AC won't blow cold - Mercedes Benz SLK Forum

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#1 Old 05-27-2019
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Radiator Fan Won't Turn Fast

Hi.

New owner of 99 SLK230 with 123K. Auto transmission. The radiator fan won't turn on high speed. It comes on and turns, but not super fast. I think this may be the reason the AC doesn't blow cold also could be making the car run hotter than necessary. Please advise on what the problem might be and how I might be able to diagnose. Thanks in advance.
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#2 Old 05-29-2019
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AC won't blow cold

My 99 slk's AC won't blow cold air. The previous owner told me that it's it's full of refrigerant and is holding pressure. However, he believes the issue is that the radiator fan doesn't kick into high speed, thus not dissipating heat sufficiently. Thoughts on where I should start to check things? Is there a way I can force the fan to kick into high speed to confirm if that's the problem? Thanks in advance.
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#3 Old 05-29-2019
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#4 Old 05-29-2019
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is the air coming out hot or outside temperature?
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#5 Old 05-29-2019
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Quote:
Originally Posted by Turdo2 View Post
is the air coming out hot or outside temperature?
The air is definitely hotter than the outside air temp.
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#6 Old 05-29-2019
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Quote:
Originally Posted by okccj View Post
The air is definitely hotter than the outside air temp.
Sounds like you have a problem with your duo valve. it's on the right side in front of the battery. It is supposed to shut off circulation of coolant to the heater core.

I dont know how to check it.. that's for the smarter among us!

You could clamp (vice-grips?) the inlet line coming out of the back of the engine and see if it makes a difference.
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#7 Old 05-29-2019
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Quote:
Originally Posted by Turdo2 View Post
Sounds like you have a problem with your duo valve. it's on the right side in front of the battery. It is supposed to shut off circulation of coolant to the heater core.

I dont know how to check it.. that's for the smarter among us!

You could clamp (vice-grips?) the inlet line coming out of the back of the engine and see if it makes a difference.
Easy to check the duo valve.
One pipe goes in, two come out.

If the two out pipes are hot when both temp wheels are on the blue setting (cold) the duo isn't working.
If dials are on blue and you pull the power lead off and push back on, you should hear an audible 'click', if not, check fuse 15 to start with.
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#8 Old 05-29-2019
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Just to add to the above. The rad fan will not normally run at full speed unless everything is desperately hot. When you turn on the AC it will turn over at a low to medium speed initially, you can open the bonnet/hood and check.
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#9 Old 05-29-2019
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It's not an AC problem as such, AC could in fact be perfectly fine.
More of a cabin air temp intake issue, otherwise there would still be an ambient temperature.
Stands to reason that the duo is open.

They are open by default, apply power (by turning the dials to blue) to close.

I had THIS - don't laugh

Also to add further .. the most common failing of the duo valve if there is power, is that one of the plungers will get stuck and that will result in one side of the cabin being hot and the other cold .. you gotta be really unlucky to have them both stuck
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#10 Old 05-29-2019
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Thanks for the replies. I followed the instruction in this video to test is my duo valve is functioning.

Per the video, I didn't hear any sound when unplugging and plugging the electrical connection. As such, I suspect a need a new duo valve? Where's the best place to buy one?
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#11 Old 05-29-2019
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#13 Old 05-29-2019
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Originally Posted by okccj View Post
Thanks for the replies. I followed the instruction in this video to test is my duo valve is functioning. https://www.youtube.com/watch?v=8jhPEfLI45s&t=38s

Per the video, I didn't hear any sound when unplugging and plugging the electrical connection. As such, I suspect a need a new duo valve? Where's the best place to buy one?

If you are certain the duo valve is not functioning and have the noted change in voltage on both sides one thing to try before spending your coin is to take off the tops, remove the plungers and clean them.
Polish them with 10000 steel wool or equivalent making sure they are super clean when reinstalling them. A light lick of fine sewing machine oil I have found helps.
Be sure to put a blanket under them so if you inadvertently drop something it gets caught before going somewhere were you cannot easily see it.
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#14 Old 05-30-2019
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Originally Posted by okccj View Post
Hi.

New owner of 99 SLK230 with 123K. Auto transmission. The radiator fan won't turn on high speed. It comes on and turns, but not super fast. I think this may be the reason the AC doesn't blow cold also could be making the car run hotter than necessary. Please advise on what the problem might be and how I might be able to diagnose. Thanks in advance.
Maybe does not help much but I have never seen/heard my radiator fan running at anything over a slow/silent speed even with car left at idle for extended periods of time. Temp never seems to get above approx 90deg.C in use.
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#15 Old 05-30-2019
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Could be a bad fan control module.
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#16 Old 05-30-2019
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Hi,

Cooling Fans are controlled by PWM Signal from Motor ECU to Fan Controller, speed selected is based on Engine Coolant Temp and A/C on / off, lots of different speeds expressed in SDS as a percentage of Flat Out .................

I have never heard any of my MB's run at full speed except when testing them on SDS, and on a very hot (UK) day with A/C on :wink

HTH, Cheers Dave
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#17 Old 05-30-2019
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Quote:
Originally Posted by Mn car guy View Post
Could be a bad fan control module.
Could be, easy enough to check for power first tho.
Run voltmeter across the plug, if power is present, all is well to that point. If no power, there's an issue elsewhere.

Check duos for opening and closing by hooking up direct to battery, if no audible click, duos are fubar. Disassembly may or may not reveal the issue.

I went that route myself before discovering that I should have had a fuse where there wasn't one .. sometimes the obvious thing is the thing that gets overlooked

Fortunately, W210 module just goes in and they're as cheap as chips over here in UK AND you get black dials

Suspect that the W210 and W202 duo valves to be the same as the R170s albeit on different bracketry but approx half the price .. (in this part of the world at least)
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#18 Old 05-30-2019
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Hello


the first day I got my 2003 230. I was checking it out alone before i bought it. I see the button for AC so i pressed it until the light came on. Heat control dials set to blue, location dial set to feet and face and all i got was hot air. I told the salesman about it. He got in the car started it, Pressed to AC button until the light was off. The AC stated to work. I looked closer to the AC button in small print it said OFF.



So when the AC light is off, the AC is really ON!
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#19 Old 05-31-2019
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Originally Posted by Dave2302 View Post
I have never heard any of my MB's run at full speed except when testing them on SDS, and on a very hot (UK) day with A/C on :wink
Clearly that was before you moved to Scotland then
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#20 Old 05-31-2019
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Quote:
Originally Posted by Kim Baril View Post
Hello


the first day I got my 2003 230. I was checking it out alone before i bought it. I see the button for AC so i pressed it until the light came on. Heat control dials set to blue, location dial set to feet and face and all i got was hot air. I told the salesman about it. He got in the car started it, Pressed to AC button until the light was off. The AC stated to work. I looked closer to the AC button in small print it said OFF.



So when the AC light is off, the AC is really ON!
Your button says AC and not EC?
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