Chapter 22 Automatic Temperature Control System Service 365
Copyright by Goodheart-Willcox Co., Inc.
Amplifi ers
To replace an amplifi er, fi rst make sure the ignition
switch is in the off position. Then remove the amplifi er
electrical and vacuum connectors. Remove the screws
holding the amplifi er and remove it from the vehicle.
Install the new amplifi er and tighten the mounting
screws. If the amplifi er is installed on the plastic case, make
sure any ground wires used are reinstalled. Then reinstall
the electrical and vacuum connectors. Start the engine and
check system operation.
Summary
Automatic temperature control system problems can
be fi xed with the application of knowledge and logic.
Common problems in electronic temperature control sys-
tems are defective input sensors and computers.
The temperature control computer stores informa-
tion about system problem in the form of trouble codes,
which identify the general HVAC system problem or
a defective component. Many electronic temperature
control systems use the HVAC control panel to display
trouble codes. Pressing a series of buttons on the control
panel causes the control panel to display trouble codes.
The technician may have to separate HVAC system codes
from other vehicle codes. Some temperature control com-
puters can be accessed through the data link connector.
Plugging a scan tool into the data link connector will
automatically place the system in self-diagnosis mode.
Trouble codes will appear as numbers on the scan tool
screen.
Some scan tools also have indicator lights. Dedicated
scan tools can be used with only one kind of electronic
temperature control system. Multi-system and generic scan
tools can be used to diagnose many manufacturer’s control
systems. All OBD II systems use the same 16-pin diagnostic
link, and generic scan tools can retrieve trouble codes from
any vehicle using the OBD II system. Scan tools can also be
used to check sensors and output devices.
Mechanical temperature control systems are usu-
ally trouble free. If all of the basic HVAC systems are
ok, check the transducer and power servo. Remember
the transducer is both input sensor and control device.
Electromechanical temperature systems operate like
mechanical systems, but use temperature sensors and an
amplifi er.
Once the problem has been located, electronic tem-
perature control system service is limited to replacing parts.
Some computers can be reprogrammed with a scan tool.
Always pay attention to electrostatic discharge problems
when servicing any electronic part.
Mechanical and electromechanical systems have
fewer parts than electronic systems. Sometimes the trans-
ducer cable must be adjusted. Parts replacement is rela-
tively simple. Always recheck HVAC system operation after
any service procedures.
Review Questions—Chapter 22
Answer the following questions using the information
provided in this chapter.
1. What should the technician check before trouble-
shooting the automatic control system?
2. The most common cause of electronic control system
problems is a defective ______.
3. To obtain trouble codes, the system must be placed in
the ______ mode.
4. A system self-test allows the self-diagnosis system to
check the operation of ______.
5. Name three things scan tools can be used to do.
6. A scan tool that can send and receive data is a ______
type.
(A) dedicated
(B) generic
(C) multi-system
(D) bi-directional
7. An OBD II data link connector has a ____ -pin
connector.
8. If a mechanical temperature control system has a prob-
lem, what two devices should the technician check?
9. On an electromechanical temperature control system
the ______ may be part of the amplifi er assembly.
10. Match the component with the service procedure most
likely to be performed to service it.
Ambient temperature sensor ___
HVAC control computer ___
Sunload sensor ___
Transducer cable ___
Passenger compartment
temperature sensor ___
Power module ___
ASE Certifi cation-Type Questions
1. All of the following statements about electronic
temperature control diagnosis are true, except:
(A) the most common cause of trouble is a sensor
defect.
(B) a defective computer can cause erratic display
readings.
(C) trouble codes can always be retrieved from a
defective computer.
(D) sometimes the only way to diagnose the com-
puter is to replace it with a known good unit.
(A) Adjust
(B) Reprogram
(C) Replace