)
Compressor coupling . . . . . . . . . . . . . . . . . . . . . . . Electromagnetic
Lubricating oil capacity . . . . . . . . . . . . . . . . . . . . . . 300 cc (18.30 in.³)

Pressure monitor switch points

High pressure
opens at ........................... 28 ± 2 (406 PSI)
closes at ........................... 22 ± 2 (319 PSI)
Low pressure
opens at ........................... 2.0 ± 0.2 (29 PSI)
closes at ........................... 2.1 ± 0.2 (30 PSI)
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 4(25)

Climate Control System Overview

General

T8009771

VN vehicle

The air inlet at the front of the cab has a replaceable

filter to clean the air which flows in through the air inlet.
The filter must be replaced regularly, see the service
manual, function group 87. The air that comes in via the
air conditioning unit is distributed around the cab. The air
is distributed so that surfaces that may be subject to cold
convection are heated up.
The air leaves the cab through the evacuation vents in
the doors and the C-pillars at the rear of the cab. The
incoming air sweeps round the windshield, up to the roof,
down the rear wall to the floor and up between the door
trim and outer panels to pass out through the openings
at the rear of the cab. This ventilation system eliminates
cold convection from the rear wall, which is particularly
important in cabs used as sleeping cabs. The non-return
valves in the air extraction vents prevent the air from going
the wrong way through the air vents and are dimensioned
to ensure that a small excess pressure is formed inside
the cab, to prevent drafts from outside.
The output of the heating system is such that the cab
temperature can be maintained at 45 C (113 F) when
the outside temperature is –20 C (–4 F).
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 5(25)

Air Distribution

T8009776

The dashboard is equipped with 4 nozzles which can be

aimed individually or closed completely. 2 nozzles on
either side of the dashboard direct air to the driver’s and
passenger’s feet and knees. The same ducts also direct
air up into the doors, through the doors and up through
6 nozzles to the side windows. The defroster, 4 nozzles
underneath the windshield and one on each side of the
dashboard, directs air onto the windows.
There is an air inlet in the center of the dashboard, where
the temperature sensor for cabin air is located. The
sensor also contains a suction fan to read off the current
temperature.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 6(25)

Structure
External Cooling Components

W8003236
1 High-low pressure monitor
2 Drier, refrigerant reservoir
3 Condenser
4 Compressor
5 Low pressure charge port
6 High pressure charge port
1 The pressure monitor is in the delivery line (fluid 4 The compressor is the working component in the air
under high pressure) before the expansion valve in conditioning (A/C) unit. It works as a pump in the
the climate unit. The purpose of the high-low pressure system, since it draws up the evaporated refrigerant
monitor is to signal to the control unit to interrupt the from the evaporator and forces it into the condenser.
current to the compressor if abnormal pressure should 5 The low pressure charge port is used for filling and
occur. draining the system.
2 In the drier, any moisture is absorbed and any particles 6 The high pressure charge port is used for filling and
which might be found in the fluid are filtered out. The draining the system.
outlet side of the drier is connected to a pipe which
goes to the base of the drier. Since the fluid level is
higher than the lower end of the pipe, the refrigerant is
prevented from leaving the drier in vapour form.
3 The purpose of the condenser is to cool the refrigerant.
The superheated refrigerant gas is cooled to
condensation by ambient air flow across the condenser
fins.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 7(25)

Structure
BAS

T8008645
1 Control Unit/control panel
2 Blower motor
3 Heater core
4 Fan speed controller
5 Stepper motor, fresh air/recirculation
6 Stepper motor, air distribution
7 Stepper motor heat control
8 Water valve
1 The control unit controls the system components 4 The fan speed controller regulates the cabin air.
depending on the values set in the control panel. The fan control is constantly supplied with 12 V and
controlled by a variable voltage of 0–6 V.
2 The blower motor draws air in through the filter in the
bulkhead and on through the evaporator or heater 5 The stepper motor controls the shutter for fresh
core. The over pressure created by the fan combined air/recirculation.
with the evacuation holes in the cab provides even air
flow distribution. 6 The stepper motor controls the shutter for directing
the outlet air.
3 The purpose of the heater core is to heat the air
entering the cab. The coolant heated by the engine 7 The stepper motor controls the water flow water valve.
flows through the heater core pipes and heats the
8 The water valve opens and closes the flow of coolant
cooling fins. If the hot coolant does not flow through
through the heater core.
the heater core, it is cooled down and will not generate
any heat.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 8(25)

