Help with auto climate malfunction

[eangler]

Techs and anyone who can help,

 

I am encountering trouble with my auto climate system on an 07 classic sierra. The only way to get it to put out warm air in any mode is to put the temp control to 90. Then it blasts out hot air. Also, when in auto, the output mode stays on vent from 60-89 degrees then changes to floor mode at 90.

 

Relevant facts:

 

1) I recently installed remote start and had the batteries disconnected for a couple days. Remote start had me snake a wire through the headliner for antenna.

 

2) during installation the bypass module went TU and clogged up the data bus.

 

3) it only recently got cold enough for me to need heat.

 

Thanks for any and all help.

[GM-Tech]

Setting the temp at 90 (or 60) overrides the auto air and basically goes into manual mode, even if it appears auto is still selected. In order to remain in automatic temp control, the temp must be set just above 60 or just below 90.

 

So what does that all mean in terms of your problem? I really don't know. You mentioned the data buss so I'm wondering if your installation has caused a problem there that is preventing control.

 

This really needs to be looked at with a scanner to see what's going on. Codes may have been set which could cause some default mode that's preventing auto control.

 

These are pretty complex systems that can't normally be diagnosed just by guessing.

[eangler]

Understood. The databus is OK now but was hosed up for a short time by the bypass module. Got a new module and the bus is fine. I am working on finding someone with a tech-2. I understand the manual function, I am using it for heat and to ascertain that the ECS has at least some control over the fan speed and temperature. It seems that the auto mode is just non functional. st looking for the low hanging fruit for what would cause (only) auto mode to be totally non-functional. Was thinking about checking all the temp sensor inputs for a start. Any other ideas? Thanks for the reply.

[GM-Tech]

What is this "bypass module" you're talking about? And what's an "ECS". Is that a made up abbreviation for something? I know GM loves their abbreviations, but I can't think of anything in relation to HVAC (or anything else for that matter) that fits ECS.

 

It might help to look at ambient sensor data.

[eangler]

What is this "bypass module" you're talking about?  And what's an "ECS".  Is that a made up abbreviation for something?  I know GM loves their abbreviations, but I can't think of anything in relation to HVAC (or anything else for that matter) that fits ECS.

 

It might help to look at ambient sensor data.

 

<{POST_SNAPBACK}>

 

 

 

The bypass module is a conponent of the remote start system. It allows remote start to function by sending ignition key present status out on the databus when the remote start is activated. ECS = environmental control system, same as HVAC. I don't know what GM calls it but in my business heat and AC are called ECS. Sorry for the confusion.

 

Can you elaborate a but on ambient sensor data? I don't have a code scanner yet. I know I need to get one eventually. I have considered pulling the dash off and measuring the resistance of the temp sensors.

[GM-Tech]

ECS = environmental control system, same as HVAC.  I don't know what GM calls it ...

 

GM calls it HVAC... Heating, Ventilation and Air Conditioning.

 

What data buss are you tied into? These vehicles can have anyone of three different types of data streaming around the vehicle. It could have just one, two or all three, depending on options. There's high speed GMLAN, low speed GMLAN and the old standby, Class 2. High Speed GMLAN is a twisted pair network and the other two are single wire but operated differently from each other. Where, exactly, did you tap into the data line?

 

It makes me very nervous when a data line is tapped into with aftermarket stuff. I'd start by disconnect that module from the data line.

 

Ambient data is displayed as a temperature on our Tech 2. I highly doubt one of those cheapy code scanners is going to allow you to look at HVAC data. I have a table that lists temperature vs. sensor resistance, but I can't post it here because as a table, a copy and paste will just be a big, unreadable mess. Besides, just because the sensor itself is reading correctly doesn't mean the data is getting to the control head correctly.

[eangler]

ECS = environmental control system, same as HVAC.  I don't know what GM calls it ...

 

GM calls it HVAC... Heating, Ventilation and Air Conditioning.

 

What data buss are you tied into? These vehicles can have anyone of three different types of data streaming around the vehicle. It could have just one, two or all three, depending on options. There's high speed GMLAN, low speed GMLAN and the old standby, Class 2. High Speed GMLAN is a twisted pair network and the other two are single wire but operated differently from each other. Where, exactly, did you tap into the data line?

 

It makes me very nervous when a data line is tapped into with aftermarket stuff. I'd start by disconnect that module from the data line.

