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Old 09-28-2006, 07:13 PM   #6
fdryer
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Join Date: Jan 2006
Location: NYC
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2003 L-Series 3.0L Sedan
Default Re: A/C Problems - '99 SL2

The a/c will extract heat from the interior of the car and send it to the condenser coil, in front of the radiator, adding to the heat from the radiator. The radiator fan must be on to either pull air through both condenser and radiator coils or blow through them both to keep a constant airflow. Normally when the a/c is on the radiator fan stays on regardless of whether you adjust for cold, medium, or hot air. The dual heat loads coming from the a/c and radiator leaves the fan on. When the a/c is off there is only the radiator to cool off and then only when water temperatures are at, say 200 degrees F., the fan turns on to cool the radiator until a lower temperature is reached whereupon the fan is switched off automatically. In this situation the fan is controlled by the ECM whose input is the engine coolant temperature sensor. The automatic fan control circuit.

The a/c running creates an extra heat load where the ECM bypasses the normal automatic radiator cooling circuit and leaves the fan on. There isn't any thermostatic control for the a/c to turn off for temperature control as you may have suspected but something else is turning off the a/c. That something else was pointed out in the previous post related to a possible refrigerant leak that created an abnormal condition-the cycling of the compressor running and then not running that you observed under the hood. Normal air conditioning will have the compressor running constantly. For a person that doesn't know much about air conditioning you certainly are observant enough to carefully describe your situation clearly. What appears to be the a/c running one moment and then not running is due to the possible lower pressure of the refrigerant that a pressure switch detects as too low to allow the a/c to continue thereby turning off the a/c and turning it back on as if there were a thermostat controlling the a/c. Not so as this isn't a normal situation but you were concerned enough to notice a less than normal cooling of the a/c system to look under the hood. You just happened upon the beginnings of a failing a/c system most would not have noticed until much later when a total absence of cooling has taken place. As an aside, some vehicles do/did have a thermostatic temperature control that indeed turns off the compressor just as you described but for one reason or another this fell out of favor. I believe our Saturns don't have this feature in favor of leaving the compressor running full time while improvements in the design of modern compressors requires less power to run continuously. To this end the blend door (in the center dash console) and the heater coil through either a lever or electronic temperature control adjusts for the amount of cold/hot air or anything in-between. The electronic temperature control is what I have in my L300 and called Auto-Temperature Control when I leave it on or revert to manual mode to leave the a/c off. Leaving it on only controls the blend door to mix the hot air from the heater coil and the cold air from the evaporator coil. It won't turn off the a/c, as in turning off the compressor.

Replacing the fan may help but that is a separate issue since you state that the fan is making a rubbing sound and doesn't appear to run correctly. Most everyone here will tell you that since the fan runs when commanded to without the a/c on and runs the instant you switch on the a/c the fan is working correctly. The replacement may be warranted due to the age of your car. If after you have the fan replaced and you still have an a/c problem take the car to a known good a/c shop for diagnosis. And for more info go to http://ackits.com to learn about car a/c. You can also read A/C Basics in the miscellaneous tech section.

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