If I'm looking at this right, the heater temp control cable opens a flap to allow hot air into the cab. This means that there is always water going through the heater core, correct? So this leads to my question: What are the pros and cons of this type of system compared to having a heater control that blocks water to the heater core? I found a real easy water valve to operate. Believe it or not, it came off of a 72 AMC Gremlin.

 

Heater only, or with A/C? Year?
Factory A/C does use the temp cable to operate a blend door to direct air either thru or around the heater core, but there is also supposed to be a vacuum operated valve in the heater hose to open/close the water flowing to the heater core. These are prone to sticking in one position, or leaking, and were frequently removed from the system rather than replaced.
Trucks with no A/C typically have a cable operated heater valve. But, around 77 or 78, there was a "Hi/Lo" ventilation system offered that used the factory A/C system, without the A/C portion.

 

1975 w/AC. AC has been deleted, but the rest is still intact

 

There should have been a vacuum operated heater control valve in one of the hoses near the heater box. If that has been deleted, then adding some sort of valve will help keep down the heat inside the cab during the summer.
With the rest of the A/C deleted, it's quite possible the factory valve wouldn't operate as it should, if you were to put on the *correct* valve. They are cheap, though. (price wise, quality depends on where it was made)

 

Thanks for the info...here is my 77 Ford with the water controll. Since I never investigated this, I am now trying to locate which valve on the cab control switch controlls this switch. I'm getting too much hot air in the cab and have been considering removing the switch but now a new one might be in order. Check out the photos https://picasaweb.google.com/1082152...eat=directlink

 

This should help... From the factory manual. Click on the thumbnail then again on the image for larger size.

http://postimage.org/gallery/u6vnoks/c9133d1e/

 

Thanks for the help!