Max A/C temperatrue control

[gw33gp]

In another thread on A/C, I posted a question about being able to control temperature while in the Max A/C position. I want to do this because it recirculates the cabin air instead of bringing in outside air. This would help keep dust, smoke, foul odors, etc. from entering the cab without having to deal with the coldest air your A/C will put out while in Max A/C.

I decided to make a new thread on it in case anyone else is interested in doing this. Several suggestions were made and helped to lead me in the right direction. I was hoping a modification to the HVAC switch would be possible to make this happen without having to add any shut off or rerouting valves to the HVAC vacuum system.

I got out my Ford repair manual and studied the vacuum and electrical routing system. According to that manual, all the vent flaps and coolant to the heater core are controlled by vacuum. It appears the blend door is controlled manually with a cable system. Electrical only controls the HVAC fan.

In the Max A/C position a vacuum line sends full vacuum to the recirculate/outside air inlet motor which puts it in the recirculate position. That same vacuum line sends full vacuum to the heater coolant control valve which shuts off the (hot) coolant to the heater core. There is a tee in the line to send vacuum to each of those vacuum motors. So, it looks like there is no way to modify the HVAC switch to do what I want.

It looks like there are two choices. One is disconnect the vacuum line going to the heater coolant control valve after the tee and plug that vacuum line to prevent a vacuum leak. This means there would always be coolant flowing through the heater core but the blend door could be used to bypass the heater core to provide outside air in to direct non-heated air through the vents. This is the way it works in all positions any way other than Max A/C. The other way is to install a shut off valve in the vacuum line going to the heater coolant control valve after the tee. That way you could leave it open if you want the Max A/C to work normally or close it if you want to be able to control the temperature in Max A/C. I don't know how accessible this vacuum line is and it is possible an extension to this line would be needed to put that shut off valve in an accessible location.

I am presuming the temperature control switch to the blend door is still functional. It is supposed to be manual and I don't see how putting the HVAC switch in Max A/C would have any affect on it. Hopefully, next weekend I will have some time to get in there and test a few things to see if my idea works. I will update the results.

 

[fred m]

Every Ford I've owned that was set up like this, where the heater control valve closes on MAX A/C, I've just unplugged the vacuum line from the heater control valve and capped it. I zip tie it up out of the way but don't remove it completely, just in case I should have to stop coolant flow to the heater core, such as if it were to start leaking. Works fine.

 

[franklin2]

Every Ford I've owned that was set up like this, where the heater control valve closes on MAX A/C, I've just unplugged the vacuum line from the heater control valve and capped it. I zip tie it up out of the way but don't remove it completely, just in case I should have to stop coolant flow to the heater core, such as if it were to start leaking. Works fine.

None of the larger Ford trucks from 1980-up to 1996 have a water valve at all. They flow hot water through the heater core at all times, and just block it off with a door. The hot tip those guys have is to get a water valve for a ranger, plumb it in to the water lines, and add a vacuum tee to the recirc damper over at the pass side hood hinge to turn the water off in max A/C mode. To compensate for any air leakage past the door during days when you need the A/C working at peak efficiency. I've got one installed on my 89 f250.

I have a 65 mustang and it also does not have a water valve for the heat. It definitely leaks air past the door, I have a regular spigot valve inline with the heater core on it. I have to get under the hood in the spring and manually shut the valve, and then open it back up in the fall.

 

[gw33gp]

I have couple vintage cars that could benefit from having a coolant shut off valve to the heater core but don't want to mess with originality. I had a 1973 FJ55 Landcruiser that had a manual shut off valve. I had to open the hood to get to it.

I did take look at the vacuum line system on my 2002 Ranger. Yes, I could disconnect the vacuum line that attaches to the vacuum motor for the coolant shut off valve that is under the alternator. I decided to make it more convenient to turn it on and off. I installed a valve in the vacuum line after the tee that goes to the vacuum motor for the inside air circulation flap. I disconnected the vacuum lines that go through the fire wall and made a loop with a vacuum line that extends to the valve that is more accessible under the dash. It works great. I can leave the valve open if I want the Max A/C to function normally or close it if I want to control temperature in Max A/C. I will try to get some pictures before I put the glove compartment back together.

It would probably be easier to just cut the vacuum line between the tee and the connection at the fire wall but I didn't want to cut that line in case for some reason I wanted to go back to original. I ended up having to do more work than I planned on. The grommet that holds the vacuum lines was not set in the firewall properly from the factory. I had to take off the right front wheel and remove the wheel well liner to get that installed properly. Once that was corrected, it was pretty easy to do the modification. Shopping for the right vacuum line and connector took a little time but it all worked out.

