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Understanding and Diagnosing Automatic Temperature Control

It is my hope that this page will help the reader to understand the components and operation of the Automatic Temperature Control system that was installed in Chrysler products between 1974 and 1978. Other years and cars may be included but I cannot confirm this.


Temperature Sensors

There are two sensors and one control. These three simply make up three variable resistors that are connected in series. There is the control unit whereby the driver selects the desired cabin temperature, the in-car sensor that reports the current air temperature of the cabin, and the ambient sensor that measures the current temperature of the air coming into the ATC airbox.

The control unit is a variable resistor (potentiometer) that goes from 200 ohms (65F) to about 1800 ohms (85F). It is at 200 when the temerature selector is at the farthest COLD position, and about 1800 ohms when the farthest HOT position is selected. To test the calibration, set the control in the center of its travel, about 75 deg and measure the resistance. The resistance should be 1000 ohms. If it is not, the control is either defective or out of calibration and should be replaced.

The in-car sensor decreases in resistance as the cabin temperature rises. The maximum resistance shoud be approximately 1850 ohms at 65F, 1400 ohms at 75F and 1070 ohms at 85F. I measured mine at about 15 C and it was about 3000 ohms. I put my finger on it and the resistance began to drop quite rapidly. I would expect that by the time it got to 40 C or so, it would be at nearly 0 ohms.

The ambient sensor also decreases in resistance as the temperature rises. The normal values for this resistor are: 650 ohms at 65F, 550 ohms at 75F and 450 ohms at 85F. If the sensor is at a constant 1500 ohms, the sensor is open and should be replaced. On my test system when I measured the sensor at about 15 C it was at about 500 ohms. I held my hand onto it for a while and it moved down very slowly. I would call this a servicable sensor.

These three things, coupled with the fact that the wiring schematic clearly shows the three resistors are wired in a simple series circuit, lead me to the following conclusion. The ATC system is set up in such a way that it will always attempt to make the total resistance of the circuit a constant value. If the resistance of the three resistors increases, the system will attempt to lower that resistance by going into HEAT mode. If the circuit resistance decreases, the system will attempt to increase it again by going into COOLING mode. The resistance of the circuit can increase or decrease in three ways.

  1. Change the setting on the control module.
  2. Change the temperature of the cabin air.
  3. Change the temperature of the incoming ambient air.

After looking at this component of the system, I am more convinced that this is a very well designed and SIMPLE system that is reliable, accurate and easy to operate and service.

Vacuum Reservoir

This is the vacuum reservoir for the ATC. It is mounted to the right inner fender. There must be vacuum present here for the system to turn on. There is a check valve in there so the orientation of those vacuum lines DOES matter. IIRC, those in the picture are backwards.

Water Control Valve

This is the hot water control valve and also the ATC delay control. This valve is mounted near the reservoir but closer to the firewall. This valve serves two purposes. The top (grey) vacuum line is used to open and close the hot water valve when needed. The green and orange lines go through a heat actuated vacuum switch. This is what keeps the ATC OFF until the engine coolant reaches 210deg F (or thereabouts). Check that there is vacuum present here when the engine is warm.

Vacuum Switches

Under the dash pad, on top of the air box, there are two vacuum actuated electric switches. These are what turn the ATC on when there is vacuum present at the above mentioned switch, with the engine warm. You can hear these things click when they turn on and off. You can pull the electrical connector and short the terminals to test their operation.

Air Door Switch & Amplifier

There is another vacuum switch on the fresh/recirculated air door mechanism, shown here with an orange and a grey (or white) vacuum line running to it. This is also an important component. In the same picture, you can see the "Amplifier" as Chrysler calls it. It's the little brain box pinned to the right end of the air. This is the only electronic part of the whole system. Everything else is switches, relays, vacuum motors etc. I don't know how to test this other than substitution. I've run across these being bad too. You can just pry this off the air box using a flat screwdriver. I may have been told these are still available new from the dealer.

Control Panel

In some cases, there can be problems with the control panel but I've never seen this. The little tits for the vacuum lines on the back of the switch can become brittle and break off and the variable resistor can become dirty but like I said, I've never seen these problems in one of these cars.


One part you might suspect as being troublesome is the servo. I've also never seen one of these go bad either. I took one apart one time and they seem very robust inside so I would not suspect this component early on either.

Chrysler New Yorker Online - Understanding and Diagnosing Automatic Temperature Control