Hi all,

From what I read in this forum the voltage on the output of the O2 sensor is supposed to be around 450 mV (fluctuating a bit) . I get only about 30 mV on both the 260E and 300E. Revving the engine decreases it to about 10 mV .

The way I measure it is that , while the engine is idling at operating temperature, I place one terminal of the digital voltmeter on Battery's negative Pole and the other terminal at the wire coming out of the O2 sensor. The voltage at the flat connector, the heater, is about 13 V. Interestingly, the voltage at the ECU pin where the O2 sensor is supposed to be connected is 450mV fixed.

I must be measuring it wrong since the cars are running fine: hovering around %50 duty cycle , EHA current normal, etc. I replaced the O2 sensors few weeks ago.

Any input will be appriciated.

Thanks.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)

Are you saying you disconnected the O2 sensor signal line to measure the voltage? That's a no-no.

The voltmeter must be installed parallel to the circuit without altering the circuit! Pull the O2 sensor signal lead out of the socket enough to connect the voltmeter probe without interrupting the circuit. Since it's a rather long pin, I've found this easy to do with some extentions that have very small clips at the ends, so I only have to pull the pin out about 1/8" inch. If you get a check engine light with an O2 sensor code, you've interrupted the circuit.

Use any good ground and the battery negative terminal is fine.

A digital voltmeter should jump around a bit near 450 mV.

If you have a scope the voltage should oscillate between about 0.2 and 0.8 at a frequency of about 0.5 to 1.5 Hertz.

Duke

Thanks Duke, I understand now.

I was just following what the Service Manual says (07.3.11 I-11/29).

I didn't think the O2 sensor can actually emit half a volt. I guess the O2 sensor is acting as a ground point for pin 8 of ECU ( which is always 450 mV) and its voltages fluctuates around few mVolts.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)

With a voltmeter you read the average output of the O2 sensor, which should be about 450 mV. With a scope you read the actual instantaneous output, which jumps back and forth between about 200 and 800 mV as the system cycles between slighly lean and slightly rich of stoichiometric.

Duke

We can also get a proper reading by disconnecting the O2 sensor plug, grounding the pin going to ECU and reading the voltage at the O2 sensor output.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)