 

Ambient data is displayed as a temperature on our Tech 2. I highly doubt one of those cheapy code scanners is going to allow you to look at HVAC data. I have a table that lists temperature vs. sensor resistance, but I can't post it here because as a table, a copy and paste will just be a big, unreadable mess. Besides, just because the sensor itself is reading correctly doesn't mean the data is getting to the control head correctly.

 

<{POST_SNAPBACK}>

 

 

 

 

Thanks for the clarification, I will use HVAC from now on.

 

It is tied into pin 2 at the connector under the dash. I tried unplugging the module and there was no effect.

 

I was able to get hold of the temperature-resistance tables for all five sensors, but do not have a good understanding of how the thing actually figures out how to maintain temperature. Can you get an an explanation of the logic in the automatic system? That might help me understand where to start looking. I put my ear up to the sensor grill in the headliner and I could hear the fan running.

[GM-Tech]

It is tied into pin 2 at the connector under the dash.  I tried unplugging the module and there was no effect. 

 

Okay, thats the old Class 2 data line. Here, you can start with this...

 

 

Air Temperature Description and OperationThe air temperature controls are divided into 5 areas:

 

• HVAC Control Components

 

• Heating and A/C Operation

 

• Automatic Operation

 

• Engine Coolant

 

• A/C Cycle

 

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:

 

Feature

Availability

 

Afterblow

No

 

Purge

No

 

Personalization

Yes

 

Actuator Calibration

Yes

 

 

Air Temperature Actuator

The air temperature actuators are 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 Temperature Sensors

The air temperature sensors are a 2-wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors:

 

• Ambient Air Temperature Sensor

 

• Inside Air Temperature Sensor Assembly

 

• Upper Left Air Temperature Sensor

 

• Upper Right Air Temperature Sensor

 

• Lower Left Air Temperature Sensor

 

• Lower Right Air Temperature Sensor

 

A signal and a low-reference circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases. The sensor signal voltage decreases as the resistance decreases. The sensor operates within a temperature range between -40°C (-40°F) to 101°C (215°F). The sensor signal varies between 0-5 volts.

 

The input of the duct air temperature sensors are different from the ambient and inside sensors. The HVAC control module converts the signal to a range between 0-255 counts. As the air temperature increases the count value will decrease.

 

If the HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted air temperature value. The default value for the ambient and inside air temperature sensors will be displayed on the scan tool. The default value for the duct air temperature sensors will not be displayed on the scan tool. The scan tool parameter for the duct air temperature sensors are the actual state of the signal circuit. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is corrected.

 

The ambient air temperature sensor mounts underhood and can be affected by city traffic, by idling, and by restarting a hot engine. Therefore, the HVAC control module filters the value of the ambient air temperature sensor for temperature display. The ambient air temperature value is updated under the following conditions:

 

Condition

Display

 

At start up with the engine off less than 2 hours

Displays last stored temperature

 

At start up with the engine off more than 2 hours

Displays real-time temperature

 

Engine coolant temperature is less than 28°C (50°F) above the ambient air temperature

Displays real-time temperature

 

Vehicle speed above 32 km/h (20 mph) for a minimum of 80 seconds

Displays real-time temperature

 

Vehicle speed above 72 km/h (45 mph)

Displays real-time temperature

 

Sensor reading is less than the last displayed value

Displays real-time temperature

 

 

The scan tool has the ability to update the displayed ambient air temperature. To update the ambient air temperature display on the HVAC control module, perform the following procedure: Simultaneously press the MODE, FRONT DEFROST and REAR DEFROST switches.

 

Turn ON the ignition.

Simultaneously press the MODE, FRONT DEFROST and REAR DEFROST switches.

 

 

Sunload Sensor Assembly

The sunload sensor is a 2-wire photo diode. The vehicle uses left and right sunload sensors. The 2 sensors are integrated into the sunload sensor assembly. Low reference and signal circuits enable the sensor to operate. As the light shining upon the sensor gets brighter, the sensor resistance increases. The sensor signal decreases as the resistance increases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts.

 

The sunload sensor provides the HVAC control module a measurement of the amount of light shining on the vehicle. Bright or high intensity light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle.

 

If the HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted sunload value. This value will not be displayed on the scan tool. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed. The scan tool parameter for the sunload sensor is the actual state of the signal circuit.

