GM Metro/Sprint 1985-1993 Repair Guide

Computer Controlled Emission Control System

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See Figure 1



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Fig. Fig. 1: Engine emission control system schematic

DESCRIPTION



This system is a feedback system that is used on all 1985-86 carbureted engines. A similar system with increased diagnostic capability is used on 1987-88 carbureted engines. The systems consist of an Electronic Control Module (ECM), oxygen sensor and a feedback carburetor.

The oxygen sensor mounted on the exhaust manifold monitors the exhaust gas air/fuel ratio and sends signals to the ECM. The ECM processes the oxygen sensor signal and controls carburetor air/fuel ratio by operating the mixture control solenoid in the carburetor. Thus the signal of the exhaust gas air/fuel ratio sensed by the oxygen sensor is fed back to the ECM and the carburetor air/fuel ratio is controlled.

Electronic Control Module (ECM)

The ECM controls the pulse air control system, fuel cut system, idle-up system and bowl vent system, as well as the feedback system. The ECM is located inside the instrument panel under the left side of the dash.

The ECM sensed parameters are as follows:



Exhaust oxygen concentration is sensed by the oxygen sensor installed in the exhaust manifold.
 
Engine coolant temperature is sensed by the thermal switch installed on the intake manifold.
 
Throttle position is sensed by the micro-switches (wide open switch and idle switch) installed on the carburetor.
 
Engine speed is computed by the ECM based on the electrical signal received from the ignition system.
 
Clutch engagement on manual transmission vehicles is sensed by the clutch switch located above the clutch pedal. The switch turns ON when the clutch pedal is depressed and OFF when released.
 
The ECM senses electric loads of the headlights, engine cooling fan, rear defogger and heater fan.
 
Altitude compensation. When the vehicle is at high altitude and the feedback system does not function, the air/fuel mixture becomes richer because of low air density. To compensate for the richer air/fuel mixture at high altitude, the high altitude compensator is ON by sensing the barometric pressure and sends a signal to the ECM. Following the signal, the ECM controls the mixture control solenoid in the carburetor, thus compensating the air/fuel mixture.
 
Engine compartment temperature compensation. Sensing the air temperature in the engine compartment the switch sends an electric signal to the ECM to compensate the air/fuel ratio of the fuel mixture. When air temperature in the engine compartment is low, the switch operates to make the mixture right. When the air temperature in the engine compartment is high, the switch stops operating and the air/fuel ratio of the mixture is not controlled.
 

SYSTEM DIAGNOSIS



Feedback System
1985-86 VEHICLES

See Figure 2

The feedback system uses a SENSOR lamp in the instrument panel to warn the driver of possible system problems. The sensor light will automatically flash at 30,000 mile intervals if the system is in proper working order.

Should the fuel consumption increase by an excessive amount, the engine stalls or is hard to start, the feedback system should be checked using the procedure below.

  1. Turn ON the cancel switch, located at the left side of the dash in the fuse box.
  2.  



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Fig. Fig. 2: Cancel switch is located at the left side of the dash in the fuse box.

  1. Turn the ignition switch ON without running the engine. At this time the SENSOR light should illuminate but not flash. If the light does not flash, check the electric circuit of the light, the light for a burned bulb and the lead wire for shorts.
  2.  
  3. After the light illuminates, start the engine and warm it up to normal operating temperature.
  4.  
  5. When engine is warm, run engine at 1500-2000 rpm. In this state and with the cancel switch ON, the SENSOR light should flash. Flashing of the light proves that the system is functioning properly.
  6.  
  7. If the light does not flash, it can be caused by a defective feedback system component.
  8.  
  9. After making sure that the SENSOR light flashes, turn the cancel switch OFF . The light should not stay illuminated.
  10.  
  11. Stop the engine.
  12.  

