A/C Compressor

I have a 1990 Caravan, almost 200K but still running good. The A/C works fine as long as the outside temperature is in the mild range, but when you really need it - the 90s and above, turning on the A/C produces a loud screeching noise (no doubt belt slipping). Belt has been replaced and appears to be tight enough. Suspect the compressor is seizing up. Correct?

If the compressor was seizing up, you would know it. The compressor’s pulley would stop turning and the belt would make a very loud noise.

You can’t make any assumptions at this point. You should take it in for a professional diagnosis.

The compressor doesn’t care what the air temperature is. When you turn on the AC, the compressor cycles on and off to maintain the correct pressure within the system. If it’s not seizing in mild temperatures it’s not seizing at warmer temperatures, either.

Are you sure the belt is tight enough? Have you observed the belt as someone switches on the AC to make sure that’s where the noise is coming from?

It’s my opinion that all AC problems should be taken to a shop with the necessary equipment and expertise to diagnose and correct the problem.

I believe the compressor DOES care what the air temperature is. Higher air temp means higher condenser temp and thus higher compressor outlet pressure. The compressor works harder at higher air temperature.

This effect from higher ambient temp would not be noticed at inital compressor engagement as is the case with the OP. The public does not see the value of posting high and low side pressures when their problems occur, most likely too technical.

As the ambient temperature increases, the static pressure within the AC system increases. This is why one must make ambient temperature corrections when reading AC pressures. So during mild temperatures, the static reading might be 50 PSI. But during hotter ambient temperatures the reading could be 70 PSI. This isn’t enough to prevent the compressor from rotating. But this increase in pressure could be enough to cause the compressor clutch to slip when the AC is engaged. So I wouldn’t suspect the compressor, but instead the compressor clutch.