Nice explanation of the TAC (Thermac) system - with pictures:
The TAC system, Fig. 82, is designed to improve carburetor operation and engine warm-up conditions. It achieves this by keeping the air entering the carburetor at a temperature of at least 100°F., or more.
Carburetor air temperature is controlled by a pair of doors, Figs. 83 thru 85, located in the air cleaner snorkel, which channel either preheated or under hood air to the carburetor.
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Preheated air is obtained by passing under hood air through ducts surrounding the exhaust manifold, causing it to pick up heat from the manifold surface. The heated air is then drawn up through a pipe to the air cleaner snorkel.
Underhood air is picked up at the air cleaner snorkel in the conventional manner.
The two air mixing doors work together so that as one opens, the other closes and vice versa. When underhood temperature is below approximately 86°F., the cold air door closes, causing the hot air door to open. Hot air from the exhaust manifold stove is then drawn into the carburetor.
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As the underhood temperature increases, the cold air door begins to open until the temperature reaches approximately 115°to 130°F, at which time the cold air door is fully open and the hot air door is fully closed.
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The doors are controlled by a vacuum motor mounted on the air cleaner snorkel. This motor, in turn, is controlled by a sensor inside the air cleaner which regulates the amount of vacuum present in the vacuum motor according to air cleaner temperature.
Whenever manifold vacuum drops below 5---8 inches, depending on the unit, the diaphragm spring in the motor will open the cold air door wide in order to provide maximum air flow.
The vacuum motor and control door assembly in the left snorkel on outside air induction units does not have a sensor and is controlled only by manifold vacuum. This snorkel remains closed until full throttle is obtained. With manifold vacuum at 6-8 inches, the door will open, allowing maximum air flow.
FUNCTIONAL TESTING
Remove air cleaner cover and install temperature gauge tool No. J-22973 or equivalent, as close as possible to temperature sensor.
Install air cleaner cover. Do not install air cleaner cover wing nut.
With engine ``OFF'', check damper door position through snorkel opening. Snorkel passage should be open, Fig. 73, if not check for damaged linkage.
Start and operate engine at idle. With air temperature below 85°F, snorkel passage should be closed, Fig. 84. When damper door begins to open snorkel passage, remove air cleaner cover and check tool reading. Reading should be between 85°F and 115°F.
With air temperature 128°F, damper door should be fully open, Fig. 85. If not check linkage for damage.
If damper door does not close completely or does not open at correct temperature, note the following vacuum motor check:
Switch engine ``OFF'' and disconnect diaphragm assembly vacuum hose from temperature sensor.
Connect a vacuum gauge and apply 7 inches Hg to vacuum diaphragm assembly through diaphragm hose. This can be done by blowing into the diaphragm hose.
Damper door should completely close snorkel passage when vacuum is applied.
If damper door does not close, check linkage for damage and diaphragm for a vacuum leak.
With vacuum applied, bend or clamp diaphragm hose. Damper door should remain in closed position. If not, replace diaphragm assembly.
VACUUM MOTOR TEST
With engine off, look into snorkel to be sure the valve plates are open.
Start engine and observe valve plates with engine idling. If equipped, the left plate will close immediately, regardless of temperature.
If the temperature in the air cleaner is below 90°F., the right valve plate will also close to outside air. With temperatures above approximately 100°F., the valve will remain open.
If vacuum motors fail to operate with vacuum directly applied, replace motor.
TEMPERATURE SENSOR TEST
With engine temperature below 85°F., tape a thermometer in air cleaner.
Install air cleaner cover and start engine.
When cold air door starts to open, remove air cleaner cover and check temperature. If temperature is below approximately 85°F., sensor must be replaced. If hoses have been removed from connectors for any reason, it is very important that they be correctly positioned for proper operation.
SERVICE & REPAIR:
VACUUM MOTOR, REPLACE
Remove air cleaner from engine.
Drill out spot welds securing vacuum motor retaining straps to snorkel tube using a 1/16 inch drill bit.
Lift then unhook linkage rod from damper door and remove vacuum motor.
To install, drill 7/64 inch hole in snorkel tube at center of vacuum motor retaining strap.
Reverse procedure to install using sheet metal screws to retain vacuum motor. Ensure that screws do not interfere with operation of damper assembly.
TEMPERATURE SENSOR, REPLACE
Remove air cleaner from engine.
Disconnect vacuum hoses from temperature sensor.
Pry temperature sensor tabs upward. Note position of temperature sensor before removal from air cleaner. The new temperature sensor must be installed in the same position as the old temperature sensor.
Remove clip and sensor from air cleaner.
Reverse procedure to install.
