A car's thermal expansion valve detects the pressure and the temperature of the refrigerant gas, when it is in liquid form. The amount of pressure detected on the thermal expansion valve determines the amount of liquid that is let through the system. This directly translates into how efficiently the air conditioning unit of your vehicle is working. When the thermal expansion valve is not working correctly, you can replace it yourself.
Step 1 - Become Familiar With Air Conditioner Unit
The easiest way to become familiar with the air conditioning unit in your particular vehicle, is to read through a current repair manual. This will help you be able to pick out the particular parts, and know where the thermal expansion valve is in your particular vehicle.
Step 2 - Remove Refrigerant from Air Conditioner
Before you do any work on your air conditioning unit, you need to discharge the system and remove the refrigerant. A refrigerant recovery machine is needed to remove the fluids, while also recycling them back into the system when the repairs are completed. Hook up the gauges to the air conditioning lines and remove the refrigerant. This is best left to professional AC technicians.
Step 3 - Disconnect Lines from Dash
From the engine compartment you will notice two lines going into the firewall to the rear of the dash. Most likely, they will be held in place by two different bolts. Use a wrench to remove the bolts and disconnect these hoses.
Step 4 - Remove Glove Box to Access Evaporator
The air conditioner evaporator is installed behind the glove compartment in your car. You need to remove it, in order to get at the expansion valve. Open up the glove compartment and use a screwdriver to remove the screws in the corner. You should be able to slide the box out. If there are any lights in the compartment, pull the box out slowly, so you do not damage the wires. Remove the wiring harness once you have access to them.
Step 5 - Remove Blower Motor
Once the glove compartment is out of the way, remove the blower motor to access the evaporator. Locate the bolts that hold it in place, and remove with a socket wrench and extension or nut driver.
Step 6 - Remove Expansion Valve
The thermal expansion valve is installed onto the evaporator. With the blower motor out of the way, slide the evaporator out of its case. Remove the two bolts that hold the thermal expansion valve onto the evaporator, and take the valve right off.
Step 7 - Install and Replace
Pour some air conditioner oil onto the O rings of the new expansion valve, and set it onto the evaporator. Line up the tubes to the correct holes, and tighten the bolts to secure it in place. Replace the evaporator, blower motor, and the glove compartment. Connect the tubes back to the firewall and tighten the two bolts. Recharge the system, by recycling the old refrigerant back into the system. Recharge is best left to AC professional technician. Start the engine and run the AC, to flush the system and start the cooling process.