Structure
MCC

T8008646
1 Control Unit/control panel
2 Blower motor
3 Heater core
4 Fan speed controller
5 Stepper motor, fresh air/recirculation
6 Stepper motor, air distribution
7 Stepper motor heat control
8 Water valve
9 Expansion valve
10 Evaporator
11 Temperature sensor
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 9(25)

1 The control unit controls the system components 9 The purpose of the expansion valve is to regulate the
depending on the values set on the control panel and flow of liquid refrigerant to the evaporator and provide
the values from the sensor. a drop in pressure so that the refrigerant will easily
absorb heat from the evaporator. The valve is an
2 The blower motor draws air in through the filter in the internally equalized expansion valve that compares
bulkhead and on through the evaporator or heater the evaporator inlet pressure to the evaporator outlet
core. The over pressure created by the fan combined pressure. This allows for a refrigerant flow consistent
with the evacuation holes in the cab provides even air with the amount of heat that needs to be absorbed
flow distribution. and to keep the evaporator from flooding (a flooded
3 The purpose of the heater core is to heat the air evaporator is highly inefficient and results in very little
entering the cab. The coolant heated by the engine cooling).
flows through the heater core pipes and heats the
10 The evaporator is the heat exchanger that allows
cooling fins. If hot coolant does not flow through the
the heat of the cab air to be absorbed by the R134a
heater core, it cools down shortly afterwards and will
refrigerant. It is located inside the climate control unit
not generate any heat.
so that cab air circulated by the blower passes through
4 The fan speed controller regulates the cabin air. it. The evaporator is constructed of stacked plates that
The fan control is constantly supplied with 12 V and are surrounded by fins. The heat of the cab air passing
controlled by a variable voltage of 0–6 V. through the fins is transferred to the fins and the plates.
The refrigerant is metered into the evaporator through
5 The stepper motor controls the shutter for fresh the expansion valve and travels through the plates of
air/recirculation. the evaporator. As the evaporator collects heat from
the cab the heat is transferred to the refrigerant which
6 The stepper motor controls the shutters for directing
boils to a vapor.
the outlet air.
7 The stepper motor controls shutter for the water flow 11 The MCC unit cuts off the compressor at 3 C (37 F)
water valve. and goes back on at 8 C (46 F). The temperature
will change to the coldest if the control nob is on
8 The water valve opens and closes the flow of coolant maximum cold. The control unit itself cuts voltage to
through the heater core. the compressor and not the sensor.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 10(25)

Structure
ECC

T8009810
1 Control Unit/control panel
2 Blower motor
3 Heater core
4 Fan speed controller
5 Stepper motor, fresh air/recirculation
6 Stepper motor, air distribution
7 Stepper motor, air bypass
8 Solenoid water valve
9 Expansion valve
10 Evaporator
11 Temperature sensor, evaporator
12 Temperature sensor, cabin temperature
13 Temperature sensor, heater core
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 11(25)

