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Discussion Starter #1
Long time reader, first time poster. First of let me thank the host, moderators and contributors of this forum. The knowledge and DIY solutions have saved me thousands of dollars. Unfortunately, I'm stumped by a problem with my air conditioner. I'll be as detailed as possible.

Car: 2001 SLK230
Miles: 100,400
Purchased: 2005 with 9,000 miles

Issue:
When I purchased the car it was winter, thus I never needed the AC. During spring I noticed the A/C would not engage unless it was at least ~77. In my ignorance I figured an A/C was a European luxury and wouldn't engage until ~77. (The compressor does not engage at all unless its over ~77 degrees outside, sometimes it has to be hotter.)

This summer I noticed a constant cycling (20 seconds on, 30 off) of the compressor and the cabin would not cool down. After researching the issue, I deducted: 1) I needed a new Evaporator Temperature Sensor, and 2) the AC compressor should engage on start up unless the ambient temperature is closer to 30 rather than closer to 80. In fact, yesterday I was driving in 90 degree heat with the AC on, I hit a rain storm the temp dropped to 68 degrees, the AC cycled off, and the windows fogged up. Once the rain storm passed and it was hot outside, you guessed it AC came back on. This has happened for the past 10 years, but again I thought the AC was temperature sensative at ~77 degrees.

Steps Taken
1) I changed the evaporator temperature sensor. The constant cycling issue has ceased and boy does it blow cold air. Unfortunately, the condenser still does not start upon car start up unless the outside temperature is above ~77.
2) My neighbor is a mechanic and he checked the R134a levels and proclaimed them good. I don't remember the high pressure reading, but the the low side was ~ 34.
3) Wondering if the ambient temperature sensor was connected to the AC and sending the wrong data to the SAM, I disconnected the ambient temperature sensor. I was hoping the default reading of -36F would either not allow the condenser to start at all or the car would know this was a default reading and the condenser would start upon start up at any temperature. The outcome was no change. Under ~77 the condenser will not come on, above it comes on within 3 seconds of starting the car.

Observations:

1) The AC condenser works like a charm, when its over ~77.
2) Over ~77 the controls on the old school 4 dial climate control work perfectly. (EC button stops the condenser and opens the vents. Under 77 the EC button opens the vents, but as stated before the condenser failed to engage so I can't tell if its sending the signal to stop it.
3) The ambient temperature sensor is not connected to the climate controls

Next steps: ???
I really have no clue. Is there a temperature sensor on the condenser that is faulty? Is it a problem with the SAM? Is there a way to reset the 4 dial climate controls? I'd hate to take it into the shop and have them say I need XYZ only to see none of the fixes work.

Any help would greatly be appreciated. I'm just baffled as to why the condenser will not start even for a second upon car start up unless its hot outside.
 

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Welcome to the forum!

First thing first, the condenser is the radiator like part in-front of you radiator. The part you are calling the condenser is actually called the compressor. Just some nomenclature.

My guess is that there is a low pressure switch and it senses the pressure is too low at low temperatures (wrongly or perhaps your pressure is low) and when it heats up the pressure increases and satisfies the switch. I could be wrong, but an idea as you've replaced the evaporator temperature sensor.
 

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Discussion Starter #4
You're correct it's the compressor. That's what I get for typing this on my phone while on my lunch break.
 

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Premium Member 2013 SLK250
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Check that the ambulant air sensor is in the right place. It has been know to fall out oif the holder and create a false reading.
 

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Discussion Starter #7
Chris,

Can the pressure switch be changed without evacuating the system? If not I might as well change out the drier.

Gbarringer,

The ambient temperature sensor works fine. I actually unplugged it to see if it would have any affect on the compressors function. It did not.

Thanks to the both of you for replying. I appreciate any and all ideas. I just hope it isn't the compressor (I don't think it is since it works fine at higher temps) or the SAM.
 

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Chris,

Can the pressure switch be changed without evacuating the system? If not I might as well change out the drier.

Gbarringer,

The ambient temperature sensor works fine. I actually unplugged it to see if it would have any affect on the compressors function. It did not.

Thanks to the both of you for replying. I appreciate any and all ideas. I just hope it isn't the compressor (I don't think it is since it works fine at higher temps) or the SAM.
No the system has to be evacuated unfortunately.
 

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Discussion Starter #10
Thank you cob.


Chris,

Thanks for the information. I guess I know what I'll be doing when the parts come in.
 

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Thank you cob.


Chris,

Thanks for the information. I guess I know what I'll be doing when the parts come in.
Let me know if you have any questions through the process. I have not worked on any SLK's AC but I have worked on a couple different cars so I might be of some help.
 

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Discussion Starter #12
Thanks. I'm guessing the drier is somewhere by the radiator accessible through the bottom of the car. B
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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On some cars (not sure about ours) you can override the low pressure switch temporarily to rule it out. It's not safe to leave this way for long, but it should give you a few moments to test. The danger of running with low pressure is starving the conpressor of refrigerant/lubricant, and your friend said it was full, right? So the risk is low....

On most cars you just disconnect the two-wire connector and put a jumper across the pins. Again, not sure on Mercedes.

You could also test both sides for+12V.
 

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Discussion Starter #14
Efair,

Correct the pressure is ideal. It seems our cars pressure switch is a three prong which makes it difficult to override. Replacing the drier and the switch seems like a cheap next step. From all the research I've done it seems the step after the pressure switch is the the SAM which is a job for an Indy
 

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Discussion Starter #16
I found the drier. I also unplugged the three prong wire and checked it. It showed that it had ground, but wouldn't show any positive from the other two prongs. It's under 68 so I wouldn't expect the compressor to engage, but I thought I'd have fire going into the pressure valve of the drier( hot on one of the prongs).
 

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Discussion Starter #17
Clarification, I was checking the plug that originates from the car and plugs into the drier. I'm assuming at least one of those prongs should be hot at all times.
 

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Discussion Starter #20
I think it's time to take it to a trusted indy. I'm just not sure why it's tempature dependent. I'll schedule an appointment next week.
 
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