O2 sensor light in 85 380SL

[Southin]

My O2 sensor light remains lit despite normal readings from the sensor itself. The nearby MB dealer could offer no explanation, except to "suggest" that there may be a fault in the car's computer, which would be a major expense to replace. When having minor service performed by a local foreign car specialist, I mentioned this problem to him and he researched it on ALLDATA. After explaining that an illuminated sensor light means that the O2 sensor requires service (no surprise there!), ALLDATA goes on the state, and I quote exactly: "After the necessary service is completed, remove the oxygen sensor light bulb." This seems a somewhat...well, low-tech solution, and also raises the question of how you know the sensor is properly functioning in the future. For the time being, I have adopted the ALLDATA solution, but wonder if it is the correct one and if that is what everyone does.

[ctaylor738]

I don't have the ETM with me, but isn't the light hard-wired to the odometer at 60K?

[RandyA]

Originally posted by Southin
My O2 sensor light remains lit despite normal readings from the sensor itself. The nearby MB dealer could offer no explanation, except to "suggest" that there may be a fault in the car's computer, which would be a major expense to replace. When having minor service performed by a local foreign car specialist, I mentioned this problem to him and he researched it on ALLDATA. After explaining that an illuminated sensor light means that the O2 sensor requires service (no surprise there!), ALLDATA goes on the state, and I quote exactly: "After the necessary service is completed, remove the oxygen sensor light bulb." This seems a somewhat...well, low-tech solution, and also raises the question of how you know the sensor is properly functioning in the future. For the time being, I have adopted the ALLDATA solution, but wonder if it is the correct one and if that is what everyone does.

I think that's the mechanic's version of ALLDATA.

The version written for Engineers, PHd's and other professionally educated would follow with. "Reinsert the bulb" by grasping between opposing digits exerting a force not to exceed 05-E12 n/m2

Removing the bulb resets the monitor....you may then reinsert the bulb.

[Southin]

Actually, I quoted from the ALLDATA printout in its entirety; there is no mention of replacing the bulb. However, the Mercedes mechanic did say that removing and replacing the bulb is the accepted procedure for resetting the sensor light. In this case, the "accepted procedure" didn't work. So...either the sensor light by design can't be reset on an 85 380SL and the Mercedes mechanic was unfamiliar with this fact (and ALLDADA is correct)...or the light can be reset but for some reason mine can't (and ALLDATA is wrong or at least misleadingly laconic). The fact is that my light won't go out, despite normal sensor reading and despite following the usual procedure for extinguishing it, and I want to know why.

[ctaylor738]

Just back to my home office and able to look at the ETM for the '83-'85 SL.

The O2 light's connection is to the speedometer. There is no connection to the sensor or to the control unit. Therefore it is impossible for the light to indicate the sensor's condition.

The last step in the instructions in the Maintenance Section for O2 sensor replacement:

"Remove oxygen sensor warning lamp bulb."

I continue to believe it is a one-shot deal at 60K on the clock.

The warning lamp in the 560SL is controlled by the CIS unit.

[Southin]

Your reply, which squares with ALLDATTA's, seems dispositive concerning this issue. I've removed the bulb and moved on with my life. Thanks.