Engine fan running

[Rockycoon75]

I have a 11 Pete with a 871. The engine fan is running constantly. Hooked up insite and it shows it's in ac mode. Threw my gauges on there and I only have 45 psi on the low side. The AC is working fine when I turn it on.
If I shut the truck off and turn the key back on the fan releases and free spins. As soon as I start it back up it goes back to locked in. Doesn't matter if the A.C. is off or on. I even tried pulling the A.C. relay. I thought maybe a switch at first but don't understand why it would only be bad with truck running.