Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

322 Chapter Fifteen
Figure 15-12.
BCM-controlled HBAC system schematic. (GM Service and Parts Operations)
up the data it needs, and displays it on its panel.
When the ECM is finished, it momentarily transmits
a 5-volt signal to declare the line idle and the
BCM opens a conversation with the next device
it needs to talk with.
The programmer controls air delivery and temperature
on instructions from the BCM, using a
series of vacuum solenoids that control the mode
door operation. The programmer also has a motor
that controls the air-mix door position to regulate
temperature. When directed to change blower
speed by the BCM, the programmer sends a variable
voltage signal to the power module, which
sends the required voltage to the blower motor.
The ECC system is the most complex of the
ones we’ve discussed, and this is reflected in the
diagnostic sequence designed into the overall
system network. When any subsystem exceeds
its programmed limits, the system sets a trouble
code and in some cases provides a backup function.
The instrument panel cluster and the ECC
panel (Figure 15-13) are used to access and control
the self-diagnostic features. When the technician
accesses the diagnostic mode, any stored
BCM and ECM codes are displayed, along
with various BCM and ECM parameters, discrete
inputs and outputs, and any BCM outputoverride
information.