After Michael's post last year explaining the FUSO A/C problems being caused by not having a coolant flow shut off valve, I bought an electric shut off valve to install on my truck. However, I haven't gotten around to it yet. So, this weekend we headed out on a road trip and I took my non-contact thermometer because I was curious about tire temp. However as I was driving I realized I could get some real world data on the AC air temp at the vents since it was hot. Temps were in the mid-90's. On the interstate, driving 63mph, early in the day on the flat with no wind, vent temp was 66 degrees. Later in the day when it was hotter, we were driving into a headwind on long hills, vent temps got as high as 73 degrees. But here is the amazing part. When we pulled off the interstate and began driving around a small town looking for a restaurant, vent temps dropped quickly to 46 degrees and even as low as 42 degrees. On time when driving slow around town it got down to 39 degrees. I think this data verifies Michaels's analysis. Once the coolant flow increases at high rpm's the heat in the heater core is sucking the cold out of the AC system to a large degree. This also explains the impression I've had of my A/C system since I got the truck. Sometimes it works fine and other times it seems to loose its oomph. I think it all has to do with coolant flow. For instance, at 40mph in 5th gear, it can hold 58 degrees at the vents. I can't imagine anything else which would explain these huge variations in output air temperature. I will be installing the electric shut off valve in the near future so I'll be able to provide 'after' temperature readings.
By the way, my A/C on/off switch quit working. It will only turn off if I physically push it down in the 'off' position. I'll have to replace it. Can anyone confirm that the dash does not need to be removed to replace the switch but that I can pull the switch out of the hole its in? The manual appears to indicate this can be done but it's not entirely clear.
Edit: for the record, the AC switch does pull out--kind of. The switch slips into the tube but there are two 'spring loaded' plastic ears, one at 12 oclock and one at 6 o clock which hold the switch into the tube. Lucky(?) for me my switch broke in two as I pulled the knob off and the top portion came out with the knob. I could then see the ears. Unfortunately, I broke off the lower one while pushing it down to get the bottom of the switch out. I think it will work with only the top ear. I have no clear idea of how the switch could be removed intact with two ears in place. Perhaps there is a tool which slides around the switch to compress the ears and allow it to be pulled out. My switch had been dodgy since I bought the truck. I think the two halves of the switch had been in the process of separating for a while.