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Discussion Starter #1
Testing A/C cooling capacity with SDS, what are the key values to determine performance level?

I see Evaporator temp slightly higher than it should be, never drops below 5 C. Air outlet (at center vent) tmperature is 8 C, which is marginal.

I can see no reference in SDS what the Refrigerant pressure should be. Is this relevant? Mine is at 6-7 bar with A/C off, then it goes up to 13-14 bar when I switch it ON and set to testing parameters… then it drops to 10-11 bar after few minutes.
 

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I can see no reference in SDS what the Refrigerant pressure should be.
The correct pressure is a function of variables which are not all known to the A/C system and Star Diagnosis: outside air temperature, barometric pressure, and humidity.

If you google "R134a pressure chart" I believe you may conclude that your system is slightly undercharged and the addition of refrigerant is in order.
 

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Discussion Starter #3
The correct pressure is a function of variables which are not all known to the A/C system and Star Diagnosis: outside air temperature, barometric pressure, and humidity.
Thanks for the explanation, Bob.

These R134a pressure charts refer to low and high pressure. Am I correct to assume that single value I read with SDS is High, while Low reading is obtained with the gauge at the service valve?

I have not serviced A/C system myself but, I might try to do it this time.
 

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Yes, Star Diagnosis shows the high pressure.

Generally, I believe proper charging procedure dictates that you observe both high and low pressures. But presuming your system is working properly, then I would be comfortable relying solely upon the high pressure reading reported by SDS.
 

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Discussion Starter #6
So I got couple 12 oz cans of r134a and low pressure gauge. My initial pressure check was right at the edge of white and green (low and filled), which is 25psi.

First can took it up to almost 40psi but the second only moved the needle slightly, to just above 40psi.

High pressure went up, center air vent temperature went down (from 8° C to 6° C) but Evaporator temp still only goes down to 5° C. All good but I feel, with ambient temperatures 76-77° F, I should try to get to 45psi.

I'm not sure if there is enough pressure in these 12 oz cans or I just need more of them to move the needle when pressure gets higher.
 

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Does your evaporator temp sensor reading make sense after the car has stood overnight? I recently replaced mine because it constantly showed something like almost 30C, causing the evaporator to freeze up and block air flow because the compressor never disengaged due to the bad reading. Apparently a fairly common issue and an easy fix.
 

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...Evaporator temp still only goes down to 5° C.
I have no experience with these late-'90s systems, but I'll guess that the compressor is not attempting to drive the temperature any lower, and the evaporator temperature will get no lower than this. Note that '97 and later cars are "variable-displacement" type compressors and have a purely mechanical control system apart from the clutch which normally is always engaged.

It's my experience using these cans of refrigerant that putting the last bit of charge takes some time, and it seems as if the can isn't actually emptying at all. I know a mechanic who would immerse a can in hot water to help empty it, and I suppose that helps.
 

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Discussion Starter #9
Does your evaporator temp sensor reading make sense after the car has stood overnight?
Yes it does, as I checked this morning, it was the same as ambient temperature. Plus it's responsive so I don't think there is a problem with it.



...but I'll guess that the compressor is not attempting to drive the temperature any lower, and the evaporator temperature will get no lower than this. Note that '97 and later cars are "variable-displacement" type compressors and have a purely mechanical control system apart from the clutch which normally is always engaged.
I got another can of R-134a and started charging some more but I wasn't paying attention to SDS high pressure readings. Just watching low pressure, which was still just above 40psi when A/C compressor started cycling on and off pretty frequently. Checked the High pressure and it would shoot up to 25 bar when compressor was on and the air stared to blow warmer out of the vent.

So I released some pressure via low pressure gauge until I got High side to 18 bar at 32 °C. Now, I was reading Outside air temp (to calculate high pressure limit) from SDS, which is different than dash display from bumper temperature sensor. I'm assuming that SDS reads outside air temp on the compressor?

Anyway, Evaporator temperature drops to 4 °C, air vent temperature at Max A/C is 5-6 °C and compressor cycles on/off bit more than it used to lately.
 

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I'm assuming that SDS reads outside air temp on the compressor?
The climate control has its own dedicated outdoor air temperature sensor mounted in the duct work in front of the windshield. The temperature you see displayed in SDS is from that sensor.

...compressor cycles on/off bit more than it used to lately.
I believe the compressor should generally never cycle off an on, since yours is a 2000 car with a variable-displacement compressor. Conditions that can cause the clutch to not engage include "EC" mode selected on the in-dash control, a refrigerant pressure outside the range of 2-28 bar, and an evaporator temperature below c. 1 degree. I don't suppose any of those conditions exist in your case, so it is not apparent what is the cause of the clutch cycling.
 