1 The control unit controls the system components 9 The purpose of the expansion valve is to regulate the
depending on the values set on the control panel and flow of liquid refrigerant to the evaporator and provide
the values from the sensor. This is so that a pleasant a drop in pressure so that the refrigerant will easily
climate can be maintained in the cab. absorb heat from the evaporator. The valve is an
internally equalized expansion valve that compares
2 The blower motor draws air in through the filter in the the evaporator inlet pressure to the evaporator outlet
bulkhead and on through the evaporator or heater pressure. This allows for a refrigerant flow consistent
core. The over pressure created by the fan combined with the amount of heat that needs to be absorbed
with the evacuation holes in the cab provides even air and to keep the evaporator from flooding (a flooded
flow distribution. evaporator is highly inefficient and results and results
in very little cooling).
3 The purpose of the heater core is to heat the air
entering the cab. The coolant heated by the engine 10 The evaporator is the heat exchanger that allows
flows through the heater core ducts and heats the the heat of the cab air to be absorbed by the R134a
cooling fans. If hot coolant does not flow through the refrigerant. It is located inside the climate control unit
heater core, it cools down shortly afterwards and will so that cab air circulated by the blower passes through
not generate any heat. it. The evaporator is constructed of stacked plates that
is surrounded by fins. The heat of the cab air passing
4 The fan speed controller regulates the cabin air.
through the fins is transferred to the fins and the plates.
The fan control is constantly supplied with 12 V and
The refrigerant is metered into the evaporator through
controlled by a variable voltage of 0–6 V.
the expansion valve and travels through the plates of
5 The stepper motor controls the shutter for fresh the evaporator. As the evaporator collects heat from
air/recirculation. the cab the heat is transferred to the refrigerant which
boils to a vapor.
6 The stepper motor controls the shutters for directing
the outlet air. 11 Only in max – cold, the ECC unit cuts off the
compressor at 3 C (37 F) and goes back on at 8 C
7 The stepper motor controls the shutter for air bypass. (46 F). The temperature will change to the coldest
if the control nob is on maximum cold. The control
8 The solenoid water valve opens and closes the flow of unit itself cuts voltage to the compressor and not the
coolant through the heater core. sensor.

12 The cab temperature transmits signals to the control

unit about the current temperature so that it can control
the system to maintain a predetermined temperature
in the cab. The temperature sensor fan draws air into
the cab so that the sensor retains the current cab
temperature.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 12(25)

Controls
BAS
All the controls are located in a panel in the center of
the dashboard.

T8008629

A Fan C Fresh Air

Position 0 Fresh air inlet fully open
Position 1
Position 2
Position 3
Position 4

B Air Distribution Fresh air inlet closed, only recirculation

Defroster only

Defroster and lower ventilation nozzle D Temperature

The control has stepless adjustment.
The heat is shut off when the control is furthest left (blue).
As the control is moved to the right (red), the heat is
increased.
Mainly lower ventilation nozzle and some defrost

Upper and lower ventilation nozzles

Only upper ventilation nozzles

Can be constructed of both settings

Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 13(25)

Controls
MCC
All the controls are located in a panel in the center of
the dashboard.

T8008630

A Fan C Fresh Air

Position 0 Fresh air inlet fully open
Position 1
Position 2
Position 3
Position 4

B Air Distribution Fresh air inlet closed, only recirculation

Defroster only

Defroster and lower ventilation nozzle D Temperature

The control has stepless adjustment.
The heat is shut off when the control is furthest left (blue).
As the control is moved to the right (red), the heat is
increased.
Mainly lower ventilation nozzle and some defrost
E Air Conditioning
The “A/C on” button should be depressed (green light
on) when the air conditioning compressor operation is
required to reach the desired cab temperature.

Upper and lower ventilation nozzles

Only upper ventilation nozzles

Can be constructed of both settings

Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 14(25)

Controls
ECC
All the controls are located in a panel in the center of
the dashboard.

T8008631

A Fan C Fresh Air

“AUT” position: The system automatically controls Fresh air inlet fully open
the fan speed.
Position 0
Position 1
Position 2
Position 3
Position 4
Fresh air inlet closed, only recirculation

B Air Distribution
“AUT” position automatic control.
Defroster only

D Temperature
Blue is maximum cold and red maximum hot. Anything in
between is temperature sensor reading.
Defroster and lower ventilation nozzle
E Air Conditioning
The “A/C off” button should be depressed (amber light
on) when the air conditioning compressor cycling is not
desired. When depressed, the control unit will still try and
Mainly lower ventilation vent and some defrost maintain the set temperature by combining air into the
climate unit, with varying heat from the heater core.

