SL/R129: AC Acting odd
03-14-2016, 06:10 PM
#1
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1999 SL500
AC Acting odd
Hi guys. New guy here.
1999 SL500 USA Spec car.
My AC is acting odd. I found an informative but old post that explains how to run a diagnostic test, which I will do in the morning after the car cools off, per the thread.
In the meantime this is the problem.
1) Cools fine in auto mode set at 72F. The air is ICE cold and blows when initially turned on.
2) After a while, the volume of air flow decreases, but the temp output is still ICE cold, however there is a considerable diminished effectiveness to maintain the set temp.
3) The odd part, the AUTO fan speed shows that it is increasing on the display as if it is trying to keep up with the increasing temps in the car and maintain the 72F setting. BUT it is doing exactly the opposite, the air flow is decreasing but the temp is still cold.
Any ideas or links to threads?
1999 SL500 USA Spec car.
My AC is acting odd. I found an informative but old post that explains how to run a diagnostic test, which I will do in the morning after the car cools off, per the thread.
In the meantime this is the problem.
1) Cools fine in auto mode set at 72F. The air is ICE cold and blows when initially turned on.
2) After a while, the volume of air flow decreases, but the temp output is still ICE cold, however there is a considerable diminished effectiveness to maintain the set temp.
3) The odd part, the AUTO fan speed shows that it is increasing on the display as if it is trying to keep up with the increasing temps in the car and maintain the 72F setting. BUT it is doing exactly the opposite, the air flow is decreasing but the temp is still cold.
Any ideas or links to threads?
03-14-2016, 08:41 PM
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94 SL 600, 98 993, 89 XJS Conv., 85 Daimlier Princess 16 Hybrid Lincoln 16 Chevy crewcab
Having recently replaced the HVAC dash control, go thru the diagnostics carefully.
The system is complex and not an easy fix for the DIY home mechanic, unless you have the system tested! The parts a easy to swap but may need to be programmed to operate fully.
The system is complex and not an easy fix for the DIY home mechanic, unless you have the system tested! The parts a easy to swap but may need to be programmed to operate fully.
04-12-2016, 02:37 AM
#3
Junior Member
ac icing at freeway speeds
Hi guys. New guy here.
1999 SL500 USA Spec car.
My AC is acting odd. I found an informative but old post that explains how to run a diagnostic test, which I will do in the morning after the car cools off, per the thread.
In the meantime this is the problem.
1) Cools fine in auto mode set at 72F. The air is ICE cold and blows when initially turned on.
2) After a while, the volume of air flow decreases, but the temp output is still ICE cold, however there is a considerable diminished effectiveness to maintain the set temp.
3) The odd part, the AUTO fan speed shows that it is increasing on the display as if it is trying to keep up with the increasing temps in the car and maintain the 72F setting. BUT it is doing exactly the opposite, the air flow is decreasing but the temp is still cold.
Any ideas or links to threads?
1999 SL500 USA Spec car.
My AC is acting odd. I found an informative but old post that explains how to run a diagnostic test, which I will do in the morning after the car cools off, per the thread.
In the meantime this is the problem.
1) Cools fine in auto mode set at 72F. The air is ICE cold and blows when initially turned on.
2) After a while, the volume of air flow decreases, but the temp output is still ICE cold, however there is a considerable diminished effectiveness to maintain the set temp.
3) The odd part, the AUTO fan speed shows that it is increasing on the display as if it is trying to keep up with the increasing temps in the car and maintain the 72F setting. BUT it is doing exactly the opposite, the air flow is decreasing but the temp is still cold.
Any ideas or links to threads?
Is this a bad evap temp sensor?
Last edited by PeterLech; 04-12-2016 at 02:40 AM.
04-12-2016, 07:21 AM
#4
Out Of Control!!
There have been numerous technical service bulletins relating to evaporator icing---maybe one of them applies to your situation---I'd call a dealer to clarify!!
04-12-2016, 12:33 PM
#5
Junior Member
Are the TSBs available online anywhere? I am kind of allergic to going to dealers after some bad experiences, most lately with a failed yaw sensor.
04-13-2016, 11:31 AM
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1998 SL500
Hi guys. New guy here.
