Air Conditioning

I have a 1999 Mercury Mountaineer with 315,000 miles and pleased to say it’s a great car. Although in recent years during hot days the A/C doesn’t blow cold at lower speeds or in traffic. It didn’t do this when it was new say 200,000 miles. I checked the coolant charge at it remains fine no leaks, I think at at some point my mechanic did a conversion to the new refrigerant R something. What can I do to keep the A/C blowing cold at lower rpms? I have to add that it will also start blowing warm while driving down the interstate as well.

Make sure the blend door inside the HVAC system is functioning correctly. This is the door that controls air temperature by directing air through either the AC evaporator or the heater core.

If the door is moving when it shouldn’t you won’t get the correct air temperature. If your system is vacuum operated I’d check for leaks or a bad actuator.

Check both your condenser (radiator looking thing in front of the radiator) and the operation of your cooling fan. If the condenser fins get too clogged up with gunk and mashed it can’t cool as efficiently. This will obviously be much worse if the fan isn’t running when it should (and it should when the AC is on).

Of course, I also have to wonder how you checked the refrigerant level. The only way to really check it is with a full manifold gauge set.

I like the idea of the blend door, i’m not not sure how to check.

This truck was made with R-134a. No need for a conversion. The blend door is under the dash before the heater core. Disassembling part of the dash would be required. A simple check to see if the blend door is stuck or bad is to check the temperature of the air out of the vents with the A/C off. If the air out of the vents is notably higher than the ambient air temps, the blend door may be an issue.

I’d check the condenser coil like cigroller mentioned. This is cheap and easy to do. A good spray from a garden hose can clear out a bunch of debris if it is blocking the coils. Also, the cooling fan has a fan clutch that may not be engaging the fan properly, preventing the fan from pulling a decent amount of air through the radiator and condenser coil at low driving speeds.

What needs to be know is the defintion of “fine” in regards to the refrigerant charge.
How is being determined, what are high and low pressures, at what RPM, etc, etc.

You’re referring to refrigerant as coolant and this generally means that someone has little to no expertise with A/C systems. That’s why I ask.

Found the problem. Turns out that when the magnetic clutch got hot (to what degree I cannot say) it would release, thus it would not turn the compressor.