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I'm assuming that SDS reads outside air temp on the compressor?
The climate control has its own dedicated outdoor air temperature sensor mounted in the duct work in front of the windshield. The temperature you see displayed in SDS is from that sensor.
The dedicated outdoor temperature sensor was eliminated in later models, I believe for model year 97 when the AC control unit changed. The later control unit gets the temp signal via the instrument cluster. In other words, the outdoor temperature reading comes from the sensor on the front bumper.
 

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Discussion Starter #12
I believe the compressor should generally never cycle off an on, since yours is a 2000 car with a variable-displacement compressor.
You got me worried with that, Bob. My compressor clutch will stay engaged all the time with temperature set to LO and Auto setting. But when I set temp to say 70-72 °F it will periodically disengage and re-engage. I'm not 100% sure but I think it's always been like that on my car. If this is a malfunction then I'd like to correct it.

I would appreciate if someone with similar MY (99-02?) could look at their A/C compressor and let me know if the clutch is ON all the time, regardless of temperature setting, with A/C on.

The dedicated outdoor temperature sensor was eliminated in later models, I believe for model year 97 when the AC control unit changed. The later control unit gets the temp signal via the instrument cluster. In other words, the outdoor temperature reading comes from the sensor on the front bumper.
I've seen info in SDS stating just that, regarding some models but, I didn't see R129 on that list.

My 2000 SL definitely uses separate temp sensors for instrument cluster and A/C control. When the car is stationary and engine running they both start with the same reading (more or less) but after few minutes the one for A/C is getting affected by engine heat much more than one on the bumper.
 

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I would appreciate if someone with similar MY (99-02?) could look at their A/C compressor and let me know if the clutch is ON all the time, regardless of temperature setting, with A/C on.
FATHERTIME, juuso, and Ears are three members that come to mind that can help with this check. All '97 and later cars have the same setup as yours.

My 2000 SL definitely uses separate temp sensors for instrument cluster and A/C control. When the car is stationary and engine running they both start with the same reading (more or less) but after few minutes the one for A/C is getting affected by engine heat much more than one on the bumper.
Despite the discrepancy in the readings, I believe there is a common sensor for outdoor temperature as juuso wrote. For '97 and later there is a serial communication link between cluster and climate control which communicates coolant and outdoor temperatures.
 

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I have never sensed my compressor cycling, and I understood it does not. The clutch is of course disengaged if the Econ switch is engaged, otherwise, it is pumping. Now, I will say I have not looked to verify...
 

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Discussion Starter #16
My 98 SL500 clutch is only on with the AC/Heat turned on I'll check the 99 SL600 later.
I know, but is the clutch spinning all the time or does it disengage from time to time? Specifically when A/C temperature setting is only few degrees lower or even equal to the outside temperature.

I have never sensed my compressor cycling, and I understood it does not. The clutch is of course disengaged if the Econ switch is engaged, otherwise, it is pumping. Now, I will say I have not looked to verify...
Could you please?
 

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Discussion Starter #17
Despite the discrepancy in the readings, I believe there is a common sensor for outdoor temperature as juuso wrote. For '97 and later there is a serial communication link between cluster and climate control which communicates coolant and outdoor temperatures.
As I mentioned earlier, I've seen this is SDS but, no reference to R129. See the screen shots.

After idling the engine for about 10-15 minutes A/C module outside temp reading can raise much more than instrument cluster… like 111 °F vs 75 °F. I'm not sure how these values could come from the same sensor.
 

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Discussion Starter #18
So, apparently, my compressor on/off cycling was caused by too much refrigerant. I removed, very slight amount, and the clutch is staying engaged…

…lesson learned from my first time A/C service experience: don't get trigger happy with the freon, read the sticker (for total weight of r134a in the system), check the static pressures on low and high side before filling.
 

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As I mentioned earlier, I've seen this is SDS but, no reference to R129. See the screen shots.

After idling the engine for about 10-15 minutes A/C module outside temp reading can raise much more than instrument cluster… like 111 °F vs 75 °F. I'm not sure how these values could come from the same sensor.
From having just done the blower/regulator, I can assure you there is no sensor in the blower intake on a ‘97 like on older R129... can’t explain as I was under the impression the A/C got its info from the same as the cluster they a serial interface like has been previously reported here in this thread by others.
 

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Discussion Starter #20
From having just done the blower/regulator, I can assure you there is no sensor in the blower intake on a ‘97 like on older R129...
Obviously you'd know firsthand, I only assumed there must be two separate sensors based on huge discrepancy between SDS and instrument cluster outside temperature. But you are all correct.

I just checked entire HVAC in EPC, based on my VIN and didn't find any sensor in there. So it remains mystery why I'm seeing what I'm seeing.
 
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