Upper and lower ventilation nozzles

Only upper ventilation nozzles

Can be constructed of both settings

Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 15(25)

Heater
A/C System Components
Heater Core
The purpose of the heater core is to heat the air coming
into the cab. The heater core is connected to the cooling
system of the engine, and is located in the upper section
of the air conditioning unit.

T8008642

Fan Motor
The purpose of the fan motor is to force the cabin air
through the climate unit. The temperature sensor is
mounted in the fan motor.

T8008639
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 16(25)

Fan Speed Controller

The fan speed controller regulates the cabin air. The fan
control is constantly supplied with 12 V and controlled by
a variable voltage of 0–6 V.

T8008638

Solenoid Water Valve

The solenoid water valve opens and closes the water
flowing through the heater core in the air conditioning
unit. Control is by means of electrical impulses from the
control unit. The solenoid water valve is only found on
vehicles with automatic air conditioning “CU-ECC”. The
solenoid water valve is constantly supplied with 12 V and
is controlled on the ground side.

T8008663
1 Iron core and spindle
2 O-ring
3 Electromagnet
4 Spring
5 Rubber seal
6 Non-return valve

Water Valve MCC/BAS

It is regulated by a linkage between the heater control
stepper motor and the water valve.

T8009529
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 17(25)

A/C System Components

Compressor
The compressor is the working component in the air
conditioning (A/C) unit. It works as a pump in the system,
since it draws up the evaporated refrigerant from the
evaporator and forces it into the condenser.

There is a mechanical safety valve on the cylinder head of

the compressor (VN), which opens when system pressure
exceeds 31 bar (450 PSI). There is a plug on top of the
compressor where air conditioning compressor oil can
be topped up.
T8008641

Compressor Coupling
The compressor coupling is electromagnetic, with a fixed
magnet. The electromagnet (1) consists of an annular
winding mounted in an iron core, which is screwed to
the compressor housing. The rotating section consists
of a pulley (2) and a hub (3), which is flexibly fixed to an
annular flange (4). The pulley rotates on the compressor,
using two ball bearings. When the engine is running,
without the air conditioning (A/C) engaged, only the
pulley (5) rotates. When the compressor is supplied with
current from the switch, temperature sensors or control
unit, the circuit in the electromagnet winding is closed
and a magnetic field is created. The magnetic force pulls
the flange towards the pulley. Friction between the pulley T8008664
and the flange force the flange to follow the rotation of the
A. Compressor disengaged
pulley, taking the compressor spindle with it.

T8008665

B. Compressor engaged
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 18(25)

Compressor Mounting and Driving

The compressor is firmly mounted on the engine. It is
driven by a poly-groove-belt run off the crankshaft pulley,
together with the alternator. Belt tension is adjusted
automatically.

T8009813

D12

Condenser
The condenser is of the parallel flow type, and consists of
two side tanks connected by flat pipes. These pipes are
provided with flanges for heat transfer. The terminals are
of the block type, sealed by double O-rings.
The condenser is permanently mounted on brackets in
front of the engine radiator. It is cooled by ambient air
drawn by the engine cooling fan and the slipstream which
flows through it.

T8009812
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 19(25)

Electroviscous Engine Fan

The engine cooling fan is requested in certain conditions.
It is used in order to keep the pressure in the air
conditioning system down and to maintain good
performance. The Electronic Climate Control (ECC) and
Manual Climate Control (MCC) inform the engine control
unit (via PID 50) when the air conditioning compressor
magnetic clutch is activated. The engine control unit then
activates the fan in the following cases:
1 The engine control unit activates the fan when the air
conditioning compressor magnetic clutch is activated
and the following conditions are met:

• The speed of the vehicle is < 45 km/h (28 mph) T8008729

On and off operation fans are still used.

