AC blows cold but not COLD - Mercedes Benz SLK Forum

SLK R172 General Discussion Discussion regarding the Mercedes SLK R172

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#1 Old 07-15-2019
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AC blows cold but not COLD

Mods sorry if this has been answered. I could SWEAR I read a thread a while back about someone with the same issue, and their solution was replacing a faulty transistor or capacitor inside/on the MAF sensor, despite it not having thrown a code, but I cannot find that thread anywhere.

My 55 is now my 4th SLK (counting the six months with the SLC 43), and they have always had ICE COLD air when turning the AC control all the way down to 60 or the blue dot. This car's AC doesn't do that. The compressor is fine and there are no detectable freon leaks according to the dealer, yet my gut tells me it is "off". It doesnt help that my wife's 2016 C300 also has ICE COLD air on its lowest setting.

I recall a thread about a similar issue where something in the MAF sensor was at fault despite no codes, but searches for AC, air conditioner, cold air, MAF, and Mass Air Flow have been fruitless. I even tried to scan through 16 pages of R172 threads.

Any help would be appreciated. Dealer won't even check anything else since there are no codes, unless I can find the relevant thread here.
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#2 Old 07-15-2019
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Put a thermometer in the air vent with it set max cold. The temperature should come out in the 50s at least.
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#3 Old 07-18-2019
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Put a thermometer in the air vent with it set max cold. The temperature should come out in the 50s at least.
If the temp is higher but the compressor is fine and freon levels are good (no leak) what would be the next logical culprit?
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#4 Old 07-18-2019
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If the temp is higher but the compressor is fine and freon levels are good (no leak) what would be the next logical culprit?
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#5 Old 07-18-2019
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I don't think that the a/c has something to do with the maf.
The dealer has the tool to test the a/c in xentry thru the guided tests.

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#6 Old 07-18-2019
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If compressor performance is ok and Freon level is correct (note that only way to check Freon on MB is to evacuate and refill by weight), a conscription in the system would be the next suspect.
There was a case of a delaminating hose on compressor low side that led to decreased cooling performance but not sure if this ever affected R172s.
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#7 Old 07-18-2019
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Definitely make some measurements over time (say every minute for 10 minutes) at the vents with a thermometer. Measure both driver and passenger vents. From a cold engine start; from a warm engine start; using min and max settings, etc. It's the transistors in the front SAM you've read about here. Also, your duo valve could be stuck open, the evap temp sensors could be failing, or the AC controller could be failing.
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#8 Old 07-19-2019
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Just wish it had thrown a code to make it easier for dealer to trace. They refuse to look at more than the basics without knowing specifically where to look. Thanks for the helpful posts guys. If I get it pinned down I will report back. Without a leak I highly doubt freon is low at only 30k miles sp I wont have that drained and refilled for now. I'll look at those other suspects first.
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#9 Old 07-24-2019
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Do you have the automated climate control? If so, set the temperatures to 72 degrees and stick a meat thermometer in the air vents. You should get a reading around 40 degrees.

Oddly enough, the A/C system won't function ideally at the lowest temperature setting. BTW, my car has ice cold air and it is nearly 10 years old. Of course, it has the digital climate control.

Assuming the system doesn't have any leaks, it probably hasn't been charged properly. If it is under or overcharged, it will not function correctly. You should take it to an independent Mercedes shop and have it checked out. It doesn't need to be evacuated if it's leak free. BTW, a leak check can be done by adding a dye to the system. It takes a week or so for the dye to work its way through the system.
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#10 Old 07-24-2019
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Around here AC is a Big Thing (all of my cars have). A good AC with a clean radiator/condenser (is the engine temp higher than normal) and a good air flow in the front, the dash vent at high fan speed should be between 35-40F. If higher and particularly if cycling then the pressures need to be checked. A good scan tool should be able to read many things about the HVAC system.

Many times the problem is a sticky air blend door.


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