What kind of ac refrigerant does a 1990 gmc sierra take?

Answer 1

Originally it would have had r12. It could have been swapped over to r134a. The high and low service ports will tell you what you have. Small threaded service ports are r12. Larger quick connect/disconnect service ports are r134a.

You should also be mindful that not all R12 to R134a conversions were done as a complete retrofit, and often a simpler method of replacing connectors, the orifice, and accumulator would be done, which would leave the original R12 schrader valves, in spite of it now being an R134a system. If in doubt, it never hurts to err on the side of caution and have a shop do a refrigerant purity test.