I have a 2010 GMC Acadia with 160,000 miles.  Owned since new.  No prior A/C work has been done.  Blowing only hot air, the compressor clutch would not engage.  Purchased a can of DIY R-134a.  Connected the hose that came with it to the low side of the system.  The gauge read “low”.  Started the engine, A/C on high, and began slowly filling the system.  Stopped every 10 seconds or so to check the gauge.  After just getting into the “filled” or green section on the gauge it jumped to the red section, which indicates that I had possibly over pressurized the system.  Borrowed a set of Manifold Gauges from a friend.  85-degree day.  The low side reads 80, high side 140.  Compressor clutch engages intermittently: on for a couple of seconds, off for a couple of seconds repeatedly.  A/C in the vehicle was cool for a few moments, then back to hot air.  When the clutch engages, the low side pressure drops to the mid 50’s, then works its way back up to 80.  Any help is appreciated.