Erratic ACC (A/C) Behavior - PeachParts Mercedes-Benz Forum

InDiesel · #1 08-02-2004, 09:18 PM

My 84 300D ACC appears to be acting erratic. Most of the time the air blows cold. However, sometimes the blower reduces speed or just stops and other times, the air becomes ambient temperature even with the temp set to max cold and the blower set on high.

I have removed the ACC and resoldered the joints. The fan still changes speed without known cause and the cold air varies in coolness as well.

Any suggestions for next steps?

Mr. FancyPants · #2 08-02-2004, 09:34 PM

How long have you owned your 123?

Benz's are known for faulty climate control systems and you might want to check in the archives for what I'm sure are many posts relating similar circumstances.

InDiesel · #3 08-02-2004, 09:48 PM

I have owned it for about a month.

InDiesel · #4 08-02-2004, 09:52 PM

BTW - In an attempt to be considerate, I have searched both in the Diesel and Tech forums for direction on this, but did not find a circumstance that was similar enough to help. I probably just did use the right search terms.

#5 08-02-2004, 09:55 PM

InDiesel,

Remove the glove box and look for a horizontal tube that goes from a plastic pipe across to the left to the temp sensor in the center of the dash. If it is gone, go to Home Depot and get a piece of foam pipe insulation like is used to keep pipes from freezing in the winter. Cut a length and slip it over the plastic pipe and the temp sensor receptor. Don't use rubber or plastic, it conducts too much heat and won't give an accurate reading.

This is very likely your problem.

Good luck,

InDiesel · #6 08-02-2004, 10:03 PM

Thanks Larry.

I'll give it a try. From the posts I read on the temp sensor tube seemed to indicate that a missing or corroded tube would actually cause the ACC to think that it was hotter than cabin temp and run the A/C colder than it should, so I didn't think it applied to me.

Thanks again and I will report my results.

Mr. FancyPants · #7 08-02-2004, 11:23 PM

InDeasel

I hope you'll enjoy your 123 300D as much as I did mine. It had a lot of miles on it before I parted with it. It no longer would pass inspection due to rust.

My AC never worked and seems I've read several posts here including 124's and 126, that have had problems with climate control systems.

The guy that posted above, (Larry Bible) is maybe one of the best guy's in the country to get good advise from.

rhino · #8 08-03-2004, 06:04 PM

Yes, I wonder if I can do a search on Larry's posts and just print them out. Put that manual up on EBay for 199.99. If all else fails, you have a great shop right up 85, Donnie et al at MB Autowerks, who sponsor this site. They did some great work on my '83 right after I bought it, at not cheap but reasonable rates.

#9 08-03-2004, 06:07 PM

No, there seems to be no pattern to how the a/c will operate if the foam tube is missing.

It's a quick thing to check, and I'll give it better than 50/50 chance that it is your problem.

Let us know,

InDiesel · #10 08-04-2004, 09:05 AM

Does the temp sensor even come in to play when the temp wheel is set on MIN (past the friction stop at the full cold position)?

autozen · #11 08-04-2004, 11:20 AM

When you go to max cool, I believe it does take ACC out of the loop, but I will have to look it up today when I go down to my shop. With the whole system off do you get heat coming from the vents intermittently? If you do the problem may be the connector at the mono valve. The female portion of the connector is kept on by the tension of the springy metal. As these connectors heat and cool thousands of times as the mono valve cycles, they lose their tension. As a result when they intermittently don't make contact, the mono valve goes to full heat. Mercedes has always designed their heating systems to go to full heat if there is a system failure. This is the default mode to keep you from freezing to death in extreme cold situations. Good luck.

Peter

#12 08-04-2004, 11:26 AM

Autozen offers good advice. I never saw the loose connector on the 123 when I was driving them, but the cars were much newer then. As these cars become 20 years or more old, things will fail due to age that did not fail when newer.

Since the monovalve is normally open, a flaky connector could definitely give varying temp and roast your butt from time to time.

Good info autozen.

InDiesel · #13 08-04-2004, 04:52 PM

Thanks to Larry and AutoZen.

As I indicated in a previous post, I will check the temp sensor tube because I would like to use the ACC in other than max cool position.

AS far as the mono valve goes, the symptoms sound like a possible candidate. The system does blow heat out of the upper vents on very rare occasions (maybe 3 times in the past month) when in max A/C (max cold and fan on high). Have not noticed heat in the floor vents when system is off.

I am not familiar with that mono valve part. Any short descriptions available where I would find and identify it?

Thanks again,
Patrick

#14 08-04-2004, 05:57 PM

It will only take five or ten minutes to pull the glove box and see if the foam hose is still present. I'll bet it has disappeared. The foam deteriorates to absolutely nothing. It vanishes in thin air. If it is in place it is probably ready to crumble.

This will also cause occasional hot air.

Good luck,

InDiesel · #15 08-04-2004, 08:48 PM

Ok, I replaced the foam hose with 1/2" pipe insulation. Larry was right, the old hose was disintegrating.

Took a test drive and seemed to get cold air with temp wheel on Min (full cold) and fan selector set to high (top fan button). The real test will be tomorrow as it was early evening and had cooled from the heat day (mid 90's). The fan however, still ran at variable speeds.

I may be dealing with more than one problem. I am starting to suspect the blower motor. when I get in the car and the system is initially turned on the blower motor runs high, but after a few mintues begins to slow down and then the speed begins to cycle back and medium high to low.

The temp issue may be another problem and may well have been resolved with the foam hose replacement. We'll see tomorrow.

Any tricks to testing the blower motor or getting it to work little better for a little while without removing the dash? How difficult is it to get to the blower motor?

Thanks,
Patrick