Choke Thermostat/Heater: Description and Operation

Fir. 38 - Typical Electric Assist Choke Installation:

Fig.39 - Electric Assist Choke Wiring Schematic:

Purpose
Most 1973-82 models are equipped with an electric assist choke to reduce hydrocarbon (HC) and carbon monoxide (CO) emissions 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 60° to 65° F, or higher. It also provides a slower choke opening at temperatures below this point.

Operation
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 degrees, 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 60-65 degrees, 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 1-1/2 minutes.

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.

Construction
The electric choke system, Figs. 38 and 39, consists of a choke cap, thermostatic spring, a bimetal temperature sensing disc (switch), and a ceramic positive temperature coefficient (PTC) heater. The choke is powered from terminal or tap of the alternator.