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89 300E; 00 E320
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2,515 Posts
Discussion Starter #1
Most of this info came from you guys, but I did go off-site for a couple of things. This is just to have it all in one place.

A balanced O2 Sensor will show 0.45 volts on average and keeps fuel mixture at 14.7 to 1.

G3/2 Heated O2 Sensor ----- Testing the Component

Mercedes Benz Model 124 with M103 engine
124.030; 1989 300E; 103.983; 3 Liter
G3/2 Heated O2-sensor In exhaust pipe, rear of transmission

G3_2 Heated Oxygen Sensor.GIF
w124 o2 sensor oem.jpg

G3/2x1 Connector, O2-sensor heater coil (2-pole) Left side of front passenger footwell
G3/2x2 Connector, O2-sensor signal (1-pole) Left side of front passenger footwell

G3_2 Heated Oxygen Sensor Connectors.GIF

Unsnap and remove carpet in front right footwell
With ignition off and key out.
Locate and disconnect O2 signal connector G3/ 2x2 (wide screwdriver gently turned should unclick it)

o2sen locate.jpg

Insert one single wire strand into connector and reconnect as tight as possible.
one single wire.jpg
insert wire 1.jpg
insert wire 2.jpg
insert wire 3]e.jpg
 

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89 300E; 00 E320
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2,515 Posts
Discussion Starter #2 (Edited)
Start engine and connect multimeter for DC voltage reading range in hundredths (.00)
hundredths.jpg

(red to wire strand just inserted)
testing red.jpg

and ((black to ground (may use door closing eye))
W124 door closing eye.GIF
testing black.jpg

Reading for a good sensor should fluctuate from 0.2 to 0.8 with 0.1 being (lean) and 0.9 being (rich); and average 0.45 volts (balanced). My readings were 0.19 to 0.72 and started cold at 0.49.
mm cold start.jpg


Pull a large vacuum hose from intake and reading should go to 0.1 (lean). Mine went to 0.06.
vac connect.jpg
vac disconnect.jpg

Could not identify fuel return line (51) without removing air cleaner so I did not check to see if the sensor would register rich if the return line were to become restricted.
W124 fuel return line.GIF


After testing is complete, put everything back the way it was.

Wiring Diagram: Electronic Engine Controls 118-1
W124 O2 sensor wiring diag.jpg
 

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W124 Moderator
86 190E 2.3L 16V, 2 95 320TE's, 02 S500
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12,756 Posts
Nice write up. Mod should put this in the DIY section

Jayare
 

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MB 260E, BMW 735i, Range Rover Classic Vogue SE
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2 Posts
nice....could you please guide me where the other end of this vacuum hose connects...thanks in advance


 

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89 300E; 00 E320
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2,515 Posts
Discussion Starter #6
^^^
Not sure where the other end connects, but it is heading back on top of the transmission beside the bowden cable if that helps.
 

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MB 260E, BMW 735i, Range Rover Classic Vogue SE
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2 Posts
Thanks got it....now i know why my upward gear shifts were rough and delayed.
 

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2008 E350 4M, 2016 Audi Allroad, 2019 Audi Q5
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5,447 Posts
Thanks got it....now i know why my upward gear shifts were rough and delayed.
Yep the modulator uses the vacuum to time the shifts correctly.
 
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