That question has been asked here quite a few times. If the system is blowing cold under variable conditions (slow and high speed, city or highway), that behavior is fine. The important thing is output temperature.

In your area, the weather hasn't been that hot. This affects cycle time greatly. If the compressor cycles frequently in high temperatures, and the vent output increases significantly in temperature, it is time for some A/C work.

A/C service in WI is expensive because of the additional state regulations (on top of federal regs). With an R134a system, refrigerant measurement is crucial. Either that, or you have to measure pressures, and the 850 doesn't have a high side port to test, that I've seen. That means you have to measure the refrigerant that goes into it. Which means, just topping off the system is no longer a possibility--R134a is too sensitive to pressures.

Monitor the output temp with a good vent temp gauge that displays degrees F. That will help you to determine whether or not there is an actual charge level problem, or if the system is in fact operating properly.
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Chris Herbst, near Chicago.