I have a 20002 GMC Sierra 2500 HD 6.6 L Diesel. When I try and turn the Air conditioning on the light blinks 3 times and turns off right away It occasionally will turn on if the the truck has been running for more than 1 hour. There have been days when its 90 degrees and it will not turn on. We have tried changing the controls inside, and that did not work. The controls inside, are computer climate control, with 3 dials, 1 for fan speed, 1 for temperature set and the other for the vent setting. What do you think it could be?