Checking Oxygen Sensor System Control Unit

Control unit function and regulation of the system can be checked by simulating the thermoswitch and oxygen sensor signals to the control unit. All tests require measuring the frequency valve duty cycle with the fuel pump jumpered, as described above.

To test the cold start enrichment function of the control unit, detach the wires from the thermoswitch and bridge the terminals of the disconnected plug with a jumper wire as shown previously in Fig. 5-2. With the fuel pump running and the ignition turned on, the duty cycle should be a steady 80% ± 2% (72° ± 2° on a dwell meter).

To test the control unit's stand-by or limp-home mode, remove wires from the thermoswitch, but do not bridge them. Run the fuel pump and turn on the ignition. The duty cycle should be a steady 50% ± 2% (45° ± 2° on a dwell meter).

To test the control unit's normal running, mixture regulating function, remove the wires from the thermoswitch, but do not bridge them. Disconnect the green wire from the oxygen sensor, and ground the green wire as indicated in Fig. 5-4. Run the fuel pump, and turn on the ignition. The duty cycle should be above 87% (78° on a dwell meter). Stop the fuel pump and turn off the ignition.

Fig. 5-4.

Oxygen sensor green wire (1) grounded (A), and connected to "D" size 1.5 volt flashlight battery for test (B). Negative (-) end of battery should contact ground.

Next, connect the green oxygen sensor wire to a 1.5 volt "D" size flashlight battery, as indicated in Fig. 5-4. The negative (-) end of the flashlight battery should contact ground. Run the fuel pump and turn on the ignition. The duty cycle should be below 20% (18° on a dwell meter).