I use a criteria of my own choosing. The conditions are easy to set up and are very quantitative if done exactly.

I believe airconditioning to start at a 40deg drop from ambient at the center duct in high blow and at at least 50mph and at equilibrium (meaning it isn't getting any cooler if you drive farther).

If this is what you have then it will drop to at least a 35deg drop when not moving and much less at idle engine speed. If you are at equilibrium and you drop the blower speed the temp drop will increase (the temp will go lower).

Really good AC will drop 50degs under these conditions. of course if the day isn't hot enough the system will regulate (cycle or otherwise) before freezing so one probably can't see a 50 deg drop off ambient on a 85 deg day as it will cycle at 40deg duct temp or higher (most of the time).

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician