1. Electrical Systems
2. ATC Reference Information

Chrysler’s Automatic Temperature Control System

I hope to provide some background information on what the system is comprised of, basic interchangeability and what is required to switch between conventional and automatic climate control.
Please feel free to make changes, update or correct information as necessary, or PM me.

The systems found in vehicles ranging from 1986 – 1993 are self-contained and are candidates for installing into a car with a conventional system. In 1994 the system relied on the body controller, and thus is beyond the scope of this article.



System Breakdown

The Early ATC System Components (1986 – 1990)
1. Control Panel
2. Power Module
3. Vacuum Module
4. Blend Door Motor
5. Cabin Temperature Sensor with Fan
6. Incoming Air Temperature Sensor

The Late ATC System Components (1991 – 1993)
1. Control Panel
2. Power Module
3. Blend Door Motor
4. Cabin Temperature Sensor with Fan
5. Incoming Air Temperature Sensor
6. Sun Sensor
7. Water Temperature Sensor
8. Re-circulation Door Actuator
9. Mode Door Actuator

Control Panel
This contains the logic that controls the system. It reads sensor data and controls actuator output.The panels from 1986-1990 look the same and function very similarly. With wiring modifications any panels in this range may be interchanged. Connectors will need to be re-pinned, and a couple wires will need to be ignored or grounded. The panels from 1990-1993 should also be possible to interchange, again with wiring modification. The early and late panels cannot be interchanged as the systems gained sensors and switched to electronic actuators instead of vacuum actuators. The 1986 and 1987 panels require constant power to recall the last used settings, 1988+ do not require constant power to remember settings.

Power Module
This is essentially just a large transistor on a heat sink which feeds the blower motor. The power module is fed PWM signals from the control panel to control the blower motor speed. It mounts in the heater box down stream from the fan. All of the heater boxes are traced in the location that must be cut out to install this module, and the screw holes are pre-drilled. The power module resides behind the glove box and depending on model may require the steel dash brace be removed to access.
(1991-1993 Power module) (1986-1990 Power module)

Vacuum Module (1986-1990)
This module is attached to the same bracket as the power module in these years. Internally it consists of four vacuum line solenoids that redirect engine vacuum to the various actuators on the heater box. This replaces the function of the manual buttons on the conventional climate control. It is fed signals by the control panel to actuate the appropriate door.In the early module, the blower relay was fed a signal from the control panel, grounding it. If this wire is not grounded the blower will run on high and not be able to be controlled by the panel. If using an older vacuum module with a newer panel, ensure the blower signal wire to the vacuum module is grounded. The second relay was to drive the AC signal relay. The later control panels would directly ground the AC signal, the earlier grounded it through this relay. If using an older module with newer panel, ignore the driver and output for AC from the module. These wires are referred to as COMP RLY and AC CLUTCH SIG in the wiring diagrams. The later heater boxes (1991+) still have the required space for the vacuum module to hang although nothing occupies that space.
(Vacuum module)

Blend Door Motor
This motor contains a series of gears between a small electric motor and a rotating shaft that slips over the blend door shaft on the bottom of the heater box, above the exhaust hump. The unit has approximately a 90 degree sweep, and its position is fed to the control panel as an analog voltage. These motors can stick if they have become wet, to fix this, disassemble the box, unscrew the gear shafts and slide off the gears. The shafts can succumb to mild rust, particularly at the ends where they are exposed and rest in the case. Cleaning this and re-greasing the gears will often result in a functional unit.Heater boxes in vehicles of pre 1991 vintage should have the two mounting locations on the bottom of the heater box already. 1991 and newer should have three. The angle of the blend door shaft appears to have changed between early and late years also. It is suggested the correct motor be used for the heater box in the vehicle, otherwise modification is more difficult requiring the shaft be angled differently and an additional mount being cut off and re-attached to the heater box in a new location. It is, however, possible.
(1991-1993 Blend door actuator) (1986-1990 Blend door actuator)

Cabin Temperature Sensor with Fan
The cabin temperature is measured by a small thermistor mounted in a plastic housing with cabin air drawn over it. A small fan draws the air over the sensor. This is often referred to the aspirator or respirator assembly.The fan motors can completely burn out, but some may fail due to the brushes becoming dirty. The small electric motor is held within a sealed rubber jacket inside the plastic frame. This is to reduce noise through vibration. The small motor has slots in the side to allow it to cool, and to allow the dust from its wearing brushes to exit. Since it is enclosed, this black dust will accumulate and may cause one or more of the brushes to stop making contact. If the fan works, but only after being pushed to start, this is likely the culprit. Spray the inside of the motor thoroughly with a cleaner such as brake clean until the liquid coming out is clear. Allow it to dry and then run the motor. This may give the motor a second lease on life.
(1990-1993 Cabin Sensor) (1986-1989 Cabin sensor. Note some are mounted horizontally)

Incoming Air Temperature Sensor
This is a simple two wire temperature sensor mounted within the air box, upstream of the blower motor. The sensor itself is held into a base which is part of the sub wiring harness. It is located behind and just above the glove box. The heater box will already have screw holes drilled and have the circle that needs to be drilled marked out.
(Ambient/Incoming air temperature sensor)

Sun Sensor (1991-1993)
This is not a temperature sensor, but a light sensor. It allows the control panel to determine how hard it will have to work to get the temperature to an acceptable level. That is, a hot car on a cloudy day may not require as high a fan speed as a hot car on a sunny day as it would cool down faster. This makes for a system that is a little more intelligent then the earlier ones. This sensor mounts on the top of the dash and would require a small hole be drilled in the top of the dashboard for installation.
(Sun sensor)

