Posted to Technical Theory Forum on 6/13/2009
HO2S aging DTC is nothing new, neither is rear HO2S'
influence on pilot Lambda control. A recent case study made
me thinking about those issues.
I have seen a few cases of pre-CAT HO2S aging DTCs. MB's way
to detect is to use what's called Tv (not the one you watch
at night.:-), it's the dwell on STFT, we have seen how this
is at work when it's designed to biased slightly rich or
lean. But, MB system doesn't bias that way. It's a Lambda
1.000 control, instead, it measures this Tv to determine if
pre-CAT HO2S is aged enough. As those aged ones will cause
larger than normal Lambda excursion.
2005 MB ML 500 came in for MIL on, not the 1st time.
"George, can you figure out this?", "Let me try" ...; BTW,
this vehicle is right before MB started using A/F sensors
SDS short test
Okay, both banks pre-CAT HO2S are switching. They say rear
HO2S can stay anywhere it feels like, right?:-) So lets say
it's okay too for now.:-)
I did look at LTFT and STFT, a little strange, but no
smoking guns there.
STFT with a little swing
Next, I went to look at Lambda control graphing, basically,
HO2S graphing on SDS. Sorry for no color and poor
resolution, the best I can show you for SDS image.
Huh? Interesting. Why B1 pre-cat HO2S say it's lean biased
and rear says it's rich? And completely opposite for B2?
I did verify sensor readings are correct comparing on-board
and off-board, they matched. SDS guided test also mention to
test HO2S heaters, I did that too.
Bx1 Heater Voltage & B1S1 Heater Current
The measurement on peak current to the heater is a little
over 500mA, both banks are the same. By now, it's obvious,
this is not a large Lambda excursion, but a pre-CAT HO2S and
after-CAT HO2S don't agree with each other. It turned out to
be a cross connected rear HO2S.
These are after "repair" testing results.
After "repair" Idle
After "repair" 2000 rpm
From this case, what I have realized. 1, Rear HO2S influence
on Lambda pilot can be quite strong (I couldn't find any
document in detail in MB WIS, I have to piece them together
by myself, I wish I didn't have to). 2, What does it mean
"delay time is too short" in DTC? I figured, it's negatively
Is there any other auto manufacture(s) also using these?
I didn't measure oxygen content, I wonder what it could
George from California
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