Feedback Carburetor System Chevrolet Sprint I

DESCRIPTION

The feedback system maintains a controlled air/fuel ratio. The system includes the Electronic Control Module (ECM), oxygen sensor, mixture control solenoid, coolant thermal switch, engine compartment thermal switch, high altitude compensator, Wide Open Throttle (WOT) microswitch and idle microswitch.

FEED BACK SYSTEM

The exhaust air/fuel ratio is monitored by the oxygen sensor, which relates this information to the ECM. Upon receiving this information, the ECM operates the mixture control solenoid as needed to control the carburetor air/fuel ratio.

ELECTRONIC CONTROL MODULE (ECM)

The ECM sends a signal to mixture control solenoid which controls carburetor air/fuel ratio. This signal is continuously cycling solenoid between on and off time (duty cycle) as a function of inputs from sensors.

Scheme 15

1. Turn cancel switch on, located near fuse block, under left side of dash. (Scheme 15)ignition on, but do not start engine. Oxygen sensor light in dash should light, but not flash.
2. If oxygen sensor light does not operate, check light circuitry. After oxygen sensor light operates, start engine and warm to normal operating temperature. (Scheme 15): Location of Cancel Switch
3. Run engine at 1500-2000 RPM and check that oxygen sensor light flashes. If it does not flash, check and adjust or replace the following: oxygen sensor, mixture control solenoid, carburetor idle mixture, thermal switch, electrical connections, ECM or microswitches.
4. After making sure oxygen sensor light flashes, turn cancel switch "OFF". Oxygen sensor light should go out.

Scheme 16

1. Warm engine to normal operating temperature. Disconnect microswitch coupler and connect ohmmeter to idle microswitch terminals. (Scheme 16)(Scheme 16): Connecting Ohmmeter to Idle Microswitch
2. Run engine at idle and make sure ohmmeter indicator is at zero. Connect tachometer to engine. Increase engine speed gradually, checking that ohmmeter indicates infinity when engine speed is 1500-2400 RPM.
3. If engine speed is not within specified range, adjust idle microswitch lever. (Scheme 17)lever down if engine speed is below range, and bend up if engine speed is above range.

Adjusting Idle Microswitch Lever. Scheme 17

Scheme 18

1. Remove air cleaner and carburetor. Connect ohmmeter to wide open throttle (WOT) microswitch. (Scheme 18)that resistance between microswitch terminals is infinity. (Scheme 18): Connecting Ohmmeter to WOT Microswitch
2. Open throttle valve gradually until ohmmeter indicates zero. Using a vernier caliper, measure clearance between throttle valve and primary carburetor bore.
3. Clearance should be.28-.33" (7.2-8.4 mm). If clearance is incorrect, bend WOT microswitch lever to adjust clearance. (Scheme 19)

Adjusting WOT Microswitch Lever. Scheme 19

MIXTURE CONTROL SOLENOID VALVE

1. Remove duty check connector cap and short the terminals with a lead wire. (Scheme 20)ignition on and off repeatedly without starting engine.
2. Using finger, touch carburetor body near Mixture Control Solenoid (MCS) valve and make sure carburetor makes small vibrations according to ignition switch operation.

Shorting Duty Check Connector Terminals. Scheme 20

COOLANT THERMAL SWITCH

1. Drain cooling system. Remove thermal switch from intake manifold. Cool switch to below 86°F (30°C). Using ohmmeter, check that there is continuity between terminals.
2. Heat switch to above 115°F (46°C). Check that there is NO continuity between terminals.
3. Wrap Teflon tape around switch threads and install switch in manifold. Refill cooling system.

ENGINE COMPARTMENT THERMAL SWITCH

Disconnect switch connector and connect ohmmeter between terminals at side of switch. (Scheme 21)that switch is on (ohmmeter indicates zero) when ambient temperature is below 45°F (7°C) and off (ohmmeter indicates infinity) when ambient temperature is above 68°F (20°C).

Checking Engine Compartment Thermal Switch. Scheme 21

HIGH ALTITUDE COMPENSATOR

1. Disconnect compensator coupler and connect ohmmeter to compensator. (Scheme 21)er 4000 ft. altitude, ohmmeter should read zero with compensator on.
2. If altitude is below 4000 ft., ohmmeter should indicate infinity with compensator off. After checking compensator, reconnect coupler.

