How does the temperature **** effect the air temp? If there is always coolant flowing, except in the presence of a closed heater valve, how does it regulate temperature? Never really gave it thought until I installed the valve.

If I turn the heat all the way up in max a/c its cool but not cold. If I turn it to cold it goes back to being cold.

 

There is a blend door that seperate hot and cold.

 

I found this diagram which explains it well

Item Part Number Description
1 18A318 Vacuum Control Motor — Panel/Defrost Door
2 18A478 Panel/Defrost Door (Full Vacuum Position)
3 — Defrost Air Flow
4 18B545 Temperature Blend Door (Full Heat Position)
5 19860 A/C Evaporator Core
6 19A813 Air Inlet Duct Door (Full Vacuum Position)
7 — Outside Air Inlet
8 18A318 Vacuum Control Motor — Air Inlet Duct Door
9 — Recirculated Air Inlet
10 19805 Blower Motor
11 18476 Heater Core
12 18A559 Floor/Panel Door (Full Vacuum Position)
13 — Floor Air Flow
14 — Vacuum Source
15 19A566 A/C Vacuum Reservoir Tank and Bracket
16 19B888 A/C-Heater Function Selector Switch
17 18A318 Vacuum Control Motor — Floor/Panel Door
18 — Panel Vent Air Flow

 

The coolant valve is supposed to shut off hot coolant flow when in the MAX A/C position if controlled properly.

The TEMP **** controls an electric motor which moves the blend door so that, based on the position of the door, airflow is directed either through the heater core to heat it or to bypass the heater core so that the core doesn't affect the air temp.

The control valve is added to prevent a 200° heat source from adding heat to the air, even when airflow is diverted, during peak cooling needs.

 

The valve is installed correctly... But the coolant in the heater core at the time of shut off is still very much hot until cold air passing by cools it.

After playing with the settings I verified everything was working correctly. I just didn't know the layout of the air ducts until I found that diagram.

 

Yep. And, since it (the core) isn't being replenished by more hot coolant, it should shed that heat rather quickly and then remain at ambient (for the plenum) for the duration.

 

I imagine this has shown up on this site previously but for those with '02 or '03 7.3's unfamiliar with the issue this is the control valve I installed in my '02 7.3 many years ago. It made for a very significant improvement in my AC function.

http://www.dieselsite.com/maxacassis...4-200373l.aspx

It works in the MAX AC setting only but I believe an additional upgrade is available that allows it to work in the other settings. The installation was easy and the result was significantly colder air.