Chevrolet Silverado / GMC Sierra. Service manual — part 2356

RECIRCULATION ACTUATOR REPLACEMENT

Fig. 9: View Of Recirculation Actuator & Screws
Courtesy of GENERAL MOTORS CORP.

1

Mode Actuator Screw (Qty: 2)

Tighten: 1.6 N.m (14 lb in)

NOTE:

Refer to Fastener Notice .

2

Mode Actuator
Procedure:
Re-calibrate the mode actuator. Refer to Actuator Recalibration.
Tip: Mode actuator cam is now accessible.

Callout

Component Name

NOTE:

In order to avoid actuator damage, DO NOT apply power to the actuator when it is not installed in
the HVAC module.

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

DESCRIPTION AND OPERATION

AIR DELIVERY DESCRIPTION AND OPERATION

The air delivery controls are divided into four areas.

HVAC Control Components

Air Speed

Air Delivery

Recirculation Operation

HVAC Control Components

HVAC Control Module

The HVAC control module is a class 2 device that interfaces between the operator and the
HVAC system to maintain air temperature and distribution settings. The battery positive voltage
circuit provides power that the control module uses for keep alive memory (KAM). If the battery
positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The
body control module (BCM), which is the vehicle mode master, provides a device on signal. The
control module supports the following features:

Preliminary Procedure

1. Remove the inflatable restraint I/P module. Refer to Inflatable Restraint

Instrument Panel Module Replacement

2. Disconnect the electrical connector from the recirculation actuator.

1

Recirculation Actuator Screw (Qty: 2)

Tighten: 2 N.m (18 lb in)

NOTE:

Refer to Fastener Notice .

2

Recirculation Actuator
Procedure:
Re-calibrate the recirculation actuator. Refer to Actuator Recalibration.

Feature

Availability

Afterblow

No

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

Mode Actuator

The mode actuator is a 5-wire bi-directional electric motor that incorporates a feedback
potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal
circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to
command the actuator movement. When the actuator is at rest, the control circuit value is 2.5
volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When
the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal
between 0-5 volts.

The HVAC control module uses a range of 0-255 counts to index the actuator position. The door
position signal voltage is converted to a 0-255 count range. When the module sets a commanded,
or targeted value, the control signal is changed to either 0 or 5 volts depending upon the direction
that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the
changing position signal is sent to the module. Once the position signal and the commanded value
are the same, the module changes the control signal to 2.5 volts.

Air Speed

The blower motor circulates air through the vehicle's interior. The vehicle operator determines
the blower motor's speed when the driver places the blower switch in one of 5 blower speeds.
The blower motor will always operate in any switch position other than OFF, as long as the
ignition switch is in the RUN position. The blower motor and mode switches are located within
the HVAC control module. The blower motor OFF input is connected in series with the HVAC
control module by the off blower motor control circuit.

Depending upon the selected speed, power is provided to the blower motor from either the
ignition 3 voltage or battery positive voltage circuits from the fuse block. The battery positive
voltage circuit only provides power when the High blower switch position is selected. Power and
ground are provided to the HVAC control module by the ignition 3 voltage and the ground
circuits.

Low Blower Speed

When the Low 1 blower speed is selected, the HVAC control module applies voltage to the
blower motor resistor assembly through the low blower motor control circuit. Voltage is divided
between 4 series resistors, a blower relay, and the blower motor to achieve the desired blower

Purge

No

Personalization

No

Actuator Calibration

Yes

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

speed. The blower motor is grounded through the ground circuit.

Medium Blower Speeds

When the Medium 1 blower speed is selected, the HVAC control module applies voltage to the
blower motor resistor assembly through the medium 1 blower motor control circuit. Voltage is
divided between 3 series resistors, a blower relay, and the blower motor to achieve the desired
blower speed. The blower motor is grounded through the ground circuit.

When the Medium 2 blower speed is selected, the HVAC control module applies voltage to the
blower motor resistor assembly through the medium 2 blower motor control circuit. Voltage is
divided between 2 series resistors, a blower relay, and the blower motor to achieve the desired
blower speed. The blower motor is grounded through the ground circuit.

When the Medium 3 blower speed is selected, the HVAC control module applies voltage to the
blower motor resistor assembly through the medium 3 blower motor control circuit. Voltage is
divided between a series resistor, a blower relay, and the blower motor to achieve the desired
blower speed. The blower motor is grounded through the ground circuit.

High Blower Speed

When the High blower speed is selected, the HVAC control module applies voltage to the blower
motor resistor assembly through the high blower motor control circuit. The voltage energizes the
blower relay, causing the blower motor to be connected directly to the battery positive voltage
circuit. The blower motor and blower motor relay are grounded through the ground circuit.

Off

When the OFF position is selected, the HVAC control module applies voltage to the fan off input
within the HVAC control module through the off blower motor control circuit. When the HVAC
control module receives this input, any A/C request will be cancelled and the A/C request LED
will turn off. Fresh air will still be brought in from the outside, but the LED indicator on the
switch will not illuminate.

Recirculation Mode

When the recirculation switch is pressed, a signal is sent from the HVAC control module to the
recirculation actuator through the recirculation door control circuit. When the logic receives the
signal, the recirculation actuator moves into the recirculation mode. This brings air from inside
the vehicle instead of fresh air from the outside. Power and ground are provided to the
recirculation actuator by the ignition 3 voltage and ground circuit. Recirculation is not available
when the HVAC control module is in the defrost position. The recirculation mode will stay on

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado