R171 Weird Climate Control / Air-Con Issue

[st13phil]

Used my wife's 2009 SLK350 for a trip to Falmouth over the last few days and observed a strange fault with the air-con. It has the proper Climate Control rather than the basic on/off air-con BTW.

Driving along on the A30 in the p*ssing rain minding our own business with the Climate Control in full auto mode and air-con (A/C) on when I noticed the screen starting to mist up. Within a minute or so we could feel the humidity level rise within the cabin and all the windows were misting badly. The air-con "ON" tell-tale was still illuminated, but clearly the air-con wasn't actually doing anything. Prodded the A/C button a couple of times to cycle it off and on again, but with no effect. Switched the demister and heated screen on so I could see out of the windows and once they cleared, switched the Climate Control back to full auto mode whereupon after a couple of minutes the misting started again. However, after another few minutes and with no further intervention, we suddenly felt the humidity level in the cabin dropping and the window misting cleared as the A/C was clearly working again.

This sequence of events happened a few times over the trip, but with no obvious pattern. Clearly the A/C decides to stop dehumidifying for some reason or another and then kicks into life again after an indeterminate time.

We've had the car from new for six-and-a-bit years and it's just started doing this in the last few weeks (according to my wife who now tells me "it's been doing this at random for a while now" ), so there's clearly a fault but the question is what?

Any ideas?

 

[GP801]

Good luck finding the cause. We have had the same issue with ours for 2-3 years now. Just happens randomly for no obvious reason. Setting it to full demist or the AC to off and back on again usually sorts it out. It has been checked out a couple of times by the dealer. No problem found. Only thing were some old codes that suggested the compressor had stopped working for a short time. If you do find out what causes it then I would be really keen to know. I think someone else on here has also had the same thing with no cause found.

 

[richard]

Have you ever had it re-gassed ?

At 6.5 years old, its probably low on gas.
I believe the gas contains the lubricant (an aircon expert here can comment), so it is worth doing.

I think Jay (Mercland in Nuneaton) can re-gas for you if he is nearby. (so can many other places, but he is great)

Richard

 

[st13phil]

Good luck finding the cause.

Since I posted here I've had a search on the SLK World forum and it would appear that a common cause is a faulty front SAM, but rarely are any fault codes stored. A number of US-based owners have been through compressor replacements (that didn't fix the problem) and have ultimately had the problem cured by replacing the front SAM.

I guess I need to go see someone who can hook it up to STAR and then take it from there...

 

[st13phil]

Have you ever had it re-gassed ?

At 6.5 years old, its probably low on gas.

Thanks Richard. I confess that I'm never convinced by re-gassing an air-con system unless the leak has been found and fixed as that's the only way the refrigerant disappears.

You're right though, it's worth checking as marginally low gas can cause some strange air-con faults.

 

[GP801]

Have you ever had it re-gassed ?

At 6.5 years old, its probably low on gas.
I believe the gas contains the lubricant (an aircon expert here can comment), so it is worth doing.

I think Jay (Mercland in Nuneaton) can re-gas for you if he is nearby. (so can many other places, but he is great)

Richard

They regassed ours even though at 8 years old it was still fully charged with gas. Made no difference at all.

 

[ItalianTuneUp]

Different car, I know, but my S-class has had slight air con misting occasionally too.

I have no idea what causes it, but could heavy rain build up in the (partially blocked) drains, and then be blown into the cabin via the air con system? My car model has a known fault that MB USA had a class action lawsuit made against them for.