Part 2
Part 2
Engine Coolant
Engine coolant is the key element of the heating system. The thermostat controls 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 heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC module. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort.
The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC module air temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.
Coolant Heater (K08)
The coolant heater function is to provide additional heat to the passenger compartment. The coolant heater burns diesel fuel, to heat up the engine coolant when the vehicle is running and will only operate during conditions where ambient temperature is below 4°C (39°F) and a fuel tank level greater than 12.5 percent. The heat of the hot engine coolant is transferred to the HVAC module to heat the passenger compartment. The coolant heater does not heat up instantly. It must go through a self test and start up procedure before normal operation. The vehicle must be running to start the unit but after the unit is no longer commanded on a two minute shut down (purge) procedure starts. The coolant flow is from the engine to the fuel operated heater through the heat exchanger back to the engine.
Battery voltage and ground is supplied to the coolant heater. The electronic control unit inside the coolant heater determines when the unit will turn ON and OFF as well as how it will function. The electronic control unit also uses GMLAN communication and the engine control module (ECM) to transfer coolant heater information that the scan tool can read. The fuel operated heater contains flame sensors to disable the glow plug once the flame is established or to abort the startup attempt if the flame is not established.
Inputs to the coolant heater electronic control unit:
* Coolant sensor
* Overheat sensor
* Combustion sensor
* GMLAN ECM
Outputs from the coolant heater electronic control unit:
* Fuel pump
* Glow plug
* Blower motor
* GMLAN ECM
The coolant heater controls the coolant temperature with 3 operating modes.
* HIGH-If coolant temperature is in a range between -40 to +75°C (-40 to +176°F), the coolant heater fuel pump will pump fuel at maximum capacity to increase the coolant temperature as fast as possible. Note: Ambient temperature must be below 4°C (39°F), fuel tank level greater than 12.5 percent and the engine should be running.
* LOW-If coolant temperature is in a range between 85-90°C (185-194°F), the coolant heater fuel pump will pump fuel at minimum capacity to increase the coolant temperature at a slower rate.
* OFF-If coolant temperature is above 90°C (195°F), the coolant heater fuel pump will stop pumping fuel and allow the remaining fuel in the combustion chamber to burn out. The coolant heater fuel pump will not start pumping fuel again until the coolant temperature reaches 75°C (167°F).
FUNCTIONAL PRINCIPLES:
* The vehicle coolant pump continuously circulates the coolant over the heat exchanger inside the fuel operated heater and throughout the coolant system.
* The coolant heater fuel pump pumps the fuel from the vehicle fuel tank to the combustion chamber.
* Coolant heater blower blows the oxygen, which is necessary for the combustion process, into the combustion chamber.
* A Coolant heater glow plug generates the evaporation energy and creates the temperature which is necessary to ignite the Air-Fuel mixture
* The heat exchanger inside the fuel operated heater transfers the energy of the combustion process into the engine coolant.
* Depending on the coolant temperature, which is detected by the coolant sensors, the heater chooses either high or low setting or gets shut off.
SELF TEST OF THE UNIT:
Before every start of the heater, the operation of the individual components is tested.
* Fuel operated heater control unit check
* Flame sensor
* Coolant sensor
* Overheating sensor
* Glow plug
* Fuel pump
* Blower motor
The fuel operated heater will only start after the self test of the heating unit is successful. Should a fault be detected, a fault notification will be output through the vehicle diagnosis.
DESCRIPTION OF SAFETY MECHANISM :
During start up the ECU is performing a random access memory (RAM), read-only memory (ROM) and electrically erasable programmable read-only memory (EEPROM) test. If failures occur during a self test of the unit, the unit will not start.
* If the power supply voltage exceeds 16 volts the unit will not start or shut off with after purge time of 120 seconds.
* If the power supply voltage goes below 10.2 V for more than 40 seconds the unit will shut off and try to restart after a purge time of 120 seconds. If the failure occurs 3 times, then unit is not going to restart till next key off.
Description of component checks:
* Coolant Heater Blower Motor-After the unit is commanded on and before normal operation the blower is tested for an open circuit. While the heater is activated the blower is tested for a short to ground.
* Flame sensor-The flame sensor is tested continuously during operation for a short to ground, short to voltage or open circuit.
* Glow plug-After the unit is commanded and before normal operation the glow plug is tested for an open circuit. While the heater is activated the glow plug is tested for a short to ground.
* Coolant Heater Fuel Pump-After the coolant heater is commanded on and before normal operation is activated, the fuel pump is tested for an open circuit. While the coolant heater is activated the fuel pump is tested for a short to ground.
* Overheating Sensor and Coolant sensor-The overheat sensor and coolant sensor are tested continuously during operation for a short to ground, short to voltage or open circuit.
FIRST START OF THE UNIT (125 seconds):
After the self test was successfully completed a first start procedure sequence is attempted.
1. The ceramic glow plug starts to heat the combustion chamber.
2. After a delay, the blower switches on. During the start procedure, the blower continuously increases blowing speed.
3. The fuel pump pumps fuel into the combustion chamber. The cycle frequency of the fuel pump is also continuously increased during the start procedure.
4. The glow plug starts to vaporize the fuel, and creates the temperature to ignite the fuel.
5. After ignition, the heater runs continuously to reach the maximum heating power.
6. After the flame sensor has detected the flame, the start procedure is complete, and the glow plug switches off.
SECOND START OF THE UNIT (125 seconds):