GM-GMC: 90 chev 1/2 ton 2wd heater and a/c, coolant level, air cond

Question
my 90 chev 1500 has very little heat.when i rev the engine the heat temp increases to good heat.the controls seem to be switching (floor to defog)just fine.is this heater vacuum controlled in any manner?Also the air cond.is fully charged but i have no cold air blowing.why does my heat temp.increase when i rev the engine?what can i do to remedy this?why does my air cond.not blow cold air yet the system is fully charged?thanks

Answer
Your problems may be related and they may also not be.

Most GM vehicles with factory air conditioning use vacuum to control the duct system.  Older GMs don't use a valve on the heater hose circuit, but it is possible that the newer GMs have a heater valve and it would likely be vacuum controlled.

If you have such a valve, it could be leaking.  Increasing engine speed will increase vacuum and the valve might open a bit more and produce more heat.

Check the coolant level in your cooling system.  Low coolant level can keep the heater from working correctly.  If the engine's thermostat has never been changed, that could also cause a low heat output.

If you are in a cold climate, your air conditioning compressor might not turn on during the winter.  Newer GMs do use the air conditioner to help with defrosting the windshield, but I cannot be certain about your vehicle.  There is also a safety pressure switch on the air conditioning system and it might be defective or disconnected and that would keep the compressor from working.