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Discussion Starter #1
It seems to me that my A/C does not cool as well as it should. It has always had lackluster performance in the 1+ year that I've owned it... and since I bought this 2009 model used, I don't know the vehicle's history or performance when new.

So expecting it to be low on refrigerant, I decided to hook up the gauges. I'm reading 34psig low side & 150psig high side at an ambient temperature of 85F and plenum outlet temperature of 49F. So now I'm thinking it may be a bit overfilled.

All I've managed to locate here is this post from 6-11-2012:

I refilled my A/C two months ago and it still is working great. Actually I think it is working better than last year. It was a 75 degree day, 23 psi on the low side and 150 psi on the high side.

Bear in mind my ambient temperature is 10 deg higher than Stretchmobile's 75F, so I'd expect a bit higher pressures. Does the service manual contain information on the system hi/lo pressures vs ambient & plenum temperatures?

~toaster
 

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Could be overcharged but then again could have too much oil in the system, the only way to make sure on a second hand car is to have it evacuated and recharged with correct amount. I top mine up from a can if required but every three or four years it's evacuated and charged professionally.
 

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These little guys only hold .993 lb of refrigerant. I would guess that it might be a touch low. The only way to be sure is to take it to a professional shop and have it evacuated and recharged. It would be quite costly to have all the equipment and proper training to do just one car.
 

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If you have 49° at the outlet...you won't do much better than that !!

I have been monitoring mine for the last month...thinking it is not working well...it will pull to 48°, but by the time it actually gets to ME, it doesn't feel very cool. I now use fan setting 3 and recirc and it's much better....I think it's time for tint :D
 

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It seems to me that my A/C does not cool as well as it should. It has always had lackluster performance in the 1+ year that I've owned it... and since I bought this 2009 model used, I don't know the vehicle's history or performance when new.

So expecting it to be low on refrigerant, I decided to hook up the gauges. I'm reading 34psig low side & 150psig high side at an ambient temperature of 85F and plenum outlet temperature of 49F. So now I'm thinking it may be a bit overfilled.

All I've managed to locate here is this post from 6-11-2012:




Bear in mind my ambient temperature is 10 deg higher than Stretchmobile's 75F, so I'd expect a bit higher pressures. Does the service manual contain information on the system hi/lo pressures vs ambient & plenum temperatures?

~toaster
The service info I have seen does not list temperatures and pressures for two reasons. The first is temperatures and pressures will vary with ambient temperature, humidity and actual heat load. The second reason is the service info assumes the technician will be using the prescribed a/c service machine which will fill the system with the correct amount of refrigerant. The German logic is inescapable. If the system is repaired correctly and filled correctly it will function correctly. How can you argue with that?

As far as proper functionality, what you need to know is whether the evaporator is flooded with refrigerant vapor. If the evaporator is not flooded the system will not cool properly. The suction line going from the evaporator back to the compressor should be within five degrees of the evaporator inlet. Essentially, the suction line coming out of the evaporator should be ice cold. If not, the refrigerant level is low.

The system will not cool properly if the heat removed from the cabin can not be released to the atmosphere. Be sure the radiator fan is running properly and the condenser is clean and not blocked with debris.

There is only so much one can check before doing a complete evacuation and recharge.
 

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I know this is pretty much off topic ... but, I've seen others post on it, and I've noticed it in my car too.

When my air conditioning doesn't seem to be putting out like it should, I run the temperature control up and down a time or two (work it). And for some reason, the results are undeniable.

Maybe dirty contacts in the control?
 

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Discussion Starter #7
If you have 49° at the outlet...you won't do much better than that !!

I have been monitoring mine for the last month...thinking it is not working well...it will pull to 48°, but by the time it actually gets to ME, it doesn't feel very cool. I now use fan setting 3 and recirc and it's much better....I think it's time for tint :D
I find I generally need to use the recirculate too. I have the sunshade closed in the hot weather, and while it cuts down on the light/glare, I don't know how much heat it really blocks. Could it be that the smart is like a mini Pacer, aka greenhouse on wheels? :eek:

~toaster
 

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Discussion Starter #8
As far as proper functionality, what you need to know is whether the evaporator is flooded with refrigerant vapor. If the evaporator is not flooded the system will not cool properly. The suction line going from the evaporator back to the compressor should be within five degrees of the evaporator inlet. Essentially, the suction line coming out of the evaporator should be ice cold. If not, the refrigerant level is low.

