I have a question about ACC in a '97 C280....

I shouldn't complain since I'm getting nice cold air... but..... up until recently if I set the A/C temp to say, 74 degrees in AUTO, the system was able to NAIL 74 degrees and keep it there - I never had to touch it. Hot or cold outside. Now I've noticed when it gets really hot outside, the A/C will cool the cabin down just fine, but it won't let up cooling. I have to manually set the temp. to about 80-82 degrees just to keep from freezing to death. At night when it cools off outside, the A/C nails the preset temp like it should. I have to jack the temp up about 10 degrees hotter during the daytime. I took it into the shop and the tech said they had error codes for a mod valve (?) and a water pump (?). They were replaced - the A/C worked a little better, but instead of a 10 degree change now it's about a 7.

Since owning the car I've had a new compressor, expansion valve, evaporator, and a few other sundry related items replaced. With this latest "failure" the A/C is sloppy at maintaining a steady temp without me messing with it. The shop said the temp sensors were ok. I thought the sensor intake on the overhead panel might be clogged, but I was told it was ok. Any thoughts? ACC computer related maybe?

Thanks,

Mark E.
Ft. Lauderdale

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Mark E.
Miami, FL
2 X 1997 C280

The in-car temp sensor in the dome lamp IS responsible for keeping the cabin within 2deg F of the desired setting on the control panel.

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If you want to check what the reading of the in-car temp sensor is, hold down the rest button while the A/C is on for at least 5 secs. Then two numbers will start to flash, the first one will be the number of what value is being displayed, the second will be the value. If you do a search, you will find a list of what the values correspond to. If I remember correctly, the first value, number 1, will be the in-car temp sensor's reading in degrees F.

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Ali Al-Chalabi

2001 CLK55
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And this is why I love this site so much! Thanks gents! Ok - to replace this temp sensor, is it a DIY job or should I take it in?

Thanks!

Mark E.
Ft. Lauderdale

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Mark E.
Miami, FL
2 X 1997 C280

Before you replace anything find out the values like was pointed out. Also notice the grill on the sensor in the upper console. Take a piece of tissue and see if the airstream holds it to the grill with the A/C running. If the little tiny aspirator fan fails the sensor gets all the heat from the roof instead of properly being cooled by the airstream the little fan provides.

Without air movement the sensor doesn't see the real inside air temp, especially with the sun beating down.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks Steve! I checked the values - with temp set at 74, the cabin temp fluctuates between 78-81 degrees. If I point the vent registers up towards the sensor intake the A/C starts blowing hot air out. This makes sense to me now - with the ac set to 74, the sensor thinks it's still hot inside the car, keeping the cool air blowing all the time. Point the vents up towards the roof, the sensor reads that it's too cold and blows hot air. Does this sound right?

Mark E.
Ft. Lauderdale

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Mark E.
Miami, FL
2 X 1997 C280

Sounds good to me.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician