Need help diagnosing an AC problem. I just bought a '98 F150 SXT over the past weekend. What happens is upon startup, the AC works normal. After driving a short distance above 20 MPH it blows warm air. The compressor trys to spin and constantly spins/stops to the point where it smells as if it is burning up. I'm an auto technician (not for Ford though), and today I recharged the system full. This didn't solve anything. I hooked up AC gauges after a drive and it seems the low side is at the same pressure as the high side, ~120. I'm not very familiar with Fords AC design and need to know what I should look at from here.