92 300E A/C Intermittent
 

I see many AC threads, none exactly like what I am experiencing. AC normally works great, blowing ice cold air. Once in a while (maybe 2 starts out of 10), the compressor doesn't engage (I can tell by the sound when it is engaged), and I get no cold air. Then usually next time I go to use the car, it works great again. I am guessing that system is charged and compressor is working since when it works, it works great. But what would cause it to not engage periodically? What is best way to troubleshoot?

My thoughts are a relay going bad, or a permissive switch (lo or hi pressure) not working intermittentaly.

Any thoughts? Thanks so much.

Same problem here (95 E320). Mine get's "tired" after 3 to 4 hours of continuos use and gets back to life If I let it rest for about 15 minutes. I'm sorry I don't have the answer but i will subscribe to this post to see If I get a solution too.

Juan

Requires voltage pin testing (MAS relay) when the system is not working correctly.

Thanks MB Doc, I have multimeter, but can you describe what pins, what voltage, etc? Thanks again.

My 1986 300E had similar symptoms. There is a high temp sensor that will disengage the compressor. If the sensor is faulty the compressor will shut off and will not engage unless the engine is shut off and restarted.

treiberg,

Did you ever fix your ac problem? I seem to have the exact same symptoms, and I'm hoping this isn't an expensive fix.

About three weeks ago my ac didn't come on one day, but was fine next time I started it up. Today it won't engage and I can't seem to get it to. Normally it's ice cold and works perfectly.

I did replace the OVP relay about a year ago to fix an ac and tach problem. Tach works fine at the moment.

Thanks

Also check your belt tension or the quality of the belt. If the belt slips at all, the compressor will be disengaged until the engine is restarted. Mine used to do that, especially if/when I drove through a puddle or if it was raining out. I never heard a belt squeal or anything. After tightening properly, the issue disappeared.

Still haven't solved this one yet. I don't think the belts are slipping, but I will check that. Here's what I have discovered: If I start my car with the AC off, then drive the car a little bit, then turn the AC while driving, it almost always comes on (about 8 times out of 10). So I have kind of learned how to deal with it, but I would really like to solve it. I would love to find a document or thread that gives a step by step on how to troubleshoot an AC system, or some type of troubleshooting flow chart. Maybe someone out there with knowledge of these Mercedes AC systems can put something together for all us who struggle with these issues.

Send me a e-mail & I will supply you a pin test for the MAS relay.

I was thinking that I had a freon pressure issue -- too low, too high, something. But here is what mine does.

1. When it works, it works fine.
2. It typically doesn't work unless I am at 30-40mph for at least a minute or two.
3. However, even on interstate it won't stay on consistently.
4. In the mornings when it is cooler outside it works great, which I would expect. This morning it was about 73* outside and the thermometer in the car showed 38 degrees out of the vent. It worked from the time I started it, with slow stop/start driving all the way through the interstate. Diagnostic showed the evap temp at 31 degrees (!?!).

I'm thinking I have some sort of sensor or switch problem as well.

(On the other hand, freon bar pressure is about 20 when at idle and at about 9-12 when at speed. Which, from the very little I do know, confuses me greatly.)

For what it's worth...

I did get the A/C working again. Not sure exactly what it was, but there were two things I did that may have helped.

1. I jostled the OVP relay around a bit. My tach has been fine, so I doubt this is it.

2. I snapped the temperature sensor back in place (number 182). I had replaced the dust filter just before my problems started, and hadn't put it back in place properly, though it was still plugged in.

We'll see if it happens again.

Hello,
I have a problem where my compressor starts and runs for a few minutes making cold air. Then the Aux fan kicks on and it runs a few minutes. Then the Fan Clutch kicks out and won't cycle again unless the car is stopped and started again. Do you think this sensor is the problem? What is it called and where is it at?

Thanx!

Les
'88 300TE

Two words - wiggle test.

Sometimes it's just dirty contacts.

Here I am one year later and ready to start more diagnosis on this problem that prompted this thread.

The interesting part is that it does not appear to be a cutout issue (like I originally thought). I can clearly see the compressor spinning, but there is no cooling. So that would point to either a bad compressor or low refrigerant. But here’s the kicker, every so often that AC will blow cold again, really cold. So that means I must have refrigerant. This happens almost every day at some point in driving the car. But I've noticed that it is less and less as time goes by. So my question is, is it possible that the compressor compresses intermittently? Would something internal in the compressor (like valves sticking) cause it to work great at times and not at all at other times? I suppose I am looking at a new compressor.

Cut a 1 Centimeter square of light paper and hold it near the grille next to the inside rear view mirror. If it does not stay in place when you take finger away while the ignition is in the on position, find and check your sample blower motor.

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.

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.

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?

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?

ACC regulator
 

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!

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

http://www.peachparts.com/shopforum/showthread.php?p=2544653&posted=1#post2544653