Feedback System Checks

Fig. 4 Oxygen Sensor Indicator Lamp:

The operation of the oxygen sensor and ECM can be checked by using the sensor indicator lamp in the instrument cluster, Fig. 4. The sensor lamp flashes automatically at 60,000 mile intervals when the engine is at normal operating temperature.
The system check can be performed at any mileage if any of the following conditions occur:
1. Excessive fuel consumption.
2. Engine tends to stall.
3. Engine is hard to start.
If any of above conditions exist, proceed as follows:

Fig. 5 Feedback Circuit Cancel Switch:

1. Turn on cancel switch located in fuse box, Fig. 5.
2. Turn on ignition without starting engine. Sensor lamp should light, but not flash. If lamp does not light, check wiring and bulb.
3. If lamp lights, start engine and allow to reach normal operating temperature.
4. With engine warmed up, increase speed to 1500--2000 RPM. Ensure sensor lamp flashes to indicate system is functioning properly. If lamp does not flash, check for the following:
a. Defective oxygen sensor.
b. Defective mixture control solenoid valve.
c. Defective carburetor or improperly adjusted idle mixture.
d. Defective thermal switch.
e. Loose or disconnected emission control system wiring.
f. Defective ECM.
g. Defective idle or wide open throttle microswitches.
5. After checking that sensor lamp flashes, shutoff cancel switch in fuse box.
6. Stop engine.

If sensor light does not flash at 60,000 or 120,000 miles, or when cancel switch in fuse panel is turned on, check feedback system operation as follows:
1. Remove ECM from instrument panel leaving connector connected.
2. Start engine and allow to idle at normal operating temperature.

Fig. 21 Oxygen Sensor Signal Test:

3. Connect voltmeter between terminals 10 (oxygen sensor signal) and 21 (ground), Fig. 21.

Fig. 22 Oxygen Sensor Troubleshooting Chart:

4. Between idling and 1500--2000 RPM, voltmeter needle should deflect between 0 and .8 volts. If voltmeter needle, indicates zero, refer to Fig. 22.
5. After confirming that oxygen sensor is properly signaling ECM, check feed back system as follows:

Fig. 23 Feedback Signal Test:

a. Connect positive lead of ohmmeter to body and negative lead to terminal 17, Fig. 23. Do not reverse connections, since damage to ECM may result.
b. Ensure ohmmeter needle deflects, which means that ECM is sending feed back signal. Turn on cancel switch in fuse box to check if sensor lamp flashes. If not, check if bulb is burned out.
6. Reinstall ECM into instrument panel, ensuring connector is securely connected.