Sorry it took so long for an update. After I had the fan clutch replaced by Prevost, we took a trip to Kerrville and it was still running hot with the dash AC on. After limping into Kerrville, I found that the belt tensioning system was only at 20 psi. I had asked Prevost to double check that but I guess they forgot. I adjusted the regulator up to 50 psi hoping that would solve the problem. I was assuming that with only the fan, 20 psi was adequate but with the added load of the dash AC compressor it was allowing the belts to slip.

Leaving Kerrville, it was running hot even without the AC on. We stopped in Fredericksburg and checked the air pressure ... still 50 psi and noted that the fan was just idling. So locked the fan in place with the new style tabs. With the fan locked in, she never ran over 195 even with the AC on. You could watch the water jacket thermostat work between 180 something and 195.

Back home a little trouble shooting pointed to a failure of the main fan relay #99.

So, now with a new fan clutch, proper air pressure and some new relays, things appear to be running fine. I am in now in the camp of it runs cooler with the dash AC on. With the AC on and the fan in high, the temperature appears to be primarily controlled by the thermostat and never goes over 195. Without the AC, the temperature appears to be primarily controlled by the fan and you can see it go over 195, the high speed fan kick in and then back down to 195 when the fan goes back to low speed.

I will send a second post with pictures of the disassembly of the locked up fan clutch and my thoughts on failure.