 

 

A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5 volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts. The PCM converts the voltage signal to a pressure value.

 

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high pressure condition exists. The PCM disables the compressor clutch if the A/C pressure is more than 2957 kPa (429 psi). The clutch will be enabled after the pressure decreases to less than 1578 kPa (229 psi).

 

 

A/C Low Pressure Switch

The A/C low pressure switch protects the A/C system from a low pressure condition that could damage the A/C compressor or cause evaporator icing. The HVAC control module applies 5 volts to the A/C low pressure switch signal circuit. The switch will open when the A/C low side pressure reaches 138-172 kPa (20-25 psi). This prevents the A/C compressor from operating. The switch will then close when A/C low pressure side reaches 275-317 kPa (40-46 psi). This enables the A/C compressor to turn back ON.

 

Heating and A/C Operation

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. The vehicle passenger can offset the passenger temperature as much as 16.7°C (30°F). Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

 

• Recirculation actuator setting

 

• Difference between inside and desired temperature

 

• Difference between ambient and desired temperature

 

• Blower motor speed setting

 

• Mode setting

 

The control module makes the following actions when automatic operation is not selected, and an air temperature setting is selected:

 

• When the air temperature switch is placed in the warmest position, the control module commands the air temperature door to divert maximum air past the heater core.

 

• When the air temperature switch is placed in the coldest position, the control module commands the air temperature door to direct air to bypass the heater core.

 

• When the air temperature switch is placed between the warmest and coldest positions, the control module monitors the following sensor inputs to determine the air temperature door position that diverts the appropriate amount of air past the heater core in order to achieve the desired temperature:

 

- Sunload

 

- Duct temperatures

 

- Ambient temperature

 

- Inside temperature

 

The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a class 2 message to the PCM for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.

 

The following conditions must be met in order for the A/C compressor clutch to turn on:

 

• The ambient air temperature is above 4°C (40°F).

 

• The A/C low pressure switch signal circuit is grounded.

 

• The A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi).

 

• The PCM receives an A/C request from the HVAC control module.

 

• The engine coolant temperature (ECT) is less than 121°C (250°F).

 

• The engine RPM is more than 550 RPM.

 

• The throttle position is less than 100%.

 

The HVAC control module monitors the A/C low pressure switch signal circuit. If the voltage signal on this circuit has no voltage drop the module will interpret this condition as a low pressure, disabling the A/C request. The A/C low pressure switch will open its internal contacts at 151 kPa (22 psi). Then close the contacts at 275 kPa (40 psi) to resume A/C operation. This switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has a low refrigerant level.

 

The PCM monitors the A/C refrigerant pressure sensor signal circuit. The voltage signal on this circuit is proportional to the refrigerant pressure inside the A/C high side pressure line. As the pressure inside the line increases, so does the voltage signal. If the pressure is above 2957 kPa (429 psi), the A/C compressor output is disabled. When the pressure lowers to 1578 kPa (229 psi), the PCM enables the compressor to operate.

 

The sensor information is used by the PCM to determine the following:

 

• The A/C high side pressure

 

• An A/C system load on the engine

 

• An excessive A/C high side pressure

 

• The heat load at the A/C condenser

 

Once engaged, the compressor clutch will be disengaged for the following conditions:

 

• The ambient air temperature is less than 4°C (40°F).

 

• The throttle position is 100%.

 

• The A/C low pressure switch is open.

 

• The A/C high side pressure is more than 2957 kPa (429 psi).

 

• The A/C low side pressure is less than 151 kPa (22 psi).

 

• The engine coolant temperature (ECT) is more than 121°C (250°F).

 

• The engine speed is more than 5500 RPM.

 

• Transmission shift

 

• The PCM detects excessive torque load.

 

• The PCM detects insufficient idle quality.

 

• The PCM detects a hard launch condition.

 

Automatic Operation

In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation.

 

To place the HVAC system in Automatic mode, the following is required:

 

• The Auto switch must be activated

 

• The air temperature switch must be in any other position other than full hot or full cold position

 

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically be adjusted to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:

 

• Monitor the following sensors:

 

- Inside Air Temperature Sensor

 

- Ambient Air Temperature Sensor

 

- Lower Left Air Temperature Sensor

 

- Lower Right Air Temperature Sensor

 

- Upper Left Air Temperature Sensor

 

- Upper Right Air Temperature Sensor

 

• Regulate blower motor speed

 

• Position the air temperature actuator

 

• Position the mode actuator

 

• Position the recirculation actuator

 

• Request A/C operation

 

Engine Coolant

Engine coolant is the essential element of the heating system. The thermostat controls the normal engine operating coolant temperature. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation.

