DESCRIPTION
Note. If choke housing has no inlet air connection, either from external heat stove or from thermactor air system, vehicle is equipped with all-electric choke. However, if housing has air inlet connection, it can still include all-electric or electric assist choke.
12-Volt System
This 12-volt choke system is controlled by engine "RUN" signal from 3-terminal oil pressure switch. Vehicles without all-electric choke systems have 1-terminal oil pressure switch or sender.
The 3-terminal switch has 2 sets of contacts. The 2 outside terminals are for ungrounded, normally open contacts, which actuates choke. These contacts complete the circuit to ground when they are closed. This permits choke heater to operate, when ignition switch is in "RUN" position.
Center terminal is for case-grounded, normally closed contacts, which actuate oil pressure or engine light.
7.2-Volt System
This choke system operates in same manner, but receives its power from stator terminal (center tap) of alternator.
ELECTRIC ASSIST CHOKE
Many Light Duty emission models use hot air choke with electric assist. Electrically-heated choke thermostat spring housing acts as an aid to fast choke release.
Heater receives its voltage either from stator terminal on alternator or from battery through oil sensor switch. Heater only operates when engine is actually running.
Electric assist choke system consists of choke cap, thermostatic spring, bimetal temperature sensing disc and positive temperature coefficient (PTC) ceramic heater. (Scheme 63)
Current is constantly supplied to temperature sensing switch. System is grounded through ground strap connected to carburetor body. At temperatures below 54°F (12°C), switch is open. No current is supplied to ceramic heater located within thermostatic spring, allowing normal choking action to occur.
At temperatures from 54-74°F (12-24°C), depending on engine requirements, switch will remain open or will close to supply current to ceramic heater. Switch will always be closed at temperatures above 74°F (24°C). As heater warms, it causes thermostatic spring to pull choke plate open within 1 to 1.5 minutes.
ALL-ELECTRIC CHOKES
Although several different all-electric chokes are used on Ford Light Duty trucks, they operate similarly to electric assist chokes. Temperature ranges may vary between models. Ceramic heater is used to act upon bimetal thermostatic spring, which opens and closes choke.
TESTING
Remove air cleaner, check choke plate and choke linkage for free operation. Remove hot air supply tube at choke housing, and install Choke Tester (LRE34618). Perform hot and cold choke function per instructions contained in tester kit.
Alternator-Powered Choke
- Disconnect electrical lead from choke cap. Turn ignition off. Connect one test lamp lead to positive battery terminal. Attach other lead to choke cap terminal. Test lamp should light. If test lamp lights, proceed to step 3). If test lamp does not light, use jumper wire to connect one end to choke clamp shroud and other end to battery negative terminal.
- Test lamp should glow. If not, connect jumper wire directly to choke cap ground pin; if lamp glows, correct poor connections between choke clamp shroud and choke cap ground pin. If lamp does not glow, replace choke cap.
- Leave test lamp connected and remove jumper wire. Test lamp should glow. If not, locate and repair open in ground circuit. Reconnect electrical lead to choke cap.
- Connect test lamp between choke cap shroud and battery negative terminal. Start engine. Test lamp should glow. If not, locate and repair open circuit between choke cap and alternator stator terminal. If no open circuit is found, check alternator output and service as required. Stop engine and remove test equipment.
Battery-Powered Choke
- Disconnect electrical lead from choke cap. Turn ignition off. Connect one test lamp lead to positive battery terminal. Attach other lead to choke cap terminal. Test lamp should light. If test lamp lights, proceed to step 3). If test lamp does not light, use jumper wire to connect one end to choke clamp shroud and other end to battery negative terminal.
- Test lamp should glow. If not, connect jumper wire directly to choke cap ground pin; if lamp glows, correct poor connections between choke clamp shroud and choke cap ground pin. If lamp does not glow, replace choke cap.
- Turn ignition off. Disconnect electrical connection from oil pressure switch and install jumper wire in electrical harness. Connect test lamp between battery negative terminal and choke cap terminal.
- Turn ignition on, but do not start engine. Test lamp should glow. If not, locate and repair open circuit (fuse, fuse link, electrical connector, etc.). Turn ignition off.
- Remove jumper wire and reconnect electrical connector to oil pressure switch. Turn ignition on, but do not start engine. Test lamp should not glow. If lamp glows, replace oil pressure switch.
- With test lamp still connected, start engine. Test lamp should glow. If not, replace oil pressure switch.
CHOKE CAP RESISTANCE
- Heat choke with choke tester for 3-5 minutes. Disconnect electrical connector from choke cap terminal. Connect ohmmeter between choke cap terminal and choke cap ground.
- Ensure metal-to-metal contact, not metal oxide-to-metal contact. Ohmmeter reading should be under 30 ohms; but more than 0 ohms. If not to specifications, repeat test. If specifications are not met after second test, replace choke cap.
- Replace air cleaner and reconnect vacuum lines. Choke cap should be quite warm. Reconnect ohmmeter as described in step 1).
- On all models, use choke tester and cool cap by directing cold air towards oval insulator (not case) around cap terminal. Ohmmeter reading should gradually vary and then register sudden increase. Stop cooling.
- Sudden increase should occur within 10 minutes after cooling began (choke tester used at maximum capacity and held close to cap). If sudden increase does not occur within 10 minutes, replace choke cap. If change does occur, warm oval insulator with choke tester.
- Ohmmeter reading should again vary and then register sudden decrease. Stop warming cap. Sudden decrease should occur within 10 minutes after warming began (choke tester used at maximum capacity and held close to cap).
- If sudden decrease does not occur within 10 minutes, replace choke cap. If change occurs, choke cap is operating properly.
