Cold engine = cold AC, warm engine = warm AC? - PeachParts Mercedes-Benz Forum

sunnyintx · #1 05-02-2006, 01:52 PM

Makes no sense to me, so I hope someone out there can offer an idea or two. I have searched the forum and don't find this problem addressed.

Just had a compressor, drier, etc. installed. Until the old compressor dies, the AC ran fine.

Now, when I start the engine, no matter what the outside temp, I get good cold air. It's great -- colder that I used to get. I continue to get good cold air until I stop the car. It doesn't matter whether I've driven 30 minutes in stop-and-go traffic or two hours on the highway. If I restart the engine in less that 10 or 15 minutes from when I stopped, i.e. while it's still hot, (like after I stop for diesel) the AC will not cool. The compressor is running -- I can feel the drag on the engine -- but cool air is NOT happening. If I let it sit to cool for a while, I often get the cold air back, but that makes being on time for much of anything a little difficult.

The mechanic who installed the compressor tells me that the magnet in the replacement compressor is not strong enough to do it's job once the engine is hot. At his suggestion, I've tried turning the AC off before I shut off the car, and not turning it back on until the car is running again, and that works perhaps 10% of the time.

None of this makes any sense to me. Any ideas? HELP!!!

Sunny

tangofox007 · #2 05-02-2006, 02:04 PM

It could be that your heat is coming on.

I had the exact same problem. What was happening was that the heater monovalve electrical connector was expanding when it got hot (like during a short stop) and the conection was broken. When the monovalve looses power, it positions to the full heat position. So cleaning and tightening up the plug on top of the monovalve might be a good place to start. You a/c is probably fine; it just can't compete with the heater running full blast.

You could also pinch a heater hose when the problem occurs and see if that makes a difference.

VegiBenz · #3 05-02-2006, 02:08 PM

In the A/C system, all of the heat absorbed by the refrigerant in the eveporator in the car is then dumped out at the condensor. After a hot drive and subsequent hot soak, the heat from the radiator transfers to the condensor and with no airflow, the condensor can get quite hot. It gets pretty warm during operation but the refrigerant is moving and there is airflow too. The hot soak probably gets it warm enough that it can't recover in the short time after a start without some pretty good airflow from driving. After a start, bring the RPM's to like 2000 and let it run like that for 30 seconds or so and see if that helps. Also, there must be airflow through the condensor for the A/C system to dump its heat so make sure your fan clutch and/or electric fan is working properly, it makes a big difference in a case like this. Eric.

sunnyintx · #4 05-02-2006, 02:27 PM

TangoFox --

This makes FAR more sense than the "weak magnet" theory...I'll take a shot at it and let you know -- thanks.

Eric --

I'll re-check, but the fan was running fine last I looked. Will the compressor run if the clutch is not working? (Probably a stupid question, but I had to ask...I know the clutch will be trashed if the compressor freezes up...) BTDT on the starting and running up to 2000 RPM anywhere from 30 secs to 10 minutes before engaging the AC -- that's not it. It's a little cool -- of course, the coils are still cool! -- but that fades to nothing pretty quickly.

I'll try these and post the results. Meanwhile, if anyone has any other insights, I'd love to hear them! It's already up in the mid-90s here in muggy NE Texas!

Sunny

tangofox007 · #5 05-02-2006, 02:34 PM

Another way to check for heater operation would be to select the "EC" mode when the problem occurs. If the air is way hotter than ambient, you will know the heater is on.

sunnyintx · #6 05-02-2006, 02:51 PM

Oops! Rats, there goes that simple answer. I've used the EC option several times just to keep air moving when the AC wasn't cooling. It's not but a tiny bit warmer than ambient air, not more than 3 to 5 degrees different. And my heater has always been intermittent at best (hot as you could want when it works, useless when it doesn't which is probably 60% of the time, and I could never find a pattern for when it would and wouldn't work!), and this AC problem is far too consistent and too new.

I just checked -- the clutch and the fan seem to be operating fine. We'll see what they're doing in a while after it's all warm. My husband reminds me that I'd have no problem if I just left the car running all the time I'm out....BIG help!

