High humidy also needs to be considered as it will result in lower AC efficiency.
So when I say you can call it "good" with a -44F to -45F drop, this is in 50% or less humidity and the OAT needs to be above 85F.
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After you get your AC system "waking up" and producing at least a -40F drop out your vents, compared to OAT, you then need to "fiddle" with the system... and keep track of what you put in it or take out of the system.
You may also want to add a 3oz can of Pag100 oil with the ice additive and let it run for 200 miles.
Then I would add 6oz of 134a and see if your dash AC temperature drops by -2F. Note: You wan to do do this at high idle so use your "smart wheel" to increase the RPM... by first turning on your cruise control... and then press the set button to raise the RPM.
Note: Cummins ISC/ISL engines should not be left idling for a long time at low idle so I hope you know this and how to use your smart wheel to raise the idle. And if you did not know this, now you do!
If you don't get a temperature drop after 15 minutes, then add another 6 oz of 134a, but not more than this.
I have found you want the recommended weight of 134a + 10%. And you loose track of how much 134a you put into your system; then you have to start over and evacuate your lines.
Note: I also think one of the reasons an RV cannot reach -44+ drop in temperature is because did not evacuate their old oil in the system. And FYI, most HVAC techs will not perform this step. Why? I which I knew. Maybe time is money and they know you will pay the bill when you get to anything better than a -40F drop.
However, there is a big difference in comfort between a -40F drop and an -44F or -45F drop! So don't let your HVAC tech off the hook if he tries to send you on your way with a big bill and only a -40F drop!
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