A/C Controls

Hi everyone! Its been quite some time but Im still tinkering with the truck when I find time. Ive switched jobs recently and considerably shortened my commute so Im planning on using the bull as a daily driver in the next couple weeks. However, since its going to hit 90F this week, Im sorting out my issue with the hvac controls. Right now its stuck in heat. There is a broken white plastic that the temperature selector clips onto behind the control panel. Is the temperature selector related to the heat/ac switch? Do I have a missing vacuum hose from the control module? I looked thru the documentation (which does a great job at explaining the refrigeration side of the ac system) but I guess I need a dummy guide as far as what Im missing behind the control panel to select AC.

Thanks,
Sam

Sam - What page of the documentation were you looking at?  On the one at Documentation/HVAC/HVAC Systems you can find factory shop manual sections that go into detail on how the systems work.  Have you looked there?

Hi Gary, yes I was looking at that page. Maybe its because I was on my phone, I will read through it on the desktop tonight. My question is about the parts labelled 19988 in this diagram. I can only locate 1 behind the controller in my truck but the tab that has broken has clips to seat two.

Yes, there are positions for two cables, but I believe there is only one cable used - the temp control cable.

Ralph, is that correct?

Maybe that isnt my problem then. I can change the temperature but its stuck on heat, even when I select AC.

It depends on the year, but on A/C models using one cable, this is correct, as the A/C side uses vacuum control actuators.

Sounds like the temperature blend door is broken. The infamous hinge breakage that happens to the door.

There is a member here who makes them if I remember correctly?

That would be the first thing I'd check. The temperature cable could also be slipping, removed or broken from inside the engine bay where it hooks to the heater-A/C box.

The vacuum controls are also a cause of failure in these trucks. However they can be fixed using O rings, and such, but I have never done the job, so do not know how easy that fix is.

Thank you both for the guidance so far. I have another question about the dash selector. When you are switching between heat and AC does it prevent you from going between hot and cold? Meaning can you put the slider to cold air while on a heat setting and visa versa? Im trying to retrace my steps to before it broke.

That's the Heat/Defrost door.

Best fix is the 3D printed doors:

https://forum.garysgaragemahal.com/Heater-Blend-Door-Alternate-Design-tp112193.html

But, those didn't exist when mine failed, so I used a modified piano hinge:

https://forum.garysgaragemahal.com/Lost-all-vent-controls-tp93943.html

(lots of links in there).

So just to clarify: I am getting heat at all times, even when I switch to AC. So far what Ive read about the blend door is that it directs the airflow to the vents, floor or defrost. Like I said before, I really dont understand all the basics of this system so I just want to make sure before I pull the dash and replace the blend door to fix this issue.

There is a cable that controls the mixture of heat/cold - my guess is that cable is broken.  There is a little "arm" near the heater box inside the engine compartment (me memory is fuzzy here, maybe it is on the inside when the dash is removed, I can't remember).   You should see the cable attached there.  Is it taut, and feel like it is attached to something (your A/C controls)?

You can try to move the hot/cold and see if the position of the arm on the heater box changes

ckuske wrote

There is a cable that controls the mixture of heat/cold - my guess is that cable is broken.  There is a little "arm" near the heater box inside the engine compartment (me memory is fuzzy here, maybe it is on the inside when the dash is removed, I can't remember).   You should see the cable attached there.  Is it taut, and feel like it is attached to something (your A/C controls)?

You can try to move the hot/cold and see if the position of the arm on the heater box changes

The temp cable or the temp door inside the heater/A/C box seems to be broke. I agree.

I did get the temp door and the "A/C register/floor/defrost door" mixed up.

It's the temp door that busts infamously on the 1997-2003/4 F-Series. not on the bullnose.

The clip that holds that cable in place is the one that is broken. I can manually pull the cable and change the intensity of the heat at the vents, but it wont go cool or cold, which is what makes me think the ac/heat selector is broken. Thats why I would like to know if the ac/heat selector is any way tied to the temperature slide, so I can narrow my focus hopefully.

I would be very happy if my blend door isnt the problem (yet), as it seems like such a pain to get to.


Broken clip for the temp slider

If your temp cable attaches to the broken part of the control panel, then you need a new control panel, and possibly the cable too.

That may fix your problems if the mechanisms isn't jammed in the heater box.  It can get jammed with debris. Leaves and junk falling into the cowl vent.

Im suspecting the same thing. Ive seen a couple control panels on ebay. What/where is the heater box?

It's outside in the engine compartment on the firewall on the passenger side.

The other end of the temp cable is on top of that box and actuates the temp blend door via a actuating rod.

The A/C and heater are connected through the firewall with half on the engine side and half behind the dash. Factory a/c models.

I can tell you even if the temp door dose work / move you will always have some warm air coming out the vents at least I do on my 81 F100 with factory AC.

IIRC there is a post to pipe in a water valve to the heater hoses going to the heater core.
If you have the right control, IIRC there are 3 different ones.
When put to vent or think max AC the vacuum water valve closes so hot water will not go to the core and you get cooler air on vent and really cold air on AC.

I forget the year controls that this trick works with but the one I have it only works on MAX AC.
Thing is when on MAX it uses more vacuum than my system can supply and the air blend door then goes to defrost.
On normal AC it will also go to defrost when under heavy throttle too.

I have replaced the check valve at the tank and it did not help.
I think the HVAC control leaks and because it is a PITA to get and replace and not that big a deal just yet I just live with it.
Dave ----

Thank you! I am going to do some more poking around thia weekend

I went through SOME of this last year.  I completely removed my AC because most of it was gone anyway, and then took off the plates in the cab for floor vents, and found the moulding for inside to fit.

On the vent door:  It is easy to access, BUT you have to remove the outer shell which can be a real PITA. Τhen you will have access to the vent door.  Τhe foam on that door degrades. Use a closed cell foam for the repair.  I never went the full route to remove mine and simply made the repair in situ, and made a new seal out of sheet metal to repair the rust in the original that divides the A/C side from the fresh air, painted, and wrapped that in closed cell foam.  That is all on my thread page.


As to the control. I have not worked on this for a long time. I have several modules. The vac on the black round unit wheel gets loose because the post breaks. I have a bit on this using a spring and a rather long screw. I do not know if it works yet as I found a totally new unit at Green sales. However, I do think this can be fabbed up un the future to provide the necessary vac control. Some of that is also in my thread.  Maybe it help.

Dave is right that some hot air will always sneak by. Replacing the foam is the only way to minimize that.

Thanks for the info! I will look through your thread to get a better idea especially when it comes to the door modifications.

I found a broken vacuum hose in the engine bay that I spliced back together to get my AC to engage finally. Its still not as cold as I remember but I do have the dash selector ordered so hopefully that will solve the temp problem. My AC compressor is cycling on and off constantly so maybe I have more problems. What a surprise. Thank you everyone for sending me in the right direction so far.