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  #1  
Old 08-15-2009, 11:03 PM
Bruester
 
Join Date: Jul 2007
Location: New Rochelle, New York
Posts: 13
A/C Compressor clutch stops after 10 min....why?

The air conditioning on my 1987 300TDT works great -- very cold -- for about 10 minutes after I start my car. Then the clutch on the A/C compressor disengages, causing the air to lose its cool. I have a 2 year old remanufactured compressor, and the AC (red) switch, expansion valve and dryer were replaced. I just recharged the system with a can of 134a (conversion was done by previous owner).

Any idea why the AC compressor clutch stops engaging after 10 minutes? It seems to happen when it's more than 85 degrees outside. Thanks
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  #2  
Old 08-15-2009, 11:12 PM
bustedbenz's Avatar
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Location: Lexington, NC or Boone, NC
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I have the same problem. Very interested in what diagnostics turn up here.
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  #3  
Old 08-15-2009, 11:32 PM
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Location: central Texas
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Do you have any way to put gauges on it ?
There may be a partial blockage and a high pressure cutoff switch... which would account for cold air but stopping after a little while...
However, I have no MB manuals for past 1985 and specialize in type one... around 1980 because the later ones are just too complicated for my simple mind....
So I do not know if you even have a high pressure cutoff switch... but gauges are a great place to start so you know more about what is happening inside the system....
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  #4  
Old 08-15-2009, 11:33 PM
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Does airflow through the center vents slow down to almost nothing before the clutch stops engaging ?
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  #5  
Old 08-16-2009, 12:14 AM
LarryBible
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On the 124 cars a sensor is on the rear of the compressor that senses compressor rotation. If lack of rotation is sensed, the clutch circuit is interrupted. Turning the igniton off and back on resets it.

The idea is to shutdown a failed compressor before it scatters metal throughout the system. Several things besides an internally failed compressor can cause the sensor to trip; a slipping belt, a slipping clutch or a worn compressor that allows the compressor innards to have excessive thrust.

Hope this helps.
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  #6  
Old 08-16-2009, 11:13 AM
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Location: Blue Point, NY
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Quote:
Originally Posted by LarryBible View Post
On the 124 cars a sensor is on the rear of the compressor that senses compressor rotation. If lack of rotation is sensed, the clutch circuit is interrupted. Turning the igniton off and back on resets it.

The idea is to shutdown a failed compressor before it scatters metal throughout the system. Several things besides an internally failed compressor can cause the sensor to trip; a slipping belt, a slipping clutch or a worn compressor that allows the compressor innards to have excessive thrust.

Hope this helps.
As a follow-on to this, I would recommend eliminating the Klima circuit that monitors these various parameters as a test.

Pull the Klima and connect a jumper between sockets 5 and 7. The compressor will start. See if it continues to run past the 10 minute point. If it does, the problem is in the Klima..........one of it's parameters is not to spec.

If it doesn't........I'd be inclined to believe the evap temp switch is NG.

The real question is whether the compressor ever comes back online after it shuts down.........if it never comes back until a restart, the problem is most certainly with the Klima.
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  #7  
Old 06-13-2010, 07:51 PM
tankowner's Avatar
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In a search I came across this thread and was intrigued because my AC compressor clutch disengages after about 10 mintues. A few things I would note are:

1. I am not getting airflow through the center vents. Lethermang alluded to this, what's the deal -last year it was plenty of cold air through the center vents.

2. I don't suspect the Klima itself (because I replaced it a couple years back), but can do the 5 to 7 pin jumper to see what happens. However . . .

3. I should note that the compressor does seem to come back on sometimes after shutting down, and without having to shut down and restart the car. Even when it is on though, the air isn't freezing cold.

I am going to put some guages on tonight and will report back. But I am wondering if anyone has any thoughts. Thanks.
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  #8  
Old 06-14-2010, 01:20 PM
mach0415's Avatar
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Location: Lawndale, NC
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FWIW i have seen several A/C compressor clutches around here in NC (non MB, although it should still be the same concept) do the same thing. You can smack the center of the clutch with a pry bar and they work again for a little while and then quit. Two things come to mind:
1.) The air gap is set on the high end of acceptable parameter and with heat, it expands to beyond specs.
2.) Clutch coil is weak, due to high resistance. With heat comes higher resistance and eventual intermittent failure.

Do you know if there is a signal at the clutch coil (at the plug) after this happens?
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  #9  
Old 06-14-2010, 01:48 PM
92 300D 2.5L OBK #59
 
Join Date: Feb 2007
Location: Central FL
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Quote:
Originally Posted by LarryBible View Post
On the 124 cars a sensor is on the rear of the compressor that senses compressor rotation. If lack of rotation is sensed, the clutch circuit is interrupted. Turning the igniton off and back on resets it.

The idea is to shutdown a failed compressor before it scatters metal throughout the system. Several things besides an internally failed compressor can cause the sensor to trip; a slipping belt, a slipping clutch or a worn compressor that allows the compressor innards to have excessive thrust.

Hope this helps.
I thought it was to save the belt considering one serpentine belt drives everything. From the CD 83-600.pdf
[IMG]file:///C:/Users/BOB/AppData/Local/Temp/moz-screenshot.png[/IMG]
Attached Thumbnails
A/C Compressor clutch stops after 10 min....why?-belt.jpg   A/C Compressor clutch stops after 10 min....why?-kilma.jpg  
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  #10  
Old 06-14-2010, 02:05 PM
LarryBible
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Saving the belt is not a bad thing and I'm not sure what it's exact purpose was, but the function is the same.
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