QuestionThe a/c system blows cold at start up but after a while blows warm at idle but blows cools while driving over 15 mph.
Answer You are correct in your statement. I am sorry if I did not explain properly.
A 03 Silverado HVAC programmer (dash unit) is able to identify a low state of charge by monitoring either a pressure transducer or the frequency of the low side cycling switch operating pressure. I do not recall the exact specifications but if the programmer sees too many cycles of the compressor in a short period of time it recognizes the fact the system is low on charge and disable the clutch.
This becomes a problem as the refrigeration oil cycles with the refrigerant. If the system does not have enough 134A it could and most likely would starve the compressor of lubrication causing catastrophic failure.
I would say of the 10 or so cars I touch a day with cooling system problems 90 percent of them are low on refrigerant. Not only are the late model systems smaller with less refrigerant (about 80 percent or less of a charge) the 134A is smaller and can easily escape slowly over the years.
The best way to find out how much is in the system is to recover it onto a scale and weight its mass. There is an AC tag on the accumulator (thermos looking thingy on the passengers firewall)that will show a full charge for single and dual units. (most of the single only vehicles likek yours will still show a dual unit spec)
At the 60 percent mark I find is when most vehicles start to perform sporadically. . If there is a way you can verify refrigerant level and it is full but still does not function I can help you from there.
Wayne
Have you verified the refrigerant level? This system sounds like it might be at 60 percent charge.
