Question for the AC gurus... - PeachParts Mercedes-Benz Forum

MBeige · #1 09-07-2007, 02:42 PM

When it's cool outside, I get cool air in my AC. But when it gets warm like what has happened last week or the other week, the AC only blows cool when engine is cold, but when it warms up the AC blows warm air. I can sense that it's trying to cool it but all it gives is warm air.

Bit of history on the 300D though. Condenser has been replaced but I didn't know much back then so the drier wasn't. It was an accident so the radiator, aux fan, etc had to also come out and be replaced. Condenser didn't leak or so they said, after the accident, prior to swapping it.

It has R12 and when it's cool outside I set it to max setting and it gets really cold, but once the outside temps get warmer like 80's to 90's F then it gets less efficient.

My friend said the monovalve may be stuck giving warm air but now that it got cooler, I got colder air out the vents. During the cold season I was able to get cool temps at the vents, I don't remember what measurements I got but it was around 12C at the vents (coldest) when ambient temps were around 30C I think.

Anyway what do you think is the culprit? Do you think the original compressor needs to go, the drier has to be replaced, or something else? Those are the two that I'm looking at because the compressor is still original while the drier wasn't changed when the condenser was. Thanks.

tangofox007 · #2 09-07-2007, 03:00 PM

Is the compressor running (clutch engaged) during these "warm air" episodes?

bugsyson · #3 09-07-2007, 03:05 PM

Sounds like you may need a new fan clutch and or the electric fans may not be working, had mine replaced (fan clutch) and AC is way cooler now. The condensers are not that good on these cars and need lots of air to work.

#4 09-07-2007, 03:17 PM

Quote:

Originally Posted by MBeige

When it's cool outside, I get cool air in my AC. But when it gets warm like what has happened last week or the other week, the AC only blows cool when engine is cold, but when it warms up the AC blows warm air. I can sense that it's trying to cool it but all it gives is warm air.

That part of your description sure sounds like a monovalve issue. You can rule out the monovalve by pinching off the heater hose (without damaging it) and seeing if it still blows warm.

Does it blow warm at highway speed, or just in traffic? If it's just in traffic, I would suspect a fan issue.

MBeige · #5 09-07-2007, 03:18 PM

Tangofox, as far as I know it does engage. Would it be possible that it doesn't engage during warm ambient temps?

Bugsyson, fans work in fact I have a steady 82-85 engine temp. What surprised me is the aux fan came on even when engine temp was reading normal.

I read that the aux fan has two inputs, engine temp and refrigerant pressure? So if the aux fan is turning on at normal engine temps then what is causing the increased refrigerant pressure?

Craig, it blows warm all the time, highway or city. I've already purchased a new monovalve in case the one I have is bad but I don't want to replace it if it's not the cause.

Here's the twist, now that it's cooler outside, I get to enjoy the cooler AC too! So would that still suspect the monovalve?

rrgrassi · #6 09-07-2007, 03:43 PM

How is the freon level? On R-12, you do not want to see any bubbles in the sight glass when the compressor is engaged. There will be bubbles each time the compressor engages, but then should clear up. Does the compressor cycle or is it on all the time?

tangofox007 · #7 09-07-2007, 04:01 PM

Quote:

Originally Posted by MBeige

Tangofox, as far as I know it does engage. Would it be possible that it doesn't engage during warm ambient temps?

The reason for my question has to do with a possible malfunction of the evaporator temperature regulator. It has nothing to do with the engine temp, although it can manifest in a way that would make it appear that it does. The evap temp regulator is designed to prevent the evaporator from freezing, so it shuts the compressor off at about 36 degrees. But a defective regulator can trip at a much higher temp, such that the a/c will work fine for a few minutes, then trip off. At which time it starts blowing ambient air, as the compressor will have disengaged.

Quote:

Originally Posted by MBeige

I read that the aux fan has two inputs, engine temp and refrigerant pressure?

What is the year of the vehicle in question? (The "refrigerant pressure" aspect is probalby wrong in any case.)

OMEGAMAN · #8 09-07-2007, 04:20 PM

Get ahold of some a/c guages. If you can tell us what the pressures are at idle and 2500 rpm we can help ypu solve this problem. Does not sound like a mono valve problem or electrical problem it sounds like it is low on refrigerant

vstech · #9 09-07-2007, 04:23 PM

first things first.
go warm up the engine, and then park it, run the a/c and report the pressures high and low.
now report out side temps and cabin temps too. if possible report humidity levels, but it's not totally important. also tell us if the compressor is engaging and disengaging while you are checking the temps.
this will give us some idea of the condition of your system.

MBeige · #10 09-07-2007, 04:56 PM

I just had the R12 freon recharged last year, one of the valves was leaking (at the alternator area - high port?). The car is the '83 300D. Tangofox thanks for the info, I'll see if I can borrow some gauges or ask a mechanic to check my system. From there I will report back. By the way where is this temp regulator?

Vstech, should I measure cabin temp after it's warmed up? Should I use the AC before I measure cabin temp? Only source for humidity levels is Yahoo local info.

vstech · #11 09-07-2007, 06:31 PM

yes, measure cabin temp after you warm up the motor, and while you are measuring the vent temp and reporting the pressures. also, see if you can measure the temp of the line returning to the compressor. at least touch it with your hand... carefull, if you grab the output line, it will be VERY HOT. if you use your hand to measure it's temp, tell me if it's cool, cold, very cold, or warm.

tangofox007 · #12 09-07-2007, 06:43 PM

Unless the car has been modified, the aux fan is actuated on the basis of a/c refrigerant temperature and nothing else. Engine temp is not a factor. The fact that the fan comes on is a good sign, as a system that is more than slightly low on refrigerant typically will not generate a temp high enough to close the switch that powers the fan.