 

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. Opening or closing the air temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.

 

Auxiliary Coolant Pump (w/HP2)

The auxiliary coolant pump circulates coolant through the engine and heater core when HVAC requires heating and the engine is HOT and OFF. The HCM will turn on the auxiliary coolant pump when the HVAC control module commands it to do so by sending a class 2 signal to the HCM.

 

A/C Cycle

Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is an very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

 

The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C system. The A/C system is mechanically protected with the use of a high pressure relief valve. If the A/C refrigerant pressure sensor were to fail or if the refrigerant system becomes restricted and refrigerant pressure continued to rise, the high pressure relief will pop open and release refrigerant from the system.

 

Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.

 

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line, to the orifice tube.

 

The orifice tube is located in the liquid line between the condenser and the evaporator. The orifice tube is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the orifice tube, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to vaporize at the orifice tube. The orifice tube also meters the amount of liquid refrigerant that can flow into the evaporator.

 

Refrigerant exiting the orifice tube flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs heat from the ambient air and draws moisture onto the evaporator. The refrigerant exits the evaporator through the suction line and back to the compressor, in a vapor state, and completing the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.

 

The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment will also change form, or condense, and is discharged from the HVAC module as water.

 

 

--------------------------------------------------------------------------------

 

Air Delivery Description and Operation

 

The air delivery description and operation is divided into 5 areas:

 

• HVAC Control Components

 

• Air Speed

 

• Air Delivery

 

• Recirculation Operation

 

• Automatic 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:

 

Feature

Availability

 

Afterblow

No

 

Purge

No

 

Personalization

Yes

 

Actuator Calibration

Yes

 

 

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.

 

 

Blower Motor Control Processor

The blower motor control processor controls the speed of the blower motor by increasing or decreasing the voltage drop on the ground side of the blower motor. The HVAC control module provides a low side pulse width modulated signal to the blower motor control processor over the blower motor speed control circuit. As the requested blower speed increases, the HVAC control module increases the amount of time that the speed signal is modulated to ground. As the requested blower speed decreases, the HVAC control module decreases the amount of time that the signal is modulated to ground.

 

Air Speed - Front Control

The blower control switch is integrated into the HVAC control module. The 2 rocker type switches provide the vehicle operator the ability to select several blower speeds. The HVAC control module uses a bar graph type display to indicate the selected blower speed. The HVAC control module provides a pulse width modulated (PWM) signal to the blower motor through the blower motor speed control circuit. The blower motor changes speed based on the received PWM signal from the HVAC control module. Power and ground are provided to the blower motor through the battery positive voltage and ground circuits. When the HVAC control module is operating in AUTO mode, the system automatically controls the blower speed. Power and ground are provided to the HVAC control module by the ignition 3 voltage and the ground circuits.

 

Air Distribution

The HVAC control module controls the distribution of air by the use of a mode actuator. The modes that may be selected are:

 

• Defrost

 

• Defog

 

• Panel

 

• BI-Level

 

• Floor

 

The mode actuator is connected to the mode door by a cam type linkage system. Depending on the position of the door, air is directed through the HVAC module and distributed through various ducts leading to the outlets in the dash. If the HVAC control module detects a fault with the mode door, the HVAC control module will try to drive the actuator for a predetermined amount of time, to defrost, which is the defaulted position for the mode door actuator. When the mode switch is placed in the defrost or defog positions the A/C is commanded on and the recirculation door is moved to the outside air position to help reduce window fogging. A/C is available in all modes and recirculation is only available in the panel and bi-level modes.

 

 

Mode Actuator

The mode actuator is an electronic stepper motor with feedback potentiometers. The HVAC control module sends signals to the mode door actuator through the mode door control circuit. Zero volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, the actuator rotation stops. A 5-volt reference signal is sent out over the 5-volt reference circuit to the mode actuator. When you select a desired mode setting, logic determines the value of the mode actuator signals. The HVAC control module's software uses this reference voltage in order to determine the position of the mode actuator through the mode door position signal circuit. The motor moves the mode door to the desired position.

