* Low-pressure switch: The AC system needs a certain pressure to function correctly. When the car is moving, the alternator generates more power, helping the compressor work harder and maintain pressure. When idling, the alternator output is lower, and if the pressure drops too low, the low-pressure switch cuts power to the compressor to prevent damage. This results in warm air when idling or stopped.
* AC clutch: The AC clutch engages the compressor. If the clutch is failing, it might not engage consistently at lower engine speeds (idling). This would lead to the compressor not working properly when the car is stopped, resulting in warm air. A weak or failing clutch may still partially engage, leading to slightly cooler-than-ambient air, but not true cold air.
* Other possibilities (less likely given the symptoms):
* Refrigerant leak: While possible, a refrigerant leak would likely result in warm air even while driving, especially if the leak is significant.
* Compressor failure: A completely failed compressor would produce no cold air at all, regardless of speed.
* Blocked condenser: A blocked condenser would reduce cooling efficiency at all speeds.
What to do:
1. Check the refrigerant level: This is the first step. A low refrigerant level can trigger the low-pressure switch. A qualified mechanic can easily check this. Do not attempt this yourself; AC refrigerant is under high pressure and can cause serious injury.
2. Have a mechanic inspect the AC clutch: They can test the clutch's engagement and determine if it's functioning correctly. This often requires specialized tools.
3. Inspect the low-pressure switch: A mechanic can test the switch to ensure it's operating within the correct pressure range.
In short, take your Pontiac Montana to a qualified mechanic specializing in automotive air conditioning. They will be able to diagnose the problem accurately and provide the necessary repair. Focusing on the low-pressure switch and AC clutch will likely solve the issue based on your description.
