This is not a blower issue, as the blower works fine. It is an issue of air temperature. I get hot air even with the compressor engaged, and I know the compressor is really not compressing because the temperature of the refrigerant lines is ambient. Then all of a sudden, randomly, I'll start getting cold air again. But there is no evidence of it being a blower/ducting/flap problem, so my eyes are on the compressor as the culprit. I've just never heard of a fully engaged compressor that can sometimes compress and sometimes is not compressing.

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K

Please reread my post. I am NOT talking about the main blower! There is a very small blower that draws air through the grille near the rear view mirror and blows it across a temperature sensor. When it does not work, the CCU is not getting a correct temperature reading, so it blows cold air at weird times.

Believe me! It will be worth your while to cut a small square of paper and take the troubleshooting step I requested.

Oh, now I understand. I will do the test and let you know.

However, if I put the temp control on max cold, does it still use the temp sensor? I would think this would override any cabin temp senor. And also, would this cause my compressor to be engaged, because I know my compressor is fully engaged (but is not doing any work based on temperature of all ac lines being near ambient) under this condition.

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K

In auto that pump runs which he is talking about.

If you put on the dot setting cold and turn fan on high and it doesnt blow hard and cold its most likely a regulator issue or a low freon issue.

Low freon almost makes sense based on not getting cold when compressor is engaged, and I need to get out my guages and check that out. However, I do know that when it intemittently starts blowing cold, I get ice cold air, so low freon would not produce that condition.

You mention a regulator, what part is that?

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K

Here is a little more information:

Static pressure on suction and discharge is around 125#.

Once the compressor engages I get a vacuum on the suction side, but the discharge stays right at 125#. Once car is shut off, the suction slowly (over several minutes) goes back to 125#.

What does this mean? is this just a low freon condition? Why doesn't the discharge pressure go up at all, not even 5 or 10 pounds? Is there blockage somewhere?

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K

The regulator supplies power to the blower motor. It is mounted under the blower motor. The regulator has 3 wires to it- black ground, red 12V, and yellow varying current dependent on setting of PB CCU.

The regulator rarely fails (usually blower motor), but it recently did on my 95 E300.

125 PSI Static pressure? Do you know exactly what refrigerant is in the system and what was the ambient temp at the time?

Temperature was about 100 degrees when I measured this.

Did some research on internet and found that vacuum on the suction indicates something is plugged. In my case, it is a sticking expansion valve. If I tap the valve, my refrigerant starts flowing again and I get my cold air back. Works until I stop and restart car, then have to tap it again. Funny, because I haven't located a thread here that diagnoses a sticking expansion valve, and I would think this might be more common.

A new expansion valve only costs about $30 - but the real cost is going to be in the R12 recovery and recharge since I don't have a recovery machine. I will call a couple of places tomorrow to find out what they'd charge for this. Until then, I'll keep tapping my expansion valve!

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K

I finally got my expansion valve changed out and the details are on this other thread:

http://www.peachparts.com/shopforum/diesel-discussion/283700-bad-expansion-valve-cause-intermittent-ac.html?posted=1#post2544653

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K