354 Auto Heating & Air Conditioning
Copyright by Goodheart-Willcox Co., Inc.
Problems in an automatic temperature control system
may seem almost impossible to correct at fi rst. However,
like any HVAC problem, they can be diagnosed and
corrected if you use a combination of knowledge and logic.
This chapter covers diagnosis and service of automatic tem-
perature control systems. Modern systems are covered as
well as older mechanical and electromechanical systems.
Studying this chapter will add to the knowledge of HVAC
control systems you have begun by studying the informa-
tion in Chapter 14.
Automatic Temperature Control
System Problems
The most common complaint about automatic
temperature control systems is failure to react when
the temperature changes. The temperature control may
remain on one setting, the system may over or under-
cool the vehicle’s interior, or may fail to switch modes
when needed. Another common defect is failure to react
to driver input, such as not changing fan speeds when
requested. Since many small differences exist between
different systems, always consult the proper service
information when troubleshooting any automatic tem-
perature control.
When troubleshooting an automatic temperature con-
trol system, keep the seven step troubleshooting process
in mind. Review the seven-step process in Chapter 15 if
necessary. In addition, you should perform the following
before troubleshooting the automatic control system.
Make sure the basic refrigeration and heating compo-
nents, blower, and air delivery systems are working.
Retrieve trouble codes when applicable.
Check service literature for common system problems
and updates.
In the following sections, automatic temperature
control system problems are grouped by type of system.
Electronic systems are covered fi rst, followed by older
mechanical and electromechanical systems.
Warning: Check the service information
for precautions when servicing vehicles
equipped with air bags. Improper procedures
may damage the air bag system or cause accidental
deployment.
Electronic Temperature Control Systems
Common problems in the control system are caused
by defects in the input sensors or the temperature control
computer. Output devices covered in earlier chapters, such
as the compressor clutch, blend door, and blower motor,
operate in the same manner as on manual HVAC systems.
When an electronic control system fails to operate,
the most common cause of trouble is a control head or
temperature sensor defect. Since modern systems use three
or four sensors, it may be diffi cult to determine which sen-
sor is defective. For sensor diagnosis, it will be necessary
to retrieve trouble codes and take ohm and volt readings at
the sensor.
If the temperature control computer is defective, it
can cause erratic or no system operation. Often the control
panel will display erratic readings and/or fail to respond
to driver input. If the basic refrigeration, heating, and air
Figure 22-1. On some older vehicles pressing a certain series
of buttons on the HVAC control panel accesses the automatic
temperature control computer. The computer will display trouble
codes and other information on the panel display. A—Entering
self-diagnostics mode. B—Requesting change in parameters.
C—Parameter data. D—Clearing codes. (General Motors)
OUTSIDE TEMP
HI FAN
AUTO
°F
°C LO FAN
ECON
AUTO
OFF
OFF ECON AUTO LO HI
ELECTRONIC CLIMATE CONTROL
OUTSIDE TEMP COOLER WARMER
OUTSIDE TEMP
°C LO FAN
AUTO
OFF OFF OFF ECON AUTO AUTO AUTO LO HI
ELECTRONIC CLIMATE CONTROL
OUTSIDE TEMP COOLER WARMER
OUTSIDE TEMP
HI FAN ECON
AUTO
OFF
OFF ECON AUTO LO HI
ELECTRONIC CLIMATE CONTROL
OUTSIDE TEMP COOLER WARMER
HI FAN
°C OFF
OFF ECON AUTO LO HI
ELECTRONIC CLIMATE CONTROL
OUTSIDE TEMP COOLER WARMER
A
B
C
D