AC Stops Blowing cool - have to turn car off then on to fix. - Mercedes-Benz Forum

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Old 08-17-2010, 09:13 AM   #1 (permalink)
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AC Stops Blowing cool - have to turn car off then on to fix.

1994 S320 -- The air-conditioning usually works fine, but if I've been driving and stop for a short time (like to get gas or a cup of coffee) sometimes the air-conditioning will not blow cold air. Its not hot, but its not cold either. I can turn the car off, remove the key from the ingnition, and restart the car, and usually all works well.

Does anyone have a solution or where I should look. When I take it to my mechanic, he cannot reproduce it.
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Old 08-17-2010, 10:10 AM   #2 (permalink)
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hmmm could be a slipping compressor or clutch
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Old 08-17-2010, 10:16 AM   #3 (permalink)
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I had a similar problem for a long time before I discovered what was causing it. The a/c would always work the first time I started the car on any day. However, like yours, once the engine was hot and I parked for 10 minutes or so, the a/c would not come back on without a second key cycle and not always then.

In my case it turned out that the a/c electromagnetic clutch on the front of the compressor had got some oil on it. When the engine was cold, the oil was thicker. Also, the electrical resistance of the clutch coil is a minimum when it's cold so it takes a bigger current and provides more clamping load between the plates. When the engine is hot, the oil was less viscous and the coil resistance increase is by about 0.4% per degC. so with a 20degC rise you lose about 8% clamping force. In addition, because the a/c system is now hot, the load on the compressor is also higher when you restart.

To check if this is your problem you could try squirting a degreasing agent into the small gap between the clutch plates. I used to use isopropyl alcohol (IPA) and this would generally fix my problem for a few weeks. Finally, I removed the outer clutch plate, did a thorough degreasing job on both plates and also lapped the outer plate flat (flatter) because it was a bit worn in places. I did this in March this year and the a/c has not failed once since.

The compressor has an rpm sender which provides a signal to the base unit. This signal is compared with the engine rpms and if they don't match, the base unit assumes the belt is slipping or the compressor has seized and de-energises the clutch. It usually requires a key cycle to clear the logged fault. If the clutch slips just a small amount, the base unit will trip it out.

Mike
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Old 08-17-2010, 10:33 AM   #4 (permalink)
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Sounds right to me

The oil on the compressor clutch sounds like the most likely problem. Like most 94 S320's, my engine leaks oil. My mechanic said its not worth spending the money to fix it unless it gets really bad. So the oil is definitely present. And yes, sometimes one time through the key cycle will not fix it, and now that I think about it, that happened during the hottest part of the day.

Is there a repair manual for this car on line anywhere?

Thanks
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Old 08-17-2010, 11:35 AM   #5 (permalink)
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Quote:
Originally Posted by mjenco View Post
........
Is there a repair manual for this car on line anywhere?

Thanks
The only on-line repair manual that I am aware of is the distilled wisdom to be found on this Forum.

Mike
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Old 08-17-2010, 03:04 PM   #6 (permalink)
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overcharged system; too much freon. give it a few pssts and it should be ok. its the pressure shutoff protection. on hot days gas expands and compressor turns off as the press is too high

and oil leaks are always worth fixing.
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Old 08-17-2010, 03:35 PM   #7 (permalink)
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Quote:
Originally Posted by gyrate View Post
overcharged system; too much freon. give it a few pssts and it should be ok. its the pressure shutoff protection. on hot days gas expands and compressor turns off as the press is too high

and oil leaks are always worth fixing.
But why should it work again after cycling the key? Cycling the key doesn't lower the temperature.

But, in any case mjenco, if you read the sensor values using this link: STAR TekInfo you will be able to see from sensor value #7 whether the system pressure is too high.

Mike
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Old 08-18-2010, 10:35 AM   #8 (permalink)
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Similar symptom with my son's 2001 Volvo S80 T6. On the Volvo forums apparently lots of folks have the problem. A/C works fine for 10-15 min's, then compressor won't come back on until engine has cooled off for awhile.

There is a simple cause and fix for their compressors -- the clutch surface has worn, and when the engine and accessories heat up the clutch won't stay engaged. His gap between the pulley and clutch was almost .9mm, and the spec calls for no more than .35mm. We removed the clutch, and inside the center of the clutch is a depressed area that presses against the shaft, with some really small spacers/washers/shims, something like .8mm, .3mm and .1mm. Removing 1 or a combo of them reduces the clearance. I removed the .8mm shim, and now the A/C has been working for the last 50 days.

ALLDATAdiy shows that shims are used to correct the gap on our compressors also, in fact the diagram shows 3 shims. But it doesn't say if they are already there, and then you simply remove a shim or two like the Volvo. It would make sense for them to already be in there, because it makes no sense to have to add any, since that would only make the gap bigger.

Our gap is 0.5 ±0.15 mm. Buy a feeler gauge and check the gap. BTW, in case you don't know, this is not the same gauge used for gapping spark plugs, and most people don't have this kind, 'cause it's seldom used. This kind has about 20 or more thin strips of metal - mine goes from .88mm to some size ridiculously thin, like .04mm, which bends like paper. This kind is also used for adjusting valves on MB diesels, which is why I bought one 30 years ago for my 220D, and now used on my 300DT.
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Old 08-18-2010, 12:41 PM   #9 (permalink)
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I found two shims when I removed the outer clutch plate but by the time I'd finished, I needed to lap a suitable washer to size in order to get what I thought was the right air gap. For want of better info, I used the data on page 30 of this link:
http://www.worldaire.com/pdf/2005Wor...talogSept1.pdf
My compressor is a Nippondenso 10PA for which this data sheet quotes 0.021" to 0.036" as the air gap (0.53 to 0.91 mm). Before I removed the plate, the gap was 0.035" (0.89 mm). I set mine to 0.028" (0.71 mm) which seems somewhat large compared to your data, but has been working just fine since March. I note that the smallest, maximum gap for any of the clutches listed on the above data sheet is 0.020" (0.5 mm)

I am quite convinced that, in my case, it was the oiled state of the plates and the fact that the surface of the outer plate was no longer flat that caused my problems rather than the gap per se.

Mike
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Old 08-18-2010, 05:44 PM   #10 (permalink)
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Check your fan belt tension and or belt, I took it in to MB they replaced the tensioner, It seems every time I accelerated hard like at a toll booth, or deep puddles, the compressor would stop. It has something to do with a speed sensor on the compressor vs engine speed on early 140s. If restarted it would go away, service guy called it quickly, and tensioners fail regularly!
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