AutoMagic Coolant Bypass
On a nice summer day, it is nice to have fresh air blowing in your face as you travel down the road. You probably have noticed that if you select "Vent" in your Super Duty, you get a blast of hot air instead. It really bugs me that I have to run the A/C just to get fresh air into the cab. I set out to fix this. Although this idea was original, I found I was not the first to do it. A side benefit is the getting the most of the A/C. You may have seen similar setups that shut the coolant off when "Max A/C" is selected. That does not accomplish my wants/desires. Neither does a manual valve.
Realizing that the Temperature Selector positioned a Blend Door, it seemed I could use the Blend Door position to operate a micro switch. This would shut off the coolant when the coolest position was selected. That would also allow blending of heated air when desired.
This system is setup to fail open. By tapping into the red vacuum line the system will only allow the coolant to be shut off in the Max A/C, A/C and Vent. It remains open in any of the settings on the right side of Off. Because this system fails open, the heater core is never shut off for any length of time. It will open with loss of electrical power or vacuum. Therefore it will open every time you shut down the truck.
Click on Pics to enlarge.
It is easier to work on this if you remove the front of the dash. Fairly easy to pull. Remove the radio, undo a couple screws and pull it out of the clips. You also need to drop the Glove Box Door. Squeeze the sides. Might remove it completely as well.
You may come up with something different... I mounted a micro switch on top of the blend door servo motor. The shaft that turns the blend door runs through to the top of the servo. It has a square center hole. I used a press fit square dowel in the shaft and mounted a plastic cam on that to operate the micro switch. There are two screws that hold the servo bracket to the HVAC housing. One is at the back and can't be seen. You have to use feel to remove it.
Servo on HVAC Unit:
I have the micro switch adjusted that it makes contact just before the blend door reaches max cold.
I tapped into a hot wire at the back of the heater fan switch. (you need to pull the dash to access) I don't recall the wire color. The same wire runs to the servo motor but that wire is of very light gage and I preferred not to use it. Once the Micro Switch is setup the rest is relatively simple.
Again you may come up with something different that a vacuum relay and vacuum bypass valve to shut off or bypass the coolant. Initially I looked for a 12 volt motorized valve.
I put a 'Tee' in the red vacuum line under the heater(behind the glove box)
I mounted the vacuum relay in the engine compartment on top of the heater box. The line from the tap runs through the firewall to the endmost port of the vacuum relay. A wire runs from the micro switch to the vacuum relay. The other electrical terminal of the relay is just to ground.
Vacuum Relay/Bypass Valve:
The bypass valve is simply cut into the heater lines. I like the 4 port valve as it allows the coolant to recirculate while the heater core is bypassed. A vacuum line runs from the second port of the vacuum relay to this bypass valve.
The coolant bypass is a '89 Ford Ranger available at most parts stores.
The micro switch is just generic from any electronic supply house.
The Vacuum Relay is problamatic. It is really a simple, inexpensive device, but finding one locally is like looking for hen's teeth. A couple of mail order places that have it are: