Chapter 22 Automatic Temperature Control System Service 363
Copyright by Goodheart-Willcox Co., Inc.
Replacing Output Devices
Remember from Chapter 14 that automatic tempera-
ture control output devices are similar to those used on
manual systems. Replacement procedures for most of these
devices are similar to the procedures for manual systems.
There are a few differences between automatic and manual
device service procedures. These are discussed in the
following sections.
Replacing Stepper Motors
Stepper motors frequently fail and must be replaced.
Replacement of most stepper motors is fairly straightfor-
ward. Begin by removing kickpanels and other interior
components as needed. Remove the electrical connec-
tor and screws or bolts holding the stepper motor to the
case, Figure 22-15. Installation is the reverse of removal.
However, before reinstalling all the interior components,
check to see if the stepper motor needs to be calibrated,
which is covered in the next section.
Recalibrating Stepper Motors
On some electronic HVAC control systems, the elec-
tronic stepper motors must be positioned so that they are
at the end of their travel before the system is re-energized.
This is sometimes called recalibrating the motors. Always
consult the service information before proceeding with a
motor recalibration. The general procedure to recalibrate
a stepper motor begins with turning the ignition switch to
the On position. Next, push the HVAC control panel AUTO
and OFF buttons at the same time. Hold both buttons for
at least fi ve seconds. You may hear the motors operating as
you hold the buttons, this is normal. After fi ve seconds, the
motors have been calibrated, and the HVAC system can be
checked for proper operation
Replacing Power Module
Instead of a blower relay, some automatic tempera-
ture control systems control the fan and compressor clutch
through a solid state power module. The power module is
located in the ductwork downstream of the blower where
it is cooled by airfl ow. The module is fi nned to dissipate as
much heat as possible. Figure 22-16 shows a typical loca-
tion of a power module.
Figure 22-14. Once the ambient air temperature sensor is
located, remove the electrical connector and mounting hard-
ware. (General Motors)
Ambient
air temperature
sensor
Retainer
Figure 22-15. Replacing stepper motor; be sure to recalibrate
the motor, if needed. (General Motors)
Figure 22-16. Some automatic temperature control systems use
power modules, which are usually located in the blower case.
Replacement is fairly simple, however, some of the modules can
be diffi cult to access. (General Motors)
Blower control
module
Connectors