I just got a 95 silverado with a 6.5 turbo motor. The a/c didn't work so I've been working on that. Got it so it blows cold now but there is another problem. When I turn on the a/c the vacuum pump does not turn. I have changed the belt and belt tensioner. Could it be just my pump is going out? I noticed when I was driving it that the check engine light comes on and will go off some times. Is there a way to chek the pump?