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  #1  
Old 04-01-2005, 08:27 AM
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dm error 2/4

Well the dreaded check engine light is flashing its face again on the W140. Need some help in the diagnostic end (really clarity). Please chirp in with any comments or suggestions.

As I understand it, DM error 2 and 4 are related in that they both use the 02 sensor to check for validity (2 is the O2 sensor inoperative, 4 is the air pump circuit inoperative, or visa versa don't remember). This rose its head about 4 months ago, so I replaced the O2 sensor with a genuine mercedes new O2 sensor. The check engine light didn't go out, flashed back on about a week later.

Anyways, I need to do more diagnostics, and am confused about the output from the x11/4 connector, pin 14, o2s I. Is it giving the O2 voltage (0 to 1 volt) or is it giving the mirrored signal translated to the reference value of car voltage for a duty cycle read (ie, if the O2 is reading 0.5v, it will give 7v if the battery voltage is 14v?.)

My first thought is to meter out the voltage signals immediately at the x3/1 connectors while the car is running but I'm at a lose on how to set up the test leads, hence I want to confirm that I should be getting the O2 sensor values from the X11/4 connector.

Further, I put the car up on a hoist, engine idling, o2 sensor disconnected and probed the connector from the O2 sensor. Was getting 0.7 to 0.9 volts out of it consistently, so it was running rich. When I probed the x11/4 lead, most of the time I got 7v but at times it would vary across the board from 0 to 3v.

Any suggestions on how to attach this?

Regards

Tower

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1995 E320 -171k-km
1995 S420 -333k-km
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  #2  
Old 04-01-2005, 10:50 AM
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'94 S500: only 793 sold!
 
Join Date: Mar 2001
Location: San Diego, CA
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You probably already know this . . .

but you can use the DTCs in Menu#6 for your DM.

If you have a DVM that can measure Duty Cycle, (I use the M0039) you can measure the Lambda (Menu#4) at pin 14; if ok, you should see an oscillation around 50% where the system corrects (a little lean, a little rich) causing the needle to oscillate around 50% point. If the signal doesn't oscillate, the system is open. That could be the fuel injection module, wiring, etc?? Has your wiring harness ever been changed??? Check its condition. That could account for these problems.

On an earlier MB, I lost the "heated" portion of the Q2 sensor due to a bad relay which supplied the battery voltage. While it's not correct, Lambda should still function.

Does the AIR pump engage in the morning on a cold start? If not then maybe the pump is frozen?? With 333km, it well could be dead.

Many times it's a broken vacuum line going the switchover valve and/or the pump itself. It can get knocked off easily causing the CEL.
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  #3  
Old 04-02-2005, 08:21 AM
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further information

Thanks jim for your support.

In answering your questions, the air pump is cycling properly for about the first 90 seconds at startup. Although that doesn't negate that the valve (name escapes me) is operating as well and sending air to the exhaust manifold.

I am attempting to interogate the system but was getting confused at the voltage readings I was receiving at the O2 sensor connection at the x11/4 terminal 14. I don't have a VOM with a duty cycle, but I was measuring the voltage at the output and was getting most of the time 7 volts static. Sometimes it did dip between 1 and 2 volts, but not very often. I was thinking that I should see between 0 and 1 volt but I'm guessing now that it is the translation to a percent of car voltage (not o2 sensor voltage) that is really being sent from the computer to the x11/4, position 14.

I'm trying to check the continuity of the wiring harness between the ECU and the o2 sensor connector beneath the car. I now have the pinouts but am trying to rig a system to check the voltage whilst the car is running. Short of pulling the ECU and probing I'm at a lose on where to make a connection. Any thoughts?

Ron

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