Choke Thermostat/Heater: Description and Operation

Fig. 37 Electric assist choke:

Most vehicles are equipped with an electric assist choke. This device aids in reducing emissions of hydrocarbons (HC) and carbon monoxide (CO) during starting and warm-up (choke on) period. The electric assist choke is designed to give a more rapid choke opening at temperatures of about 54° F to 74° F, or higher. It also provides a slower choke opening at temperatures below this point.
The electric assist choke system does not change any carburetor service procedures and cannot be adjusted. If system is found out of calibration the heater control switch and/or choke unit must be replaced.
The electric choke system, Fig. 37, consists of a choke cap, thermostatic spring, a bi-metal temperature sensing disc (switch), and a ceramic positive temperature coefficient (PTC) heater. The choke is powered from terminal or tap of the alternator. Current is constantly supplied to the ambient temperature switch. The system is grounded through a ground strap connected to the carburetor body. At temperatures below approximately 60° F, the switch opens and no current is supplied to the ceramic heater located within the thermostatic spring. Normal thermostatic spring choking action then occurs. At temperatures above approximately 54°---74° F the temperature sensing switch closes and current is supplied to the ceramic heater. As the heater warms, it causes the thermostatic spring to pull the choke plates open within 1---1.5 minutes.

Testing

1. Disconnect stator lead at connector leading from choke cap and connect a test light in series with stator lead and ground.
2. With engine running, test light should glow.
3. If light does not glow, repair or replace either the alternator or choke wire.
4. With engine running at normal operating temperature, place test light in series with choke terminal and alternator lead. If light does not glow, replace choke cap assembly.