O2 sensor test and inverse duty cycle - PeachParts Mercedes-Benz Forum

LeaUK · #4 04-06-2005, 03:05 PM

O2 sensor testing

Quote:

The duty cycle is a synthesised signal

Are you describing the signal measured at the diag connector or at the Lambda? If the diag signal represents a synthesised 'input' then can it be used to measure how much compensation the system is making? In a closed loop system, if only the output is measured, we cannot tell how much adjustment has been made.

Quote:

If you have a scope, you can eavesdrop on the actual O2 sensor output by pulling the O2 sensor connector out just enough to get a scope test lead attached with the other scope test lead to ground. The wave form should jump quickly between about 0.15 and 0.88 volt and back with a frequency of about 1-3 Hz at idle.

Looking at the graph for the O2 sensor, I assume this is due to the extremely high transfer ratio about the stoichiometric value. Is this true?

I have a scope and will be doing this check. But I'm not sure what I will achieve. If the closed loop system is running correctly I would expect a reading of 50%. I think my engine is a little lean and so as it's closed loop am I correct in thinking I'll still see 50%. I was hoping to find out how lean.

Which signal reveals more, the diag synth sig or the O2 sensor output direct. Whilst typing this, I'm starting to think the diag o/p....

Failing Cat?

Not only this, but my Cat is potentially failing CO emissions (at tailpipe), these are:

Heated Cat -
Co 0.2-0.3% (UK limit 0.3%)
HC 100ppm
Lambda 0.97-1.02

With such a good Lambda reading I'm guessing that the O2 sensor and control system are performing fine, but my 12 year old Cat is simply starting to give up. Hence I want to check the O2 sensor with a scope just to see what's really going on.

Can anyone recommend a test procedure by any chance?

Thanks
Lea