Water Temperature Sensor (1991–1993)
This is a small temperature sensor that determines the heater core temperature. It looks similar to a crimp-on ring terminal and is screwed into the engine compartment side of the heater core, between the two water inlets. This sensor allows the control panel to know if the engine is hot yet, thus the system will not be operating the blower at full speed with a cold engine in an attempt to warm up the cabin. It will then wait until the coolant temperature is high enough to warm the cabin.
(Water temperature sensor)

Re-circulation Door Actuator (1991-1993)
This replaces the pneumatic actuator on the far right side of the heater box, accessible from behind the glove box. The motor mounts on a bracket that uses two existing screws on the heater box. This motor has no position sensor like the blend door or mode door actuator, and thus should not be powered when not installed or it may run past the end of its range.
(Electronic recirc door actuator)

Mode Door Actuator (1991-1993)
This replaces the dual action pneumatic actuator visible from the bottom of the heater box, left of center. This actuator has a position sensor as it is not simply an on/off, thus is similar in nature to the blend door actuator.
(Electronic mode door actuator)



Installation

To install an ATC system on a car with conventional climate control is not beyond the scope of many with reasonable technical knowledge.

Installation 1986-1990
For the older heater boxes with the two-tab blend door actuator mounts, any system from 1986-1990 should be appropriate. These systems require very little modification to install, and can be installed without removing the heater box. Using the existing box requires some cutting and drilling, the fait-of-heart may prefer to switch over the entire heater box, in which case the first three steps may be skipped.
1. The power/vacuum module must be mounted by cutting a hole in the heater box where marked. It is affixed with 4 screws. A rotary tool with a cutting wheel works well for this. Alternatively, a keyhole saw or similar may be employed. Be careful the tool used does not penetrate too far into the box, the AC evaporator is behind this wall, no more then about three inches back. The seven-way vacuum harness connector can be disconnected from the existing control panel and connected to the vacuum module.
2. The mode door actuator must be mounted in place of the existing cable and arm attached to the shaft. To remove the arm, simply undo the 10mm nut holding the arm in place. The cable should then be removable from the box via a clip, similar to at the control panel. The mode door actuator is held in with two screws, but make sure to line up the shaft first. Attempt to rotate the door shaft to the approximate angle currently set on the actuator, then wiggle the actuator back and forth till the shaft seats on the shaft, then proceed to screw it down.
3. The ambient sensor is easy to install, it simply requires a hole drilled in the heater box in the marked location.
4. The control panel only requires two screws and drops rite in the opening.
5. The wiring is mostly self contained. Wiring diagrams are below. The wiring requiring only these external connections:

• Ground – Body ground and electrical ground may be tied together. Ensure the ground for the heater motor wire is sufficient as it will handle high amperage. This can be grounded through the existing control panels black ground wire or to the body.
  
• Switched Power – The ATC systems used two sources of power, the blower motor was fed by the blower circuit’s dark green wire. This can be tapped from the existing connector that feeds the blower motor, or anywhere between there and the fuse box in the main harness. This wire is not available at the existing control panel connector. The second power source is used for the control panel, the fan on the cabin temperature sensor and the actuator and vacuum module. This may be supplied by the blower feed, but be warned there will be voltage drop when the fan is on high. It seems to be acceptable none the less. A better circuit would maybe be the switched radio power or the circuit feeding the instrument cluster.
  
• AC Output – This wire can be tapped into from the connector to the existing control panel. The feed for this will either come from the Power/Vac module (1986-1987) or directly from the control panel (1988+)
  
• Illumination – This is the orange wire and can be tapped from the existing control panel connector
  
• Intensity Feed – This is the yellow wire (yellow with black trace in later years) that will dim the display panel when the lights are turned on. This wire is optional. If connected it will need to be tapped from the headlamp switch.
  
• Constant Power Feed (1986-1987 only) – This allows the control panel to remember the last settings, ie. C/F, Temp, Fan Speed. The later panels did not require this power source. This wire may be tapped off the stereo constant power feed or elsewhere.
  
• Running an entire system test is advisable when the swap is complete to ensure all wires are connected correctly.
  

Installation 1991-1993
The later years employed electrical actuators exclusively instead of pneumatic and electrical as was used in earlier years. It is recommended that the entire heater box be attained. The heater boxes from C and Y body cars (Imperial, 5th Ave) is not likely to fit the K/J/P/G/A/H body cars, but the actuators, brackets and possibly some other parts may need to be switched over. It is likely that the heater box will need to be removed to swap over all the parts. Otherwise the instructions are very similar to those listed above. The wiring is still self-contained, and need only be connected as explained above.
In the later cars that were made for the three-mount actuator, it may be advisable to switch out the entire heater box with one from an older car with an ATC system (1986-1990). This would avoid the possible headaches associated with swapping the actuators over to the other box.



Diagnostic Test
The system is equipped with a built in diagnostic test feature. By depressing AUTO, FLOOR and DEFROST simultaneously, the display will begin to flash and begin its test. The blower will go on high and the AC will cycle. The test will check the actuator(s) by moving them back and forth, check the vacuum module (if equipped), check that enabling the AC actually pulls the voltage on that pin down to zero volts among a few other things. If no codes are present, the control will go back to displaying a temperature. If there was a fault detected, its code will be displayed as a two digit number. Pressing panel will continue the test and display any additional codes if present. The test will only check the system is electrically functional. This test does not check system efficiency, so a low AC charge, plugged heater core, burned out blower motor, vacuum leak and similar conditions will not be detected.


Wiring Diagrams

1986-1987


1988


1989-1990


1991


1992


1993