 

 

Front Defrost

When defrost is selected, the A/C compressor is activated. The A/C compressor clutch will engage when ambient temperatures are above 3°C (38°F). The blower motor will be activated, regardless of the coolant temperature. The HVAC control module will override the auxiliary HVAC control module so a high volume of air is delivered to the front defrost vents. The rear window defogger does not affect the HVAC system.

 

Recirculation Operation

The HVAC control module controls the air intake through the recirculation actuator. The recirculation switch closes the recirculation door in order to circulate the air within the vehicle. The outside air switch opens the recirculation door in order to route outside air into the vehicle. Regardless of the blower motor switch position, recirculation is available only in the panel and bi-level mode switch positions. Including the OFF position. The mode switch must be placed in either the panel or bi-level position before the blower motor switch is placed in the OFF position. In order to reduce windshield fogging, outside air is circulated when the mode switch is in the defrost or defog positions. If the recirculation switch is pressed into the ON position when the mode switch is in an unavailable mode position, then the recirculation switch LED will flash 3 times. If the HVAC control module detects a fault with the recirc door the HVAC control module will try to drive the actuator for a predetermined amount of time, to outside air, which is the defaulted position for the recirculation actuator.

 

Automatic Operation

In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation.

 

To place the HVAC system in Automatic mode, the following is required:

 

• The Auto switch must be activated.

 

• The air temperature switch must be in any other position other than full hot or full cold position.

 

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically be adjusted to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:

 

• Monitor the following sensors:

 

- Inside air temperature sensor

 

- Ambient Air Temperature Sensor

 

- Lower Left Air Temperature Sensor

 

- Lower Right Air Temperature Sensor

 

- Upper Left Air Temperature Sensor

 

- Upper Right Air Temperature Sensor

 

• Regulate blower motor speed

 

• Position the air temperature actuator

 

• Position the mode actuator

 

• Position the recirculation actuator

 

• Request A/C operation

[eangler]

Thanks! That will keep me busy for a while.

[chew]

ECS = environmental control system, same as HVAC. I don't know what GM calls it ...

 

GM calls it HVAC... Heating, Ventilation and Air Conditioning.

 

What data buss are you tied into? These vehicles can have anyone of three different types of data streaming around the vehicle. It could have just one, two or all three, depending on options. There's high speed GMLAN, low speed GMLAN and the old standby, Class 2. High Speed GMLAN is a twisted pair network and the other two are single wire but operated differently from each other. Where, exactly, did you tap into the data line?

 

It makes me very nervous when a data line is tapped into with aftermarket stuff. I'd start by disconnect that module from the data line.

 

Ambient data is displayed as a temperature on our Tech 2. I highly doubt one of those cheapy code scanners is going to allow you to look at HVAC data. I have a table that lists temperature vs. sensor resistance, but I can't post it here because as a table, a copy and paste will just be a big, unreadable mess. Besides, just because the sensor itself is reading correctly doesn't mean the data is getting to the control head correctly.

 

 

 

What brand/type of scan tool would you recommend to be useful? Someone at work mentioned 'AutoXray'?.? And I saw they had several different models. Are they worth having? Thanks!

[cnw20]

Why does the A/C light show ON when using Auto and when it is cold outside and you only need heat? Thanks.

[spock]

Why does the A/C light show ON when using Auto and when it is cold outside and you only need heat? Thanks.

This has been puzzling me on my '04 since new. If I push the A/C button it will turn off the compressor but it shouldn't come on in the first place if the ambient temp. is below 40F and not in defog mode. GM Tech/Anyone know any easy fixes for this?

[m0ntecarloss]

This perhaps may not be helpful, but is quick to try. Pull the fuses for the HVAC module - I believe there are two. (or just disconnect the battery again) Wait for a few minutes so that the keep alive memory is allowed to erase.

 

Connect battery or fuses, start the vehicle and let it run for a few minutes without touching anything on the HVAC unit. Maybe you didn't touch anything when you reconnected your battery after your install but its worth a shot. According to the manuals I have - the first minute after the memory is wiped out is used to recalibrate the device...

 

I am in the middle of swapping my manual controls to auto controls. Right now I have no sensors. I just swapped in the blower control module that controls fan speed and wired it up. What you are seeing is exactly the same operation as my truck in its current phase.