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The Rich-Running Engine and the Bad O2 Sensor Ground CircuitPosted 4/10/2000By Kent Klingensmith
Vehicle: 1988 through 1991 Oldsmobile Delta 88 FWD 3.8L V6 SFI C. System: Emissions/PCM/Fuel Symptom/Problem: Engine runs rich. Block learn and integrator are at, or greater than, 150 while in closed loop. May have code 44. The problem is that the oxygen (O2) sensor bias voltage is greater than .45 volts. This is usually caused by a bad O2 sensor ground circuit, or in uncommon occurrences, may be caused by a defective powertrain control module (PCM).
O2 Sensor Background The PCM compares the voltage between the O2 sensor ground and signal circuits, and the difference between the two is what the PCM "sees" as the actual O2 sensor voltage. The O2 sensor ground circuit is a solid tan wire that goes from pin D3 to a location under the ignition module. This connection is on the same stud as the negative battery cable. Since system charging and cranking currents also go through this connection, there is a potential for problems with this ground. This is the reason the test needs to be done with the engine running because the alternator current is what causes the incorrect voltage on the O2 sensor ground circuit. Test and Fix: The first step is to check the continuity of the O2 sensor harness and ground circuits. This can quickly be done by disconnecting the O2 sensor and measuring the voltage on the purple wire coming from the PCM. This should be done with the engine running and the ground for the meter needs to be a solid connection to the engine block. The PCM, O2 sensor signal, and O2 sensor ground circuits are all good if the reading is in the normal range of between .32V and .45V. If the reading is high, plug the O2 sensor back in and check the voltage between the engine block and the O2 sensor ground (circuit 413) by backprobing at pin D3 of the orange 32-cavity connector on the PCM. The voltage should be less than 25 mV with the engine running. If the voltage is higher, the O2 sensor ground circuit has a problem and needs to be repaired. You may be wondering why the meter needs to be grounded at the engine block. Since the testing is being done on a running engine, the voltage source is the alternator and the battery is actually a load. If there is a voltage drop between the battery and the engine, that voltage can affect the reading of the O2 sensor bias voltage. If the testing described above shows a high O2 sensor bias voltage and the O2 sensor ground circuit is OK, check the O2 sensor signal circuit for a short to voltage. This can be done by disconnecting the PCM and the O2 sensor, then turn on the ignition and measuring the voltage on pin D2 of the orange 32-cavity PCM connector. If voltage is present, the wiring needs repair. If no voltage is present, replace the PCM.
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