BasharHalabi96

Hello,
My ac recently stopped blowing cold air.
i went to a shop that changed my compressor oil using an ac service machine. The ac worked back fine for 2 days
After that it stopped. Went back to him. He sucked the ac freon and then charged the system back and the ac worked again fine.

He diagnosed my problem as bad compressor.
i bought a new compressor and condenser.
i still have the same problem.

when i start my car in the morning the ac works fine for about 30 mimutes and then the air becomes gradually warmer until it gets blowing warm air.
every day the same issue is repeated.
Can anyone help me this issue is getting me frustrated.
i have no leaks at all.

kajtek1

Sounds like you dumped lot of money in the system without troubleshooting,
MB AC systems operate with about dozens of different sensors, so sledge hammer approach has high error margin.
For W212 you need a scanner to read the sensors. This is the best help site I know, even it shows older models, where you could read the values on car display.
Start with sensor calibration test, doing it 1st thing in the morning before engine start and before sun might warm up the cabin
.Than on hot afternoon when you enter parked car, display the refrigerant pressure, start the engine and observe how the pressure change in next 20 minutes.
90 seconds of pushing the buttons can save you lot of money by not replacing good parts.
Post the values here if you need help with interpretation.
https://www.mercedesmedic.com/ac-air...le-codes-list/

Arrie

When it stops blowing cold does the fan in the front of the car run at very fast speed at the same time?

Arrie

And does this happen when it is very hot outside?

CaliBenzDriver

It seems your system wants to work, it does every day. How clean is your cabin air filter... 5 years old or like 6 month old?

Without normal air circulation, the evap. may freeze the TXV shut.

Remove filter for simple testing if you have too

BasharHalabi96

when the ac does not work or stops working my main radiator fan would not be working at all. Basically when the ac blows cold air yes the fan starts rotating at high speed

BasharHalabi96

the filter is less than 6 months old. And i do not have any fault codes on the scanner tool.
A diagnostic scanner i used once had a long list of possible errors for the ac compressor not to start and all it checked them and all were fine

Chris_tA_4

I had this same problem with my 2010 w212. Never was able to determine the culprit before I sold it but I was thinking it’s the expansion valve getting stuck. Whenever I revved the motor in park it would make the ac cool.

kajtek1

This is worthless without a picture.

Arrie

I have posted about this issue before in other threads and some readers do not believe this saying it cannot be overcharged from the factory. I had the same problem with my 2010 E550. Out of the blue after several years it suddenly did not give cool air when started up and idling. Had to rev it up or go driving to get A/C blow cool again. I first thought the system is low in refrigerant but adding more in the system did not help but it made the fan in front run full speed, screaming.

I then let refrigerant out and this totally fixed the problem. I had a pressure gauge and in this gauge the low side pressure was within the green about third way up from low edge of the green area. I let pressure go down to that low limit of that green area. This happened years ago and it is still working. I don't remember if the problem surfaced during a very hot day but I see a lot more posts like this in the forum and it could be related to the very hot weather in some areas of the country.

Hot ambient temperature makes the system pressure higher and if the pressure is set to the high limit at factory and they did not set that limit for the high enough ambient temperature the system can end up over charged and cooling is lost. Too high charge will not allow proper evaporation at the expansion valve and coil. The system OP has could be on this charge limit as his system works but intermittently it does not. Would be good info to know if these bad days are very hot days in the area he lives in.

MBNUT1

Yes my experience with the AC not coming on is when the engine and outside air are hot. Generally if I turn the AC on right away with a cold engine it will work.

Arrie

Go get a can of a134 refrigerant. At Walmart it cost only about $6. Get a refrigerant service hose with a gauge in it from a car parts store. I don’t know if Walmart sells them. That hose will cost about $8-10.

Set car running on idle and set A/C to max cool and let it run like this for a few minutes. Then with the hose with closed valve connect the hose to the low pressure side service port and record the pressure in your system.

Then let out some refrigerant by opening the valve on the hose, i.e. you have not connected the hose to the can of refrigerant yet. Monitor the air temperature coming out from the vents.

DO NOT LET THE REFRIGERANT SPILL ON THE PAINTED CAR SURFACES!!!

If lowering the charge in your system does not work you can re-charge the system back to the pressure you recorded before letting any refrigerant out.

This method fixed my car.

ygmn

OR take it to a MB AC specialist... like a dealer?

kajtek1

But it is tough choice.
Spend $50 on scanner and do troubleshooting in 90 seconds, while keeping the scanner or
Go to the dealer and spend $200 for their troubleshooting, what will give you 4-figures job proposal.

CaliBenzDriver

Watch this what happens when A/C systems are overcharged.