• The outside temperature is > 40 C (104 F)

• The engine speed > 1500 rpm

2 The engine control unit activates the fan when the air
conditioning compressor magnetic clutch is activated
and the following conditions are met:

• The speed of the vehicle is < 45 km/h (28 mph)

• The outside temperature is > 30 C (86 F)

• The engine speed > 1200 rpm

3 The engine control unit activates the fan when the air
conditioning compressor magnetic clutch is activated
and the following conditions are met:

• The speed of the vehicle is < 20 km/h (12 mph)

• The outside temperature is > 30 C (86 F)

4 The engine control unit activates the fan when the air
conditioning compressor magnetic clutch is activated
and the following conditions are met:
• The speed of the vehicle is < 10 km/h (6 mph)

• The outside temperature is > 20 C (68 F)

5 The engine control unit activates the fan when the air
conditioning compressor magnetic clutch is activated
and the following conditions are met:
•The ECC and MCC control units transmit a request to the
vehicle control unit to activate the engine cooling fan (via
hard wired signal) if the pressure in the air conditioning
system exceeds 20 bar (290 PSI). The vehicle control unit
transmits the request onwards to the engine control unit
(via J1939 message) which activates the engine cooling
fan.
If the air conditioning pressure falls below 17 bar (246
PSI), the control units transmit a request to switch off the
engine cooling fan.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 20(25)

Refrigerant Hoses
The refrigerant hoses consist of a polyamide inner surface
with rubber exterior.

The hoses have block-fitting unions, sealed with double

O-rings.

T8008165

Receiver Drier - Refrigerant Reservoir

The drier absorbs any moisture which might be found in
the system, and is also intended to filter and store liquid
refrigerant before it is forced into the expansion valve.
The moisture-absorbent material which the refrigerant is
forced to pass through, before it reaches the outlet, is
integrated in a filter body in the reservoir.
The refrigerant mainly arrives at the drier in fluid form. The
outlet goes through a pipe in the bottom of the container,
to prevent any gaseous refrigerant from passing the drier
and reaching the expansion valve. As long as there is
enough refrigerant in the system, no gas can reach the
expansion valve.

T8009814

High-low Pressure Monitor

The high-low pressure monitor is in the delivery line (fluid
under high pressure) before the expansion valve in the
climate unit.
The high-low pressure monitor tells the control unit if
abnormal pressure is reached and disengages the air
conditioning compressor.

W8003237
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 21(25)

Expansion Valve
The purpose of the expansion valve is to regulate the flow
of liquid refrigerant to the evaporator and provide a drop
in pressure so that the refrigerant will easily absorb heat
from the evaporator. The valve is an internally equalized
expansion valve that compares the evaporator inlet
pressure to the evaporator outlet pressure. This allows
for a refrigerant flow consistent with the amount of heat
that needs to be absorbed and to keep the evaporator
from flooding (a flooded evaporator is highly inefficient
and results and results in very little cooling).
T8008670
The valve is connected to the evaporator inlet and outlet 1 Ball valve
tubes and on the tube coming from the receiver/dryer and 2 Diaphragm
the tube leading to the compressor (see illustrator). It is 3 Capillary pipe
located in the engine compartment next to the bulkhead 4 Pressure equalization connection
where the climate control unit tubes pass through. 5 Compression spring

The amount of refrigerant allowed through the valve is

controlled by a push rod and a ball. A diaphragm either
forces the push rod down which moves the ball away from
its seat or pulls the push rod up which allows a spring to
push the ball closer to its seat. Pressure on the bottom
of the diaphragm comes directly from the pressure of
the refrigerant leaving the evaporator. Pressure on top
of the diaphragm comes from a gas filled tube that is
surrounded by the refrigerant leaving the evaporator.
Pressure on both sides of the diaphragm causes it to
move up or down depending on the refrigerant conditions
of temperature and pressure.
T8008671
As the evaporator outlet refrigerant pressure increases
(indicating less refrigerant is needed) it forces the
diaphragm upwards to close the valve more. If that
pressure decreases, the diaphragm moves downward,
pushing the push rod down, opening the valve more.