1987-88 VEHICLES
  1. Turn the ignition key ON . The CHECK ENGINE light should illuminate but not flash.
  2.  
  3. Reach under the dash and turn the diagnosis switch ON . If the CHECK ENGINE light remains ON steady or goes out, the ECM is defective or poor contact exists between the ECM wires.
  4.  
  5. If the CHECK ENGINE light flashes, codes are stored in the system. Record the flashes and check the diagnostic code list. (Refer to "Diagnostic Trouble Codes-Feedback System" in this Section for more information.)
  6.  
  7. Turn the diagnosis switch OFF after all codes have been recorded.
  8.  
  9. Stored fault codes may be erased from memory at any time by removing power from the ECM for at least 30 seconds. This time period must be increased as the temperature drops. It may be necessary to clear stored codes during diagnosis to check for any recurrence during a test drive, but the stored codes must be written down when retrieved. The codes may still be required for subsequent troubleshooting. Whenever a repair is complete, the stored codes must be erased and the vehicle test driven to confirm correct operation and repair.
  10.  

COMPONENT TESTING



Oxygen Sensor
  1. Warm the engine to normal operating temperature.
  2.  
  3. Disconnect the connector of the oxygen sensor.
  4.  
  5. Connect a voltmeter between the sensor side terminal and ground.
  6.  

Never apply voltage to the oxygen sensor as it may cause damage to the sensor.

  1. While keeping the engine running at 1500-2000 rpm, push the lever on the wide open micro switch down and ensure the reading on the voltmeter is about 0.8 volts.
  2.  
  3. With the engine running at 1000-1500 rpm, disconnect the vacuum hose at the intake manifold under the wide open switch. Ensure the reading on the voltmeter is about zero volts.
  4.  
  5. After checking, reconnect the vacuum hose to the intake manifold and connector to the oxygen sensor.
  6.  

Idle Micro Switch

See Figures 3 and 4



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Fig. Fig. 3: Checking the idle micro switch



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Fig. Fig. 4: Idle micro switch lever

  1. Warm the engine to normal operating temperature.
  2.  
  3. Disconnect the micro switch connector and connect an ohmmeter to the top two terminals.
  4.  
  5. Run the engine at idle and make sure that the reading is zero.
  6.  
  7. After connecting a tachometer to the engine, increase engine rpm gradually from idle speed and make sure that engine rpm is between 1500-2400 rpm when ohmmeter indicates infinity. If it is not within specified range, make adjustment by bending lever. Bend lever down when engine rpm is below specification and up when over specification.
  8.  

Wide Open Throttle Switch

See Figures 5, 6 and 7

  1. Remove air cleaner and carburetor
  2.  



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Fig. Fig. 5: Checking the wide open micro switch



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Fig. Fig. 6: Checking clearance between throttle valve and carburetor bore



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Fig. Fig. 7: Wide open throttle micro switch lever

  1. Connect an ohmmeter to wide open micro switch terminals and check for resistance. Resistance should be infinity.
  2.  
  3. Open throttle valve gradually until the ohmmeter indicates zero ohms. Then, using a vernier caliper, measure the clearance between throttle valve and carburetor bore. Clearance should be within 0.29-0.33 in. (7.2-8.4mm). If the clearance is out of specification, make adjustment by bending the wide open micro switch lever.
  4.  

Mixture Control Solenoid Valve
  1. Remove the cap of the duty check connector located in the engine compartment behind the water reservoir tank and the battery. Connect the terminals with a jumper wire.
  2.  
  3. Turn ignition ON and OFF repeatedly without starting the engine.
  4.  
  5. Touch the carburetor body near the mixture control solenoid valve and make sure that carburetor makes small vibrations according to the operation of the ignition switch.
  6.  

Thermal Switch
  1. Disconnect the thermal switch electrical connector.
  2.  
  3. With coolant temperature below 86°F (30°C), check that there is continuity between the terminals.
  4.  
  5. With coolant temperature above 116°F (46°C), check that there is no continuity between the terminals.
  6.  

Thermal Engine Compartment Switch
  1. Disconnect switch connector and connect ohmmeter between terminals on switch side.
  2.  
  3. With atmospheric temperature is 44°F (7°C) ohmmeter should read zero ohms.
  4.  
  5. With atmospheric temperature is 67°F (19°C) ohmmeter should read infinity.
  6.  

High Altitude Compensator
  1. Disconnect the compensator electrical connector and connect an ohmmeter between the terminals.
  2.  
  3. At an altitude above 4000 feet, the ohmmeter should read zero ohms.
  4.  
  5. At an altitude below 4000 feet, the ohmmeter should read infinity.
  6.  

 
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