Hi fleetwin. Welcome to the forum. The easiest thing to do is find another similar car and see how it compares. According to two Chrysler trainers from many years ago, AC systems are only intended to lower air temperature 20 degrees. The real comfort comes from the removal of the humidity.
Some systems will have a duct temperature of as low as 40 degres which is actually a bad thing. The humidity condenses and collects on the cold evaporator in the heater box. That moisture runs into a drain pan and drips onto the ground. There are mechanical or electronic controls to slow the flow of refrigerant to prevent the evaporator from getting colder than around 40 degrees because that moisture would freeze into a block of ice and block air flow. The symptom would be real good cooling for a few minutes, then no more cold air from the ducts.
One thing you can check for is excessive frost on one of the hoses under the hood near the firewall. A liquid must absorb a lot of heat to turn into a vapor. It is supposed to evaporate in the, ... Uhm, ... Well, ... Evaporator, hence the name of the part. The point of evaporating is where it gets real cold, and you want that to be inside the car. Two things can cause that to occur in the wrong place. First, if there is not enough charge in the system, the liquid will vaporize in the hose leading to the firewall. Where that occurs you will see water condensation on the hose, or more likely, frost. Some water on the hose is normal because the refrigerant will be much colder that outside air, but a lot of frost is not normal.
If the system is over-charged, the evaporator will be full of liquid refrigerant. It will be cold but the real cooling will take place in the hose going from the firewall to the compressor. Again, the point where the liquid turns to a vapor will be evident by the frost on the hose.
The second cause of cooling in the wrong place is a restriction in the refrigerant's path. There is a designed-in restriction that adjusts to control how much cold liquid enters the evaporator, (to prevent over-cooling and freeze up), but unintended restrictions cause a point of pressure drop which promotes vaporization and cooling. You don't want that cooling to take place under the hood. There will usally be frost buildup on the location of a restriction. A small canister called the receiver / drier connected to one of the hoses has a filter and moisture absorber that can become blocked and is a common place to look for blockage.
The evaporator must be held at a temperature above freezing for water, and 40 degrees is typical. Since you are mixing it with much hotter air, it is impossible for the air coming from the ducts to be that cold. The less the volume of air flow, the easier is will be for the temperature to be lowered closer to 40 degrees. Measure the duct temperature with the fan on high speed, then run it on the lowest speed for a few minute and measure the temperature again. At the higher speed, the larger air volume will result in a higher duct temperature but there should not be a significant difference. The closer the two readings are to each other, the more cooling capacity the system has and that indicates it is working properly. If the temperature is a lot higher on the faster fan speed, that indicates the system doesn't have much capacity to remove heat from the air. If no restriction is obvious, look at the temperature-sensing device on the evaporator. It used to be a copper bulb on the end of a long copper tube. It was filled with a gas that expanded in response to increased temperature and would push on the valve that controlled how much refrigerant entered the evaporator. Newer systems often use an electronic sensor, then a computer turns the compressor on and off to control the flow rate of the refrigerant. If that sensor is not in solid contact with the evaporator, or is mispositioned from its intended location, it will not sense temperature correctly. Usually it will not get cold enough so the system mistakenly over-cools and freezes up.
There have been some problems with the valves that control the flow of refrigerant but they usually intermittently cause no cooling or over-cooling when they stick. The valve itself does not get out of adjustment. It just responds to the command from the sensing device.
Friday, June 4th, 2010 AT 5:41 PM