An overcharged A/C system will have too much liquid refrigerant and reduce the useful condenser surface available for cooling. When that happens the system cooling is poor and you're left with a HOTTER LIQUID line - I personally keep that line as cooled as it got to hold its pressure.

When undercharged the liquid line can get bubbles of uncondensed refrigerant... that are practically useless for evaporative cooling.
​​​​​

We understand the system can get the most cooling done when the entire condenser is used and no gas bubbles are pushed through.

Higher temperatures translate into higher pressures that when combined in the extreme strain the compressor needlessly.

++++++
Wikipedia offers well written explanations about A/C efficiencies of heat exchanger and total subcooling.

"Another widespread application of subcooling is boosting and economising. Inversely to superheating, subcooling, or the amount of heat withdrawn from the liquid refrigerant on the subcooling process, manifests itself as an increase on the refrigeration capacity of the system. This means that any extra heat removal after the condensation (subcooling) allows a higher ratio of heat absorption on further stages of the cycle. ..."

"Subcooling phenomena is intimately related to efficiency in refrigeration systems..."

" The subcooling principle behind all these applications is the fact that, in terms of heat transfer, all the subcooling is directly added to the cooling capacity of the refrigerant as superheating would be directly deducted"...

- Keep it cool: prevent heat above ambient from needlessly entering your circuit.👍

Arrie

Just measured the A/C system pressures in my cars after sitting all night and not started yet for today. The E550 measures 95 psi on low side and the high side shows 110 psi. This probably is a gauge error as when the system sits for long enough the pressures should be the same in both sides.

When running on idle the pressures go to 38 psi on low and 180 psi on high side. A/C set to MAX COOL.

On my S550 the low side pressure is 88 psi before starting the engine and it drops down to 35 psi when engine is on idle with A/C on MAX COOLING. I do not know where the high side service port is on the S550 so I don't have that pressure info. Ambient temperature during this test was 90 F on S550 cluster display and 94F on the E550 display.

The very easy way to troubleshoot this is to first measure the low side pressure in the system, then let some refrigerant out especially if the pressure reading is higher than what I measured in my E550 (I believe A/C systems are pretty much the same between E550 and E350) and see if it helps and be not surprised if it actually does. If letting refrigerant out does not help the very easy way to return the system back to what it was is to add refrigerant pressure back to the same level what it was before letting any out.

A bottle of R-134a with the hose and gauge cost about $30. It is easy to use and simpler than buying separate hose.

There are very strong opinions about this in the forum stating that the system cannot be over charged from the factory but I strongly disagree. There are several other issues with these cars that were made wrong at the factory. Why could this not be one of those items? Mine was and I strongly believe there are others.

kajtek1

Too much speculation.
You did not post ambient temperature, but 35/180 psi pressure on hot day is bad undercharge.
I avoid hooking up pressure gauges to the system as that can lead to valve failure.
Since the car has high pressure sensor, who is not only accurate, but can be read with clean fingers, no need to get greasy.
https://www.jamcoautoparts.com/syste...shootingchart/
As for overcharge, the cars do have high pressure limit switch (or program), so the system will still work, but will cycle on/off, what will lower its efficiency.
Again, something you can troubleshoot in just seconds with proper scanner.

CaliBenzDriver

HEAT EXCHANGER FOR EXTRA EFFICIENCY...

... Once you become aware that overcharge directly leads to poor heat transfers, you try to optimize that process.

Dumping a little juice for high heat locales makes perfect sense as a quick fix. Helping pipes stay cool with IHX etc... is an extension of this reasoning.

MBNUT1

I have the refrigerant and hose that I bought for my wife's Mazda (the problem there was a bad wire to the clutch). Interesting that one tends to run high pressure so maybe I will play with it to work up my courage to mess with the Mercedes.

Arrie

There is nothing special with it. Read and record the low side pressure & ambient temperature one morning before you start the car. Then start and record the low side pressure when the engine is on idle and A/C on MAX COOL. Let it run 10 min before taking the pressure reading.

From here you can just let some refrigerant out and follow the air temp coming out from the vents. If your system does not work you probably don't have condensation on the piping surfaces where you connect the hose but if humidity is very low you may not get that even if the cooling works. Anyway, the piping surface should get cool though.

If your pressure readings are about the same as mine then you may have a different problem but as the system mostly works you may just have the exact same over charge issue that I had.

If you let a lot of refrigerant out and then add it back also add some oil charge as with refrigerant also lubrication oil comes out. Refrigerant and oil are well mixed together in the system so you don't need to add oil if you don't add more refrigerant but if you add lots of refrigerant you need to also add oil so the lubrication is not compromised.

