Choke Thermostat/Heater: Description and Operation

The electric assist choke reduces carbon monoxide (CO) and hydrocarbon (HC) emissions during warm-up. It provides supplemental heat to the choke bi-metal to speed up choke valve opening after the choke cover interior reaches a pre-determined temperature.

Fig. 19 Electric choke assembly. 4-151 engines:

4-151 ENGINES
The main components of this electric assist choke system, Fig. 19, consist of the thermostatic coil, ceramic resistor, cover, bi-metal snap disc and contact spring. The ceramic resistor is divided into a small center section for gradual heating and a large outer section for rapid heating of the thermostatic coil. The electric actuated ceramic resistor heats the thermostatic coil, gradually relaxing coil tension and allowing the choke valve to open.
When air temperature is below 50 degrees F., electric current applied to the small section of the ceramic resistor allows slow opening of the choke valve for good engine warm-up. As the small section of the ceramic resistor continues to heat, a bi-metal disc causes the spring loaded contact to close and apply electric current to the large section of the ceramic resistor which increases the heat flow to the thermostatic coil for more rapid opening of the choke valve.
When air temperature is between 50-70 degrees F., electric current applied to the small section or both to the large and small sections, produce the required amount of heat to control the choke valve for good engine operation within this temperature range.
When air temperature is above 70 degrees F., current is applied to the small section of the ceramic resistor and through the spring contact to the large section of the ceramic to provide rapid heating of the thermostatic coil for quicker choke valve opening when leaner air/fuel mixtures are required at warmer temperatures.