As the evaporator outlet refrigerant temperature increases

(indicating more refrigerant is needed) the diaphragm is
forced downward, opening the valve. If the temperature
decreases, the pressure on top of the diaphragm
decreases, and the valve closes more.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 22(25)

Evaporator
The purpose of the evaporator is to absorb heat from the
air which is in, or is drawn into the cab. The evaporator
is a loop of aluminum tube, with aluminum fins. The inlet
and outlet pipes are equipped with O-ring unions for the
expansion valve. When in operation water condenses on
the outside of the evaporator since it is colder than the
surrounding air. Condensation is collected in the base of
the climate unit and is drained through a pipe in the front
section of the air conditioning unit.

T8009815

Control Panel/Control Unit

The control unit is integrated into the control panel which
controls the temperature within the set values. The input
signals come from the temperature sensors. The output
signals control the water valve, compressor and stepper
motors.
The desired temperature, fan speed, ventilation position
and recirculation are set using the control panel controls.
The climate unit fan speed slows down when the desired
temperature is reached.

T8008658

Temperature Sensors
The temperature sensors contains a temperature
dependent resistor. As the temperature increases, the
resistance falls. When the resistance in the temperature
sensors falls, a signal is transmitted to the control unit.
The temperature sensors are found on vehicles with air
conditioning.
The temperature sensors are positioned as follows:
1 In the center of the dashboard for the cab air
temperature. The sensor also contains a suction fan to T8009808
draw cab air across the temperature sensor. 1.
2 Above the heater core, for the air temperature in the air
conditioning unit above the heater core.
3 Behind the evaporator, for the evaporator temperature.

T8008634

2 and 3.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 23(25)

Stepper Motor
There are three stepper motors to set in the climate unit:
1 The shutter which mixes fresh air with recirculated air.
2 The shutter which distributes outlet air.

Depending on variant ECC, MCC or BAS:

3 ECC: The shutter which controls the air bypass.

MCC, BAS: The shutter which controls air mixing and
the valves that control water flow through the heater
core.

T8008728

Stepper Motor Function

The stepper motor converts electric signals to a stepped,
rotating motion.
The stepper motor is constructed of:

• Rotors with permanent magnets.

• Two stator windings with central power supply.

T8008643

Stepper motor.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 24(25)

Turning to the Right

Layout Diagram

T8008689

T8008690

Turning to the Left

Layout diagram

T8008691

T8008692

The stator windings are supplied with 12 V direct current different combinations. In this way a rotating magnetic
at A and B. Each winding has three cables, six cables in field is created which turns the rotor, step by step. The
total, four of which are ground cables. The control unit control unit stores the stepper motor settings on the
continually controls the grounding of the stator winding in ignition key to position “0”.
Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 874 21 25(25)

O-rings
The O-rings in the air conditioning system seal the pipe
joints and the connections. Lubricate new O-rings using
compressor oil before fitting.

Note: O-rings can only be used once and therefore must

not be reinstalled or reused.

Air Conditioning Compressor Oil

WARNING
Air conditioning compressor oil can be hazardous to
health. Use protective gloves and protective goggles
when handling the opened container. Ensure there
is good ventilation.

Note: The air conditioning compressor oil is hygroscopic

(absorbs moisture from the air) and must be stored in a
well sealed container.

CAUTION
The oil plug for the air conditioning compressor must
never be removed if there is refrigerant in the unit. The
oil and refrigerant will discharge.

Note: Only use PAG SP15 air conditioning compressor

oil, Volvo P/N: 3917742.

Note: Open connections must be plugged when working

on the A/C system for longer than 15 minutes.

The compressor is filled with 300 cc of oil when installed

in the factory. Too much oil will impair cooling. Not
enough oil damages the compressor. The compressor
is supplied from the stores filled with 300 cc of air
conditioning compressor oil. When the air conditioning
unit has been in operation for a while, traces of oil will
be found on all the components of the unit, since the
oil follows the refrigerant as it circulates. Carry out the
following when repairing or replacing the system: If the
hose has ruptured on the high pressure side, most of the
oil has probably escaped. There is no need to refill the
oil if a new compressor is to be installed. If using the old
compressor, first drain off all the oil and then fill with new
oil (300 cc). Apply compressor oil to the O-ring on the
plug and tighten. Tighten to 10 ±2 Nm (7 ±2 ft-lb).