System Overview
Let’s begin our discussion of the climate control system from a high-level view. The above illustration is the block diagram layout for this system. Keep in mind that this is from a design perspective that shows the sub-parts of the overall system.
Note: I’ve taken the liberty to make some minor changes to the original drawing so that I can depict the differences between MTC and ATC.
The blocks that are colored in are the actual differences in the ATC. Everything else shown in white color are common to both the MTC and ATC systems.
Control Unit
The control unit is the actual controller in the dash which a person operates to control different functions for heating and cooling. On the MTC it is referred to as the push control module and it is an analog system. On the ATC it is called an auto amplifier and is a digital system with built-in self diagnostics.
In the block diagram, inputs to the control unit are shown with arrows pointing to it. Outputs from the control unit are shown with arrows pointing away from it. This is the main unit that we will be replacing and rewiring. Refer to the pictures in post #3 for reference of each of the control units.
Potentio Temperature Control (PTC)
The PTC is the control where you change the COLD->HOT settings on the control unit. For the MTC it is the slider control
and for the ATC it is setpoint control
. The PTC are different on both systems and it is built into the control unit. It will not be modified.
Ambient Sensor
The ambient sensor detects outside temperature by way of a thermal resistor (thermistor) located in the lower front of the radiator. As outdoor temperature rises the resistance decreases. And as outdoor temperature falls the resistance increases. This relationship between temperature and resistance is known as negative temperature coefficient (NTC).
In-Vehicle Sensor
The in-vehicle sensor detects internal cabin temperature by way of a thermal resistor (thermistor) located behind the center dash piece. A vacuum hose is connected between the in-vehicle sensor and the aspirator. This allows internal cabin air to pass through the sensor to detect temperature. This sensor has the exact electrical specifications as the ambient sensor.
Aspirator
The aspirator is attached to the frontside of the heater core unit. As air is discharged from the heater unit it leaves behind a vacuum within the aspirator. A hose runs from the aspirator to the in-vehicle sensor to draw in cabin air for temperature sensing.
Sunload Sensor
The sunload sensor detect the level of sunshine entering the inside cabin by means of a photodiode. The signal is used to balance the temperature control setting.
Thermal Transmitter
The thermal transmitter detects the coolant temperature and controls fan blower speeds based on “hot soak” or “cold soak”. The idea is to delay sudden hot and cold air blasts to the occupants. This sensor is the exact same one used for the temperature gauge. On my car, it is the bottom sensor pointed to in the following picture,
Fan Control
In the MTC, the intake blower fan speed is controlled by the operator changing the fan switch position. That in turn causes a different resistor value to feed the negative side of the blower motor.
In the ATC, when the system is set to AUTO mode, the intake blower fan speed is automatically controlled by the Fan Control Amplifier located on the cooling unit. The final fan speed is actually based on the feedback obtained from the various sensors and the PTC settings. The AUTO mode can be overriden by the manual fan speed switches on the control unit.
...I'll stop here for the moment and will continue later....
Note: Some of these photos are subject to change on this or other posts as they get expanded and edited. Anyone with better pictures, please send them to me. Thanks. js