1999 SL500 USA Spec car.
My AC is acting odd. I found an informative but old post that explains how to run a diagnostic test, which I will do in the morning after the car cools off, per the thread.
In the meantime this is the problem.
1) Cools fine in auto mode set at 72F. The air is ICE cold and blows when initially turned on.
2) After a while, the volume of air flow decreases, but the temp output is still ICE cold, however there is a considerable diminished effectiveness to maintain the set temp.
3) The odd part, the AUTO fan speed shows that it is increasing on the display as if it is trying to keep up with the increasing temps in the car and maintain the 72F setting. BUT it is doing exactly the opposite, the air flow is decreasing but the temp is still cold.
Any ideas or links to threads?
1999 SL500 USA Spec car.
My AC is acting odd. I found an informative but old post that explains how to run a diagnostic test, which I will do in the morning after the car cools off, per the thread.
In the meantime this is the problem.
1) Cools fine in auto mode set at 72F. The air is ICE cold and blows when initially turned on.
2) After a while, the volume of air flow decreases, but the temp output is still ICE cold, however there is a considerable diminished effectiveness to maintain the set temp.
3) The odd part, the AUTO fan speed shows that it is increasing on the display as if it is trying to keep up with the increasing temps in the car and maintain the 72F setting. BUT it is doing exactly the opposite, the air flow is decreasing but the temp is still cold.
Any ideas or links to threads?
The good news is you can do it yourself. Harbor Freight has a good manifold/gauge set for $60 (sometimes less), and you can buy r134a for $12 to $15 per 12 oz. can.
The web is loaded with DIY tutorials and videos on how to check the refrigerant charge and add more if needed....
---------------------------------------------------
HF also has a very capable vacuum pump for $100 (again often for less). Buy that and with a little knowledge you will have all you need to repair anything that could go wrong. I have had that same pump for years and it works just fine--used it last New Yearsv Eve day to install a new HVAC system in my house.
Last edited by cliffyk; 04-15-2016 at 11:49 AM.
04-15-2016, 10:29 AM
#7
Member
Not an expert myself, but seems like a failed register (?).
Very recently, my SL600 (1997) A/C started acting funny and had to replace the said failed register. At the same time, had the fan blower unit (pretty large stuff) replaced due to parts failing that resulted in funny sound.
Now all is well again.
w/ smiles
Jimmy
Very recently, my SL600 (1997) A/C started acting funny and had to replace the said failed register. At the same time, had the fan blower unit (pretty large stuff) replaced due to parts failing that resulted in funny sound.
Now all is well again.
w/ smiles
Jimmy
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04-15-2016, 12:35 PM
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1998 SL500
Somewhat counter-intuitively an undercharged refrigeration system can result in increased cooling capacity--albeit briefly and with a big gotcha.
In a moderately undercharged auto AC system, when the operating pressure at the expansion valve drops to < 58 psi, the vaporization temperature of the r134a refrigerant drops to 32° F or less--this creates the "..air is ICE cold..." condition reported by the OP.
However when this happens moisture in the air being cooled forms frost on the evaporator coil and fins. In a now runaway process¹ the frost builds to a layer of ice that eventually blocks air flow through the evaporator--this is the reported "...volume of air flow decreases, but the temp output is still ICE cold..."
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¹ - Not only does the ice buildup block air flow over the coil reducing cooling capacity, it is also as Eskimo's know an effective thermal insulator--thus insulating the coil and preventing efficient absorption of heat from the passing air--allowing the frost to build into a thicker and thicker layer of ice.
In a moderately undercharged auto AC system, when the operating pressure at the expansion valve drops to < 58 psi, the vaporization temperature of the r134a refrigerant drops to 32° F or less--this creates the "..air is ICE cold..." condition reported by the OP.
However when this happens moisture in the air being cooled forms frost on the evaporator coil and fins. In a now runaway process¹ the frost builds to a layer of ice that eventually blocks air flow through the evaporator--this is the reported "...volume of air flow decreases, but the temp output is still ICE cold..."
--------------------------------------
¹ - Not only does the ice buildup block air flow over the coil reducing cooling capacity, it is also as Eskimo's know an effective thermal insulator--thus insulating the coil and preventing efficient absorption of heat from the passing air--allowing the frost to build into a thicker and thicker layer of ice.