 

[RonD]

Sounds good

I would have done it a bit differently, if the goal was to cut off outside air in dusty conditions
I would have used an inline check valve on the re-circulation vent "motor", so it works normally when system vacuum is applied
The vacuum switch would be T'ed between source vacuum and vent "motor", so you could select "inside air only" in any climate control setting, so even in heater mode

So basically like the older systems that had a separate slider/switch for fresh or re-circulated air selection

But what you did does virtually the same thing

 

[gw33gp]

I got some photos. This first one is through the opening of the glove compartment. You can see the inside air recirculation flap that is closed and the vacuum motor that opens and closes it. You can also see the white vacuum lines that goes from the control switch (from the left) to the tee and from there to the air recirculation control motor in one direction and in the other direction on through the fire wall to the coolant shut off control motor. The two shiny vacuum lines are the ones I installed that loop to the valve that I also installed. The dusty black line that runs beside the white line to the left is the vacuum source line that goes though the firewall and to the HVAC control switch.

This next one shows the valve handle (yellow) that I installed to shut off the vacuum to the coolant shut off valve when I want to control the temperature when in Max A/C. You can also see the bottom of the inside air circulation control motor with the white vacuum line connecting to it. Again, you can see the shiny black vacuum lines that loop to the yellow handled shut off valve.

This last one is a close up of where I tied into the vacuum line at the fire wall connector that goes to the coolant shut off valve. The larger black tubing connector, behind the white firewall connector, is an elbow that slips onto the line that goes through the firewall and on down to the coolant shut off valve. The other shiny black line that is connected to the semi clear tube that ties into the white vacuum tube running up to the tee (HVAC control switch and recirculate motor). Again, both shiny black lines loop up to the yellow handled valve.

If any of this is not clear, let me know and I will try a better explanation.

 

[gw33gp]

Sounds good

I would have done it a bit differently, if the goal was to cut off outside air in dusty conditions
I would have used an inline check valve on the re-circulation vent "motor", so it works normally when system vacuum is applied
The vacuum switch would be T'ed between source vacuum and vent "motor", so you could select "inside air only" in any climate control setting, so even in heater mode

So basically like the older systems that had a separate slider/switch for fresh or re-circulated air selection

But what you did does virtually the same thing

Ron, I may be missing something about your approach, but when you apply vacuum to the recirculate motor, it also applies vacuum to the coolant shut off valve (shuts it off) and you loose control of the temperature again. You would still have to have a control valve on the vacuum line to the coolant control valve motor.

I was considering being able to shut off the coolant to the heater core in any HVAC position but it became complicated and I wanted to keep things as simple as possible.

 

[RonD]

That's what the in line check valve is for, like these: https://www.amazon.com/vacuum-check-valve/s?k=vacuum+check+valve

So you can apply vacuum to the fresh air vent without it sending vacuum backwards into the system, but vacuum from the system can still activate the vent

System vacuum line Max AC--------"T" for engine bay bypass----->>>check valve>>>-------new "T"------>>>vent motor
Added vacuum source line------------------vacuum switch--------------------------------------->>>/

So basically just a fresh air/re-cirulated air switch, but system works exactly the same


So you still would NOT have temp control in MAX AC, but you wouldn't need it, because you could turn off fresh air in any other selection, including regular AC

 

[gw33gp]

Okay, I understand your approach. You feel controlling the recirculation motor is the best way. If I were go to the extra work required to do this, I would do it to control the heater coolant shut off valve. Your method could also be used to control the heater coolant shut off valve.

Some people may be able to benefit from being able to recirculate air in the cabin in any HVAC setting but for me it would be useful only in the A/C setting to keep dust out. In my opinion being able to control the heater coolant shut off valve would have greater value.

I am pretty sure that having hot coolant flowing through the heater core has an affect on the fresh air coming out of the vents in the HVAC system. Even if the blend door seals perfectly, the heat soak affect of the heater core on the surrounding can raise the air temperature of the air flowing through it. It may only be a degree or two when the air is flowing through but that changes when air stops flowing through it. I have always been annoyed with the blast of hot air in my face, with the HVAC control in the dash vent position, when restarting my Ranger after it has been sitting for short time with the engine up to temperature. It also takes a while for the air temperature to back drop down without the A/C on. I think if the heater coolant valve was shut off, the heat soak effect would be eliminated and this blast of hot air would not occur.

I am not going to go to the trouble to do this because I wanted a simple solution. I think what I did is the simplest approach and will settle for just being able to control the temperature when in Max A/C mode.