My rear ac is blowing hot air and the front is cold in my 1998 GMC Savana Explorer Conversion Van

If you have owners manual check that your climate control settings are as you expect them to be, if that makes sense. Some on Fixya do not realise they gave climate control , they think they have normal A/C and try to correct chimerical problems. Please check and repost

I have a 2000 GMC Savana Van SLT with front and rear AC/Heat. My front would blow hot while the rear blew cold with AC on. It turned out to be low on Freon. It's hard to know if you have the right amount of freon installed because pressures vary depending on the ambient temperatures. So the best way is to remove all the freon then reinstall the recommended amount as listed on the evaporator housing near the firewall. This worked fine for me... works like new after installing the correct amount of freon.

Generally you would need to check the level of freon in the system, and probably the expansion valve in the rear unit. You should have one hot and one cold ac line going to each evaporator.

Could be low on freon, a faulty expansion valve on the rear system, or other issues. Can you tell if the rear ac lines are hot and cold ?

Could be a blockage in the lines to the rear unit.
Might be low on freon too. Or a blend door problem.
Can you check under the floor and see if one of the lines is cold ?

My rear a/c blows but the passager side vents don't blow can you tell what is wrong with it.

Once again, it could be the temperature doors not operating correctly on the rear hvac module.

My first guess would be the low side pressure switch, If you have checked all fuses?

If the front is still blowing air but not air conditioned air than it is likely that a flap that controls airflow to the front has failed. The rear would still work as it is controlled by an auxiliary control.

If there is no air coming out of the front at all I would suspect a blower motor as the rear has it's own blower.

Either way a scan tool that is able to access the HVAC system is going to be the only to accurately diagnose this problem.

Typical symptoms of being a little low on refrigerant.