Chapter 22 Automatic Temperature Control System Service 355
Copyright by Goodheart-Willcox Co., Inc.
delivery components are working properly, check the elec-
trical and vacuum supply to the control module. If these
check out ok, begin diagnosis of the control module. Often
a defective temperature control computer will be unable to
provide trouble codes. In this case, the only way to make
sure the temperature control computer is the problem is to
replace it with a known good unit.
Electronic Control System Self-Diagnosis
When an electronic temperature control system
develops a problem, the temperature control computer stores
information about the problem as a trouble code. Trouble
codes identify either the general HVAC system problem or a
defective component. The exact nature of the problem iden-
tifi ed by the trouble code varies with the system. The com-
puter may also indicate a problem by turning on a warning
light on the HVAC control panel. On many modern vehicles,
a scan tool must be used to retrieve trouble codes.
Reading Codes Using the HVAC Control Panel
To obtain trouble codes, the system must be placed
in the self-diagnosis mode. Many electronic temperature
control systems use the LEDs or other lights on the con-
trol panel to display trouble codes. Always consult the
vehicle service information for correct code information.
The service information will explain the correct retrieval
method and the meaning of each trouble code.
Many electronic temperature controls can be placed
in the self-diagnosis mode by pressing a certain series of
buttons on the HVAC control panel. After the buttons are
pressed, the control panel will display the trouble codes.
Figure 22-1 shows how a typical HVAC control panel
displays trouble codes. This system can display all vehicle
trouble codes. The technician may have to separate the
HVAC system codes from engine and other codes. Some
systems allow the technician to access powertrain, anti-lock
brake/traction control, air bag, and HVAC codes separately
by pressing certain buttons on the HVAC control panel. A
typical sequence for separating HVAC and other computer
codes is shown in Figure 22-2.
The self-diagnostic system of some temperature con-
trol computers can perform a self-test on the output devices
by operating the HVAC system in all modes. The vehicle
service information will give directions for performing the
self-test. Compare the system operation against the manual
chart to ensure the system is operating correctly.
Reading Codes Using a Scan Tool
To get the most out of the self-diagnostic capabilities
of an electronic temperature control system, you should
have a scan tool, Figure 22-3. Scan tools are commonly
used to retrieve trouble codes. The codes will appear as
numbers on the scan tool screen. Many scan tools can be
used to perform other diagnostic activities, such as read-
ing the input and output voltages of the compressor clutch,
blower motor, relays, and output solenoids. These readings
can be compared with factory specifi cations to determine
whether or not the component is defective.
Some scan tools can override the control module
and directly operate the compressor clutch, blower, or
solenoids. Forcing a component to operate allows the
technician to determine the condition of the component
and related wiring. Most modern scan tools can be used
to reprogram the temperature control system module. This
procedure is explained later in this chapter.
Figure 22-2. This fi gure illustrates the steps in using the dashboard HVAC control panel to access the automatic temperature controls
self-diagnostic system. (General Motors)
LIGHTING? (ALLANTE)
IPC? (ELDORADO/SEVILLE)
SIR?
OFF
LO
LO
LO
LO
HI
HI
HI
HI OFF
OFF
OFF
OFF
BCM? BCM?
ECM?
ENTER DIAGNOSTICS ENTER DIAGNOSTICS
PRESS
“OFF” & “WARMER” “OFF” & “WARMER”
SEGMENT
CHECK
DIAGNOSTIC CODE
DISPLAY DISPLAY
DIAGNOSTICS—BASIC OPERATION
ENTER DIAGNOSTICS BY SIMULTANEOUSLY PRESSING CCP “OFF” AND
“WARMER” BUTTONS UNTIL ALL DISPLAYS ARE LIT.
DIAGNOSTIC CODE LEVEL DISPLAYS ECM CODES FOLLOWED BY BCM
CODES, LIGHTING CODES (ALLANTE ONLY), AND SIR CODES.
TO PROCEED TO THE DESIRED TEST LEVEL PRESS AND RELEASE
THE INDICATED BUTTON.
PRESS “OFF” TO RETURN TO THE NEXT SELECTION IN THE
PREVIOUS TEST LEVEL.
EXIT DIAGNOSTICS BY PRESSING “RESET” ON THE DRIVER
INFORMATION CENTER.