I have a 99 Yukon and the a/c is not working. The ac clutch is not engaging. I jumped out the high and low pressure switches 1 at a time and found it to work when the low pressure is bypassed. So I went and bought a new switch figuring it was bad and the same thing happens with the new on. I pick up a second one thinking maybe it was defective but same thing. What could be the issue? Could it be low on Freon? I do not have gauges to check. It is about to get hot so want to get this fixed.