Noticed that sometimes my AC does not cool well at all on hot days. Checked AC High/Low pressures and they appear fine. Problem is most pronounced at low speeds and was able to confirm that my radiator fans are working at both high and low speeds. However, when I notice that the AC is not blowing cold, the high fan mode is not engaged, only the low fan mode. If the car is sitting in a drive through for a while the high fan mode does eventually kick on and the AC seems to work much better. Anyone know what triggers the high fans to begin operating?