Voltage reading coming from o2 sensor plug, ECU side? M103 in a '91 190e.

[RolandTiangco]

Ran codes on my '91 190e 2.6L after getting a check engine light and a subtle stumble at idle, and got code #17 "oxygen sensor shorted to positive or ground." I recently installed a new Bosch o2 sensor during some exhaust work, but haven't put on many miles since.

So I got out the multimeter and tested the o2 sensor connectors beneath the passenger carpet. I tested the voltage being generated by the o2 sensor. I did so by connecting a jumper wire on the single sensor-wire at the connector, and used the multimeter to measure voltage generated by the o2 sensor. I kept getting a steady reading of .49 volts no matter what conditions, rpm, or temp, which was odd. After probing at various points on the jumper wire, I realized that this reading was actually coming from the ecu-harness side, and not from the o2 sensor. I removed the jumper wire, disconnecting the o2 sensor entirely, and tested voltage coming from the harness (that the sensor plugs into) and learned that it's producing .47-.49 volts. I didn't expect to get a positive voltage reading from the ecu-side of the plug. Is this normal?

Any input would be greatly appreciated.


Roland
NYC

[97 SL320]

Some cars ( all ? ) use a bias voltage from the ECU so the O2 sensor can pull the voltage down or raise it. I'm not sure of the specific voltage you car will have but it sounds about right.

[RolandTiangco]

Ah this makes sense. Thank you for the input!

[Frank Reiner]

The O2 sensor is a Voltage Source

See this from Wikipedia:
Operation of the probe
Zirconia sensor
The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere.

An output voltage of 0.2 V (200 mV) DC represents a "lean mixture" of fuel and oxygen, where the amount of oxygen entering the cylinder is sufficient to fully oxidize the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2). An output voltage of 0.8 V (800 mV) DC represents a "rich mixture", one which is high in unburned fuel and low in remaining oxygen. The ideal setpoint is approximately 0.45 V (450 mV) DC. This is where the quantities of air and fuel are in the optimum ratio, which is ~0.5% lean of the stoichiometric point, such that the exhaust output contains minimal carbon monoxide.

The O2 sensor is to be tested out-of-circuit. Connect voltmeter to the lead from the sensor and to ground (or the ground wire of the sensor).

[97 SL320]

It's a desert topping and a floor wax Frank. . . .

I'll see you. . .

The O2 sensor is a voltage source ( chemical battery ). If exhaust oxygen is low / mixture too rich , there will be an imbalance on either side of the sensor that will generate an electrical current. If exhaust oxygen is high / mixture lean , the two sides balance each other out and there won't be any current generated.


And raise you. . .

Systems apply a bias voltage so the ECM knows the sensor is hot and it's output can be used for mixture adjustments. ( Closed loop )

"Using the data " and diagram on page 3 of the link below.

(AC Delco as in General Motors )
https://www.acdelcotechconnect.com/pdf/TechConnect+MayJune+2012+F.pdf

[RolandTiangco]

Haha, great response.
Thanks Frank - I'm familiar with how an O2 Sensor works.
97SL320 - Thanks for reading, I believe you are correct. The reading I was getting is indeed the bias voltage.