El Cid

For last few months my Automatic HVAC on my 2010 seems to be operating strangely. The AC and heat both work.
For six years plus, no problems.
However, it seems now that the system wants to discharge cold air from all the upper vents at higher fan speed and continuously.
I usually have temp set on 68 or 70. Now the outside temp can be as low as 50 and the HVAC is pushing cold air out of upper vents. If I raise Auto temp to 76 or so, it will stop it, but then gets too warm in car.

Dealership did the DAS/Xentry tests and everything showed normal.
SA says the Auto system is usually 10-15 degrees variable.

I have set it on manual and it seems to respond better that way.
When on Auto, the display does not show Fan speed nor Mode, so no idea how fast fan is running or where discharge is being directed.

Thanks,

thefisch

I often have an issue where it seems to run too high of a fan speed and too cold for the temperature needed. I've gotten used to bumping the fan speed down and that seems to work without having to adjust the temp. Every now and then I've got to raise the temp.

I attribute some of the issue to the vent positioning - since it is often hot in FL I have the vents pointed right at me. I know I could move the vents but it's just easier to hit the fan speed.

El Cid

In my car, if you have it on Auto there is no way to adjust the fan speed or mode. You have to turn Auto off and use "manual."
My issue is that the Auto function doesn't seem to work as well as it used to.

blancawilliams

I also face this issue.

http://www.aspenheating.ca/

homeyclaus

Somewhere in this forum there is the key combination for the HVAC diagnostics. You will likely find it's the input air temperature sensor reading high. I had a Nissan Maxima that did this.

El Cid

Thanks, but the Star diagnostics thread is 45 pages long and very confusing to me. The dealership did the Xentry diagnostics in October and the HVAC portion tested normal.
Are you talking about something different?
What does the input air temperature sensor read? Where is it located?
What about the "thermostat" that reads the inside temp of car to automatically adjust the system? I assume there is such a thing.
Maybe they just go bad over time? Car only has 25,000 miles on it, but HVAC has always been on Auto.
Was by the dealership last week and mentioned situation to SA again and he said the Auto system may read 10-15 degrees off.
Thanks for your help.

homeyclaus

Yes - there is a diagnostic mode for the hvac on its own that tells the values of all of its sensors. One of them will be reading out of range and causing the system to heat or cool the air too much as a result. The systems aren't complicated, there is input from three sensors that tells the hvac computer what the various temperatures are, and it sets the vents, fan speeds and ac compressor from that.

They do go bad over time - they're thermistors, exposed to air rushing by them, vibrations, etc. Whether the system is set to auto or not has nothing to do with their lifespan.

Google mercedes ac diagnostic mode and you'll see videos of people doing this. I'll wager one of the temp readings will say "99 degrees" meaning it's no longer conducting. I am not sure where those sensors are located on your particular model.

cetialpha5

Yeah, there's definitely temperature sensors inside. Last scan I did showed a bad temperature sensor in the passenger footwell. Haven't gotten around to fixing it yet because it doesn't seem to affect the temperature that much. I think there's also a blend valve and I think in the old model if it went bad, it wouldn't necessarily produce a code.

El Cid

Tried that, but couldn't find one specifically addressing sensors or too much cold air being produced.
I will get the dealer to do the checks, but would kind of like to be able to tell them specifically what to check.
Thanks.

El Cid

Anyone know where the sensors are located in 2010 E350 Sedan?
Could vacuming possibly help? In case dust, etc. inside.
I think the sun sensor is the little thing on the dash close to the windshield.

El Cid

Don't think it is the "sun sensor" as last night, very dark country roads and still blows cold air out of upper vents. It was 64 outside and set Auto on 74 inside.
Actually I changed Auto from 68 to 70 to 71 to 72 to 74. Each time, warm air out of upper vents for about one minute, then back to cold again.

Andrew Barnes

I seen same whether on auto or not…. Was at the actual middle air vents the tiny slide from blue or red or no color…!It has an Auto light at top. If you roll push the switch to furtherest up you’ll get a red light come on. Vents are set to Auto. That scrolling switch can roll back Now it’s opening and closing auto choosing which vent to use and allows for a auto temp move and all is ok. 2nd thing I saw was when low gas and guy didn’t do it right so check by relays no idiot hasn’t put an extra wire into fuse box bridging the relay hence fan on max no matter what! Saw this on email now and know it’s older but thought maybe worth an update.
Hope that makes sense.
Andrew B