1998 Suburban A/C

My Son's 1998 Suburban has front and rear A/C. He said the back units work but the front blows hot air. What I would like to know is: does this system use a solenoid in the A/C coolant line or does it control
the hot air by closing a duct in the air system? Or could the orfice jet be bad in the front.
He also said he was charging the system and put 1 can of 134 in it and that is when this happened, Maybe he just needs more 134. I thought the front unit would work before the rear in this case.
Any thoughts would be appreciated.
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With the AC on, feel the tubes entering the front of dash. If they're cold, the problem is the temp door motor not working.
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