The rear a/c on my 1996 gmc suburban is blowing hot air, the front blows cold. where is the heater control valve for the rear heater core? i have taken the panels off of the condenser and the ducts for the rear but i cant find the valve that controls the hot water to the back.

Could be the controller or a faulty blend door.
The blend doors direct air thru the heater core and evaporator along with the vents. The controller uses electric motors to change the direction of air flow.

To clarify - - if the truck has rear and heat, you have two heater cores and two evaporator cores. The evap cores carry the freon.
If the leak is in the evap core in the front, the price is probably right.

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 ?

possible heater hot water control valve. if equipped would be found on one of the heater hoses under the hood. also try cafeully pinching the heater hoses to stop water flow.

Hi John,
Your right with your diagnostics, you have a blend door problem. If your switching to cool air and it is still hot then you also have a heat exchange unit fault, this is the component that changes the water flow from hot to cool by shutting off. If it is not operating correctly this will give you constant hot air no matter where you temperature switch is positioned.
As for the warm air while the engine is cold, sounds like the blend door is still open to atmosphere tempreture and if it is hot outside will pull that hot air into the inside of the vehicle.
Have the heat exchange valve checked, and the only way the check blend door operation is to remove the wiring from the blend doors and have the circuit tested. If the circuit is ok then the blend doors will need to be replaced.
Hope this helps

You are going to need to use a multimeter and have a professional test the relay voltages. This should probably only take an hour, make sure they have the proper electrical schematic for your vehicle year make and model.

Find the two heater hoses going through the firewall to the heater core both need to be hot at operating temp. If both are hot the problem will be inside with the heater control. I only one heater hose is hot the
problem will be the heater core. I am assuming you have bled all the air out of the cooling system.

Sounds like there may be an air pocket in the system.
Try 'burping' the system. If this still doesn't fix the problem, check the temperature of the heater core hoses (in and out) The inlet should be fairly cool, the other should be piping hot. If they're both cold; the heater core has an internal blockage.

Next, replace the thermostat with a winter (180-195 F) derivative.
Bleed the system accordingly, and keep me posted!

its the blend door behind the glove box. I had the same problem

it has a thermal expansion valve on the rear evap core top