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Old 02-14-2006, 05:48 PM
Secret Squirrel's Avatar
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Join Date: Mar 2005
Location: Commerce City, CO
Posts: 74
Smile Oxygen Sensor grounded to chassis on 300SEL?

I went to put a new O2 sensor on today and the old one had a big copper wire with a red jacket attached to a screw into the chassis. It made it a real chore to even turn the wrench with that dang wire smashed down against the threads.

Why would someone have done this? Some type of bypass on the O2 sensor? It wasn't getting as good of mileage as I thought it should have... 17-19 on long cruising trips and occasionally the check engine light would come on after it hadn't been driven for a couple weeks and needed some carbon burned out of the system.

But why the ugly ground wire attachment??

(2) 1988 300 SEL W126
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Old 02-14-2006, 06:54 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The sensor voltage of the O2 sensor is very small by comparison to other sensor readings. In many instances a .2v voltage drop is inconsequential. In a O2 sensor it is s deal breaker. An O2 sensor voltage of .5v needs to reference the same ground as the control unit. One of the surest ways to do that is make real sure both are referenced to the same thing: chassis ground.

In newer 4 wire O2 systems the ground for the sensor is brought back to the control unit to assure they are the same. I presume someone was just making sure.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 02-15-2006, 12:46 PM
Secret Squirrel's Avatar
Registered User
Join Date: Mar 2005
Location: Commerce City, CO
Posts: 74
Steve thanks for the reply,

So is this something I should worry about? I had put it all back together (without the wire) and it's running fine as far as I can tell... no check engine light or anything.

(2) 1988 300 SEL W126
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