W211 Humidity Sensor - PeachParts Mercedes-Benz Forum

shertex · #1 09-05-2017, 08:41 AM

One of our members helped me figure a weird HVAC symptom on my CDI that happens from time to time....namely, the AC cutting out on a long trip during certain temperature and humidity conditions. Turns out there's a TSB on the issue: problem is easily solved by installing the humidity sensor behind the cabin filter. It was not installed in the W211 from MY06 forward. So it's next on my to-do list.

Any idea why MB decided to delete it? Just a cost saving measure or something else?

Apparently it not only solves the problem but also enables the AC system to work more efficiently.

torsionbar · #2 09-07-2017, 02:14 PM

Have a link to the TSB? Is this only for CDI engine models? Or also Bluetec? Or all 211 sedans?

shertex · #3 09-08-2017, 07:48 AM

Here's the TSB.

Diseasel300 · #4 09-08-2017, 09:38 AM

Is the fan still running or does it shut off as well? I'd expect with the evaporator frozen up that the fan would be running high speed but not blowing any air out the vents. Should be pretty easy to troubleshoot if so. If it is "cutting out", I'd be looking elsewhere.

shertex · #5 09-08-2017, 09:48 AM

Notice that the service bulletin says that reduced airflow is one of the symptoms.

Diseasel300 · #6 09-08-2017, 09:55 AM

Reduced airflow is an effect of the evaporator freezing. The fan will still be running, likely at a higher speed to try to maintain cabin temp. Next time it does it, manually set the fan on high and see if you have little or no airflow.

Before getting carried away with adding a sensor that may or may not fix your issue, it would be worth checking (or replacing) the cabin air filter and the refrigerant charge. Low refrigerant will cause evaporator freezing in a Manhattan minute, especially on a long trip.

I'm frankly shocked that the W211 doesn't have a "freeze stat" in the evaporator area or if it does, why it doesn't prevent the evaporator from becoming a glacier. The entire purpose of it is to keep the evaporator from frosting. Even the W123/W126es had one. When the evaporator reaches the trigger point, the compressor shuts off to allow the fan to melt the frost.

torsionbar · #7 09-08-2017, 10:47 AM

Quote:

Originally Posted by shertex

Here's the TSB.

Awesome thank you.

ILUVMILS · #8 09-08-2017, 12:13 PM

In most cases, frozen evaporators are caused by the evap temp sensor reporting a slightly higher temp than actually exists. This results in the compressor running constantly. I've seen this occur not only on 211's but other models as well...........

shertex · #9 09-08-2017, 04:20 PM

The improved efficiency alone is worth the price of the part. I'd be surprised if refrigerant is low....AC is ice cold.

shertex · #10 09-19-2017, 04:08 PM

Sensor installed and coded. Oddly, two of the settings were already set to humidity dependent (as opposed to fixed value), even though there was no humidity sensor installed in the car.

Ran a diagnostic and no fault codes, so I assume the car like the sensor.

Now I'll watch for my dramatic improvements in fuel economy.