Choke Thermostat/Heater: Description and Operation

Electric Choke System:

In the electric choke system, a bimetal choke spring is heated by an electric PTC heater (Positive Temperature Coefficient heater). As the bimetal spring is heated after engine start-up, the bimetal spring opens the choke valve gradually by thermal expansion. This pushes down the stopper lever (attached to the fast idle cam). The lower the temperature when the engine is started, the tighter the bimetal spring closes the choke valve, thus improving cold weather starting.

CHOKE OPENER OPERATION
If the engine has been started with the throttle valve lever on the highest fast idle cam step, then the engine speed will increase with the engine coolant temperature to a RPM higher than desired (over speed). In order to prevent such an over speed, the choke opener is provided.

Choke Breaker System:

CHOKE BREAKER OPERATION
The choke breaker utilizes two diaphragms to create a two stage operation. At temperatures below 65F~ (18~C) only the first stage ("A") Diaphragm receives vacuum, this only allows the diaphragm to pull the choke linkage as far as the stopper on the second stage ("B") diaphragm. This opens the choke to the first stage position. If the engine coolant temperature is above 65~F, or as the engine warms up to 65~F, vacuum is routed to the second stage diaphragm as the thermovalve closes. With the thermovalve closed vacuum is allowed to build to draw back the secondary ("B") diaphragm. This withdraws the stopper that the first stage is resting on, thus allowing the first stage diaphragm to pull the choke linkage to its second stage position.

ELECTRIC AUTO CHOKE RELAY
The electric choke relay is normally closed (ON) and it opens when its coil is energized. During engine cranking, or for approx. 80 seconds at an engine coolant temperature of 14 to 64~F (-10 to 18~C), the ECU energizes the electric choke relay coil. This prevents heating of the electric choke heater, until the engine has started.