Scheme 22

Scheme 23

1. Warm engine to normal operating temperature. Disconnect oxygen sensor connector.
2. Connect voltmeter between oxygen sensor side terminal (of disconnected connector) and ground. (Scheme 22)T apply voltage to oxygen sensor. (Scheme 22): Connecting Voltmeter to Oxygen Sensor
3. With engine at 1500-2000 RPM, turn WOT microswitch on by moving microswitch lever. (Scheme 23)that voltmeter reads about.8 volts. (Scheme 23): Moving WOT Microswitch Lever
4. With engine at 1000-1500 RPM, disconnect vacuum hose at intake manifold. Check that voltmeter reads near zero. Reconnect vacuum hose and oxygen sensor connector.

ECM GROUND CIRCUITS CHECK

1. To check that ECM is grounded to body and transaxle, turn ignition switch off. Disconnect coupler from ECM (under left side of dash).
2. Measure resistance between terminal No. 1 (Black wire) and body and terminal No. 19 (Black/Green wire) and body. If resistance is zero, ECM is properly grounded.
3. If resistance is not zero, check body ground at left kick panel and transaxle ground near gear shift guide case. After ensuring ECM is grounded, reconnect ECM coupler.

Scheme 24

1. Disconnect ECM coupler. Turn ignition on, but do not start engine. Connect voltmeter between terminal No. 2 of disconnected coupler (on wiring harness side) and body ground. (Scheme 24)voltage. (Scheme 24): Checking Sprint ECM Power Circuits
2. Repeat voltage check for terminal Nos. 3, 9, 12, 13, 16 and 18. If measured voltage between each terminal and body is about 12 volts, each circuit is in good condition.
3. Check clutch switch operation as follows: Connect voltmeter between terminal No. 13 and ground. Depress clutch pedal fully. Voltage should drop to zero, indicating clutch switch is on and terminal is grounded. Reconnect coupler to ECM.

SENSOR CHECKING

1. Turn ignition off. Disconnect coupler from ECM. Connect ohmmeter between terminal No. 4 (coolant thermal switch) of disconnected coupler and terminal No. 19 (ground). Measure resistance.
2. Repeat resistance check with terminal Nos. 5 (idle microswitch), 6 (engine compartment thermal switch), 14 (high altitude compensator) and 15 (WOT microswitch). See «SENSOR RESISTANCE CHART»(/chevrolet/sprint/i-1984-1988/remont/testing-diagnostics/#feedback-carburetor-system) .
3. If ohmmeter readings are not as specified, sensor or lead wire may be faulty. If ohmmeter readings are as specified, the sensor and lead wire circuit are in good condition. Reconnect coupler to ECM after sensor check is completed.

OXYGEN SENSOR SIGNAL CHECK

1. Remove ECM from instrument panel without disconnecting coupler. (Connect coupler if disconnected.) Warm engine to normal operating temperature and keep at idle.
2. Connect voltmeter between terminal Nos. 10 (oxygen sensor ground) and 19 (ground). If voltmeter indicator deflects between zero and .8 volts when engine speed varies from idle to 1500-2000 RPM, feedback system is operating properly.
3. If voltmeter indicator remains at zero, check for the following: oxygen sensor lead wire disconnected, intake system leak, air/fuel ratio too lean, pulse air constantly sent to exhaust, voltmeter inner resistance too small, or faulty oxygen sensor.
4. If voltmeter does not deflect from about .8 volts, check for the following: engine not warmed up (choke operating), defective thermal switch, defective WOT microswitch, or disconnected mixture control solenoid lead wire.
5. Connect ohmmeter negative (-) lead to terminal No. 20 and positive (+) lead to body ground. If ohmmeter indicator deflects, the ECM is sending out feedback signal and system is operating properly.
6. Turn cancel switch on. Cancel switch should cause sensor light to flash. If sensor light does not flash, check to see if bulb is burned out. Install ECM to instrument panel, making sure coupler is connected.

Sensor & Circuit Check. Scheme 25

SENSOR RESISTANCE CHART

See also:
• SENSOR RESISTANCE CHART