I did a quick test today to familiarize myself with measuring duty cycle - as I'm chasing driveablity issues. After reading several threads, I assumed that by connecting my meter leads red - pin 3, black - pin 2 on the capped, round drivers fender mounted port, I'd get some useable numbers.

Key on, engine off, I get 85%. But I do not have a CA car as far as I can tell, only 1 O2 sensor, no LED by the battery...

Also, now I may have connected my leads backwards on my meter. I had to flip them as my first try reading was 15% and that sounded very wrong.

With the car running, it goes to 99.7% or so.

Can someone point me in the right direction here? Being a late model car, I suspect something differs from the typical method of reading it?

I also have a homemade LED/Switch with banana plugs to read codes with, if that's required.

I have the same year and model. You have to put it into 'Diagnostic Mode' before you can read/adjust duty cycle.

Push and hold the button down on the LED tester for 4 seconds and release. You should then see the fluctuating duty cycle.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible

Ach Ha!



I'll check it in the morning. Thanks boss

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http://i54.tinypic.com/1z20won.jpg
- 1993 w201 2.3L Auto -
- 2002 GMC Sierra Denali AWD Quadrasteer -

ha, couldn't wait. Got it in diag mode - now reads 70%

cold idle = 50%

warmed up = 81.9% little to no fluctuation

Indicitave of a problem?

Thanks guys!!

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http://i54.tinypic.com/1z20won.jpg
- 1993 w201 2.3L Auto -
- 2002 GMC Sierra Denali AWD Quadrasteer -

Yes. If its not fluctuating, most likely O2 sensor (if you haven't changed it). Next look at ECU.

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http://i193.photobucket.com/albums/z...-RESIZED-1.jpg
1991 300E - 212K and rising fast...

great, i'll grab a generic bosch 3 wire(?) tomorrow from the parts store and wire it up, and try again!

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http://i54.tinypic.com/1z20won.jpg
- 1993 w201 2.3L Auto -
- 2002 GMC Sierra Denali AWD Quadrasteer -

Yep that one works great. Do a search for the exact part number. And is like $30-40

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http://i193.photobucket.com/albums/z...-RESIZED-1.jpg
1991 300E - 212K and rising fast...

snagged a 1990 Ford Mustang 5.0 GT upstream sensor... didn't have a generic.

I THINK (read online) it's the same one - can anyone confirm?

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http://i54.tinypic.com/1z20won.jpg
- 1993 w201 2.3L Auto -
- 2002 GMC Sierra Denali AWD Quadrasteer -

Yes that's the one, it should be a three wire O2. It will splice right in to the factory harness.

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http://i193.photobucket.com/albums/z...-RESIZED-1.jpg
1991 300E - 212K and rising fast...

Hmmm... before I replace this guy, I spliced my meter inline to the O2, on the MV scale, and I'm getting .450-.410 MV at idle, fully warmed up.

Should I go ahead and replace the O2 still?

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http://i54.tinypic.com/1z20won.jpg
- 1993 w201 2.3L Auto -
- 2002 GMC Sierra Denali AWD Quadrasteer -

Hmm. That is in the proper range for a good O2 sensor. Is it fluctuating up & down as well?

If your duty cycle is not fluctuating when warm that would imply the car is NOT in closed loop. I really want to say ECU. I had the same problem last year, my cold duty cycle was fine, but when warm it never went into closed loop. Failed emissions horribly. I'm not sure if there is a way to test the ECU output. Got any good junkyards around?

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http://i193.photobucket.com/albums/z...-RESIZED-1.jpg
1991 300E - 212K and rising fast...