#7 05-02-2006, 02:55 PM

Quote:

Originally Posted by sunnyintx

And my heater has always been intermittent at best (hot as you could want when it works, useless when it doesn't which is probably 60% of the time, and I could never find a pattern for when it would and wouldn't work!), and this AC problem is far too consistent and too new.

This is starting to sound like a problem with the climate control unit. They normally do not fail all at once, but a little at a time. Both the heat and AC are controlled by this unit, I don't think you can predict what they are going to do once they start acting flaky.

tangofox007 · #8 05-02-2006, 03:10 PM

Quote:

Originally Posted by Craig

This is starting to sound like a problem with the climate control unit. They normally do not fail all at once, but a little at a time. Both the heat and AC are controlled by this unit, I don't think you can predict what they are going to do once they start acting flaky.

The fact that the compressor remains on would seem to exclude the ACC as the culprit, at least on the a/c side. The a/c itself has only two operating modes, on or off.

You might check the temp of the a/c suction hose under the hood the next time the problem occurs. See if it's cold or not. I would be tempted to plug a heater hose, so the heat won't work even if it is so commanded.

sunnyintx · #9 05-02-2006, 03:13 PM

Arrrgggh! I was really hoping not to have to pull and re-solder that little bugger!! That sounds like a weekend's work, if I don't screw it up in the process.

This problem with the AC is extremely consistent, while the heater problem is not -- anything else it might be? (I know -- someone is going to accuse me of "looking for my contact under the street lamp where there's light instead of looking for it where I dropped it, because it's dark over there!")

tangofox007 · #10 05-02-2006, 03:30 PM

The problems that you are having with the heat are consistent with a torn diaphragm in the monovalve. Which could also contribute to the heat working when it is not supposed to work. So....park your car right under street lamp and remove the four screws on the top of the monovalve and pull the inards out. A torn diaphragm will be easy to spot. This is a ten minute task.

sunnyintx · #11 05-02-2006, 03:46 PM

Will do. I'd better order a rebuild kit -- she's my daily driver, so once I open her, I need to fix her!

#12 05-02-2006, 07:58 PM

Quote:

Originally Posted by tangofox007

The fact that the compressor remains on would seem to exclude the ACC as the culprit, at least on the a/c side. The a/c itself has only two operating modes, on or off.

You might check the temp of the a/c suction hose under the hood the next time the problem occurs. See if it's cold or not. I would be tempted to plug a heater hose, so the heat won't work even if it is so commanded.

I agree, that you need to determine if the compressor is engaging, and if it is making the hoses cold. If the compressor is engaging, but the air in the cabin is not cold I would look at the monovalve.

BTW, when the heat is "not working" is the fan blowing cold air, or is the fan not working at all? If the fan is not working 60% of the time, I still think you need to look at the control unit (independently of the AC issue). If the heat is blowing cold, it sounds more like the monovalve.

sunnyintx · #13 05-02-2006, 09:37 PM

The fan is dependable EXCEPT on EC. Any other setting, the fan blows without stopping, whether I'm trying for heat of AC. Before this new compressor, if I was trying for AC, it always blew cool. With heat, one never knew if the air would be cold or warm, but mostly cold. It was always a delightful surprise when it was warm! And somewhere I read that even on heat, the compressor is on, so I never paid much attention to the possibility that I might be overcoming the heat with "coolth". I suppose it's possible.

Before you ask, on EC it seems to have it's own mind about how long it ought to blow. The fan always comes on when I hit the button, but after anywhere from a minute to 5 minutes, it will quit. If I then hit any other button on the climate control, the fan will come on, and stay on. If I switch back to EC, it will again run for a while, and then stop. It's just strange that way, but it's never been that much of an inconvenience.

tangofox007 · #14 05-02-2006, 10:33 PM

Quote:

Originally Posted by sunnyintx

The fan is dependable EXCEPT on EC.

Resoldering the CCU could well solve that problem. It's not a difficult job; just reflow the existing solder joints where the horizontal and vertical boards join.

One more question. When you had the compressor replaced, did you switch from R-12 to R-134A?

sunnyintx · #15 05-03-2006, 09:37 AM

Not by choice, but yes, it went from R-12 to R-134. In fact, my first thought was the high pressure switch because R-134 needs higher pressure, but that doesn't appear to be the problem.