Most people are not aware that the Montero Sport sold in the US does not come with a heater control valve system installed, this means that the hot coolant is always flowing through the heater core, warming up the dash, vents, etc. This has an unintended side affect of increasing the air temp when the A/C is running. I discovered this when my heater core blew and I ran a bypass, suddenly my A/C was blowing super chilly air.

As promised in another thread, I'm posting my DIY touchy heater control valve system. It's not perfect but it helps keep the A/C ice cold during the summer months while still providing an easy method of activating the heater core for extra de-fogging capability for those humid mornings/nights here in Louisiana. Firstly, if you are going to attempt this, I highly suggest getting an electric solenoid valve for this, using vacuum power has it's drawbacks which I will get to later on. You will also need to have the intake plenum removed in order to have significantly easier access to the heater core inlet/outlet.

Here is my simple MS paint diagram:




And here is what it looks like installed under the hood. Before anyone mentions it, I know the ground cable is unnecessarily long, I made the entire harness inside my house at my solder station and transplanted it to the engine bay. This ground was intended to be on the left of the solenoid on the firewall but I wrote down the wrong measurement. I can probably also shorten the length of vacuum hose by teeing off one of the other lines but I wanted it to be independent from anything that is responsible for keeping the engine running. There was a capped vacuum port on the far right side of the plenum anyways, so it worked out. I also used an EGR solenoid from another Montero Sport for this system.




As I briefly mentioned earlier, there is a few drawbacks to using vacuum to control this setup.

(1): Spontaneous heater valve actuation. Sometimes it seems the pressure from the coolant is enough to force it's way past the plunger in the valve, however when routed backwards, the vacuum pressure is not strong enough to open the valve. (lesser of two evils situation)

(2): The vacuum solenoid has to be energized constantly during the "off" season. Since the EGR solenoid on our sports is a "normally open" valve, this means that the solenoid has to be constantly energized in order for the heater valve to be closed. If it were a normally closed solenoid, it would just be energized during the winter instead, perpetuating the issue.

(3): To turn off the heater, the engine must be turned off. This is due to the vacuum pressure being maintained in the short loop between the solenoid and heater valve as there is no way to bleed this vacuum until the engine is shut off.
The Solution: A second solenoid with a delayed relay or sufficient capacitor must be installed in-between the first solenoid and heater valve that is by design, a normally closed solenoid. Additionally a one-way diode should be installed on this circuit to bypass the capacitor for instant actuation when the heater is turned on. This would allow the system to hold vacuum until the 1st solenoid is shut off, at which point the second solenoid would release the vacuum. This setup insures that there is no vac leak to cause idle/running issues for any brief moment when the system is shut off.

TLDR: Just avoid a vacuum controlled system and shoot straight for an electric heater valve, just make sure it can withstand continuous usage, most of the generic devices are normally open or closed, meaning, one way or another, the valve will need to be continuously energized during one of the seasons. Make sure to use a relay for this system so you don't accidentally leave it on overnight and drain the battery!

 

UPDATE!

I replaced the vacuum solenoid with another one that seemed to be in better condition and now the system works well enough. It got rid of the spontaneous operation of the valve, still have to shut off the engine to relieve the vacuum so it will shut off however. Still beats having to open the hood every time you want to change the valve position!