A/C problem below 1200RPM - 1982 300TD Turbo - PeachParts Mercedes-Benz Forum

brettbolander · #1 07-01-2008, 11:25 PM

I browsed the forum topics, but couldn’t find a way to search them, so please forgive me if this question is redundant.
I just flushed my A/C system with Hecat Flush, replaced all o-rings (assembled with Nylog), replaced the compressor (used new seals), receiver/drier, expansion valve and the short hose that goes between the condenser & receiver/drier. I added the appropriate amount of oil upon assembly, and took it to the shop to have it charged with R12.
The shop said it held vacuum well and that the compressor was working properly. The pressures for the R12 are according to spec.
When the car is cold, at start-up, the A/C blows very cold. However, once the engine reaches operating temperature (approx. 80-86 degrees or so) the A/C starts blowing hot UNLESS you up the RPMs to around 1200RPM. Then it will blow cold all day long. In other words, I drove at highway speeds for over an hour and it never blew hot. As cold as could be. So much so that it gave me goosebumps and was uncomfortable for my hands.
I have checked the vacuum pump, and I have 20" at idle. I have also checked the pods under the dash that I could reach (I found 4 of them) and they all held vacuum fine.
Any ideas of what the problem could be? It seems like such specific symptoms that I figured that someone here may know the problem, or at least point me in the right direction.
My wife will be driving the car about 1500 miles in a week and a half, so I am hoping I can have it solved by then. Any suggestions are greatly appreciated!

300SD81 · #2 07-01-2008, 11:32 PM

The condenser probably isn't getting enough airflow with your not moving, or the compressor isn't flowing enough at low RPMs.

#3 07-01-2008, 11:33 PM

I don't think it's a AC problem, I think the monovalve is failing and allowing the heat to overwhelm the AC at lower rpm. The usual solution is a monovalve rebuild kit that replaces the internals. If you search on monovalve, you will find lots of discussions of unwanted heat.

Edit: Just to clarify, is it really blowing hot or just ambient air temperature?

brettbolander · #4 07-01-2008, 11:52 PM

Yes, it’s really blowing hot air, not ambient. I have a Monovalve rebuild kit (from previous owner.) How confident are you that this is the issue? I will gladly pull it & rebuild it, but want to be fairly confident that it is the issue before going through the trouble. The interesting thing is that the Monovalve was rebuilt by our MB diesel mechanic when we first bought the car about 6 or 7 years ago. About 70,000 miles ago. What are the chances that it has failed again after such a short time? I find it hard to think that as the previous response has suggested, that there is an issue with the condenser or compressor, but obviously I am very new to this, and realize I could be wrong about that. Thanks for your input. Did I give you enough new info to help in the diagnosis?

Thanks!

#5 07-02-2008, 12:13 AM

If it's blowing hot air, it means the heat is operating. The most common problem for unwanted heat related to engine speed is the monovalve. It could also be a climate control problem of a loose connection, but it is unlikely to be related to engine speed.

If you have a rebuild kit, it only takes about five minutes to install with a very minimal loss of coolant. I would not be surprised if it needs rebuilding after 6-7 years, it's a pretty common failure.

Edit: you do not have to remove the monovalve to install the kit, just remove the top cover (4 screws) and replace the internal plunger.

JimmyL · #6 07-02-2008, 12:21 AM

I bet Craig is all over this one......

#7 07-02-2008, 12:27 AM

Quote:

Originally Posted by JimmyL

I bet Craig is all over this one......

It seems obvious, but you're never 100% sure until you try it.

brettbolander · #8 07-02-2008, 12:30 AM

Thanks, guys! I will rebuild the Monovalve in the morning and report back about the results. I really appreciate all of the prompt & valuable input!

rrgrassi · #9 07-02-2008, 01:20 PM

IIRC, the defrost setting makes the fan go to high speed and the monovalve open, giving hot air.

The defrost flap is behind the drivers side kick panel, and you can tie it down, so that air is redirected back to the center vents. All my pods are shot, so mine are tied down until I get extra cash and time to fix the properly. The vacuum is also disconnected. Leaking pods create one heck of a vacuum leak!