The system will not cool properly if the heat removed from the cabin can not be released to the atmosphere. Be sure the radiator fan is running properly and the condenser is clean and not blocked with debris.

There is only so much one can check before doing a complete evacuation and recharge.
Yeah, I should have clamped a thermocouple onto the suction line to have gotten a good temperature reading there, but for some reason it did not occur to me. :|

So where are good spots to access the evaporator for temperature measurements? I assume the suction line by the low-side port is right by the evaporator outlet... but where do I access the inlet... from under the 'hood' or inside the vehicle?

The fan was running the whole time, compressor was cycling (I think), and the condenser is clean... as well as the cabin air filter. I did notice the high-side pressure slowly climbed up to 230 once the car warmed up, which makes sense as the heat from the radiator would decrease the ability of the condenser to dump its heat. I also wonder about the position of the service flap hanging on its hooks and affecting the airflow through the condenser & radiator.

Do you recommend taking system measurements at idle, or at a somewhat higher rpm?

~toaster
 

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Discussion Starter #9
I know this is pretty much off topic ... but, I've seen others post on it, and I've noticed it in my car too.

When my air conditioning doesn't seem to be putting out like it should, I run the temperature control up and down a time or two (work it). And for some reason, the results are undeniable.

Maybe dirty contacts in the control?
I've come across that too in the forums. And a report that it cools better if the temperature control is set just a smidge warmer than max cold.

I guess I need to play with this and see if it has any effect.

~toaster.
 

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Hey, Toaster, I had some of these classic issues with the A.C. on my car. My auto temperature control system was replaced under warranty. That part has been revised several times. The controller stores fault codes only readable by a Mercedes STAR machine. We don't see this problem mentioned much on later model year cars, so the controller may well solve your issues.
I live in the mid-south with very high humidity. I've learned to use re-circ. due to the humidity all the time. The lowest setting is max cool with no heat added to control the inside temp by the controller. The heat works the same way when set to max. Anything other than max is moderated by the controller.
Another thing that has affected some is poor conductivity with contacts in the connector of the plenum sensor in the passenger foot well on the left hand side. CRC 2-26 is good for improving electrical properties.
When working properly, it cools really well.
By the way, the temp. control lever rheostat is known to lose contact with a resultant change of air temperature to the warm side. Moving the lever up or down one notch and back restores temperature control and operation back to normal. The wiping action of the contact in the control itself is a self cleaning feature of the control. It is the nature of the beast. :D That only happens occasionally to mine.
 

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So where are good spots to access the evaporator for temperature measurements? I assume the suction line by the low-side port is right by the evaporator outlet... but where do I access the inlet... from under the 'hood' or inside the vehicle?

Do you recommend taking system measurements at idle, or at a somewhat higher rpm?

~toaster
The reason I stated the suction line should be cold and sweating is due to lack of access to the evaporator inlet. The TXV (thermal expansion valve) is under the front service flap and tight against the bulkhead wall. The evaporator lines that connect to the TXV are inside the HVAC case. There is a small plastic cover on the lower right of the HVAC case that can be removed to disconnect the heater hoses from the heater core in the case but the evaporator lines are toward the center of the car from the heater hoses and I do not think they are accessible, but I have never looked.

The system pressures are very dependent on heat load, as I stated. They will give you a indication of something seriously out of whack but hard and fast numbers are a waste of time. The important thing is whether the system is full and moving heat out of the cabin into the surrounding atmosphere.

In another post you mentioned the sunshade. It does make a difference. I had the shade out of my car for a few days. When my brain started melting and running out my ears I made it a point to reinstall it today rather than wait two days until Saturday.
 

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I have never checked my a/c system because it always has worked as well as I would expect it to. Today I stuck a thermometer in the center air duct just because I was curious. Ambient temperature was 85 degF, sunshade closed, system on recirculate. Going down the road vent temp was about 40 to 42 degrees. At a stop for a longer than a minute with the sun streaming in the windows the vent temp went up to 46 to 48 degrees I think, since I could not look directly at the scale on the thermometer. Start moving again and the temperature comes back down because more air is moving across the condenser to shed more heat. Sitting at a stop in the shade with no direct sunlight the vent temp only goes up to 42 to 44 degrees, due to less heat load. I do know what the system pressures are and I don't particularly care. They will vary with the heat load in a similar fashion.
 
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