Arrie

I think I did post ambient temperature and you can read it if you care to read the whole post.

What comes to your expertise saying my A/C system is under charged I can only say that the systems in both of my cars work very well with the charge levels I measured. And I don't care how much you drum for getting the scanner. I don't think the scanner would help much with this unless the compressor is bad and the scanner would somehow tell that but the compressor is not bad as the system mostly works and as MBNUT says it always works when the car is not hot and the outside temperature is not too hot.

I have troubleshoot all sorts of issues as my job for all my life and I have learned one thing: Sometimes the solution is not what the books say. You need to take the solution and go with it no matter how unorthodox it is and how the solution was sometimes found by trial-and-error and then TRY TO FIGURE OUT THE THEORY BEHIND THE SOLUTION LATER OUTSIDE THE BOOKS THAT TELL YOU DIFFERENTLY/WRONG!

In this case if the refrigerant charge pressure is at high limit and it causes the system not work properly then just a few psi lower pressure can make it work again. This method worked for me and it was simply letting refrigerant out (after first adding that did not help and made fan in front run fast) and feeling the air temperature thru the vents. Went also too low with it and then added it back to the readings I measured today.

This is a prime example of German engineering as if the problem really is (I believe it is) over charge in the A/C system it will never get fixed by the MB dealer service. They will take the car in and start changing components in it (customer pays everything) and in the end of the day say it works fine as they test it in their garage in the nice 75F temperature but when the customer takes the car home it again will not work in hot environment. Take it back to dealer and they cannot fix it only because they do everything what MB in Germany tells them to do. There is no way they can deviate from what Germany says, like the amount of refrigerant fill in the system.

Some people go by books, I go by what works and then change the books, which takes an act of God in case of a German company.

juanmor40

I would have phrased it a bit differently: most of the time the solution IS on the books, sometimes is the reader's misinterpretation or the incorrect book, and rarely a failure in the theory. The theory did not make it into the books out of nowhere, but after being validated by experts/specialists/appropriate groups in their field.

I have also found out "solutions" by trial-and-error over my career, but they will never leave my desk, garage or laboratory before they are understood by the team and myself. If the solution is against the accepted understanding, the higher the scrutiny so we all understand the future forms of failure. That is how science and engineering have made progress over time; otherwise, it is anarchy.

I think we are here trying to help each other with our best understanding of the issues.

kajtek1

You posted "temperature posted on cluster display"
For the guy who claims to be problems solver, I see you are problems producer.
It doesn't take much knowledge to understand than on parked car, the front sensor is affected by engine heat, even when the sun exposure is not the issue.
Than when you have a point that the system will work with low refrigerant, this is not what you suppose to advise to somebody who already has a problem.
The sledge-hammer advise can be right only to some point.
That is why smart people do troubleshooting.

Arrie

Cluster temperature is ambient temperature when the reading is taken before the engine hasn’t run for one minute yet.

You say smart people do troubleshooting so STAY THE HECK AWAY FROM IT THEN!!!

You are another BOTUS but hanging on the damn scanners to be all mighty problem solvers. Scanner is a tool for the real problem solver who needs to be able to step away from that magic box with buttons.

You obviously either read or understand or both poorly.

Arrie

So for checking what I wrote earlier I today spend about 6 hours inspecting/researching the A/C issue at hand. As I stated earlier my low side refrigerant pressure was running at 35 psi when engine at idle and I did not know where the high side service port in my S550 is but today I found it at the front right upper corner of the radiator. Measurement today gave me 38 psi on low side and 180 psi on high side. Ambient temperature at 95F. I then added refrigerant as the expert so strongly says my system is low in charge. Slowly adding R-134a to a point low side pressure reads 45 psi and high side 250 psi. Looked fine first but then suddenly high side pressure went up to 450 psi with low side going down to 40 psi. The system very obviously was over charged. I could even see the compressor piston strokes in the high side pressure gauge.

Quickly let refrigerant out and set the low side pressure to 40 psi with the high side at about 210 psi. Went to drive and lost A/C. Barely any cooling noticeable in the air coming out of the vents. The iCarsoft scanner showed refrigerant pressure at 26 bar when it normally has been at around 13-16 bar. Drove back home and let refrigerant out to get the 35 psi on the low side pressure again. High side reads 180 -190 psi. Air from vents 40F. Went to drive and refrigerant pressure by the scanner reads about 11-12 bar. Pressure goes up to 14.5-15 bar when engine on idle. This is back to normal well working A/C on my car with the seriously undercharged system by your opinion.

I go by what works and in my two cars it seems the low side pressure in the A/C system needs to be below 40 psi for it wo work during very hot days.

.