Lately, my Defrost is worse On than none at all.

The windshield constantly fogs and even the side-glasses fog over so bad that I can't see to pull out.

When I got home last night, outside temperature in the upper thirties, I was playing around with the various switch settings and feeling of the Defrost vents when I noticed ice building up on the inside above the vents.

With the engine at normal operating temperature, 180°, and the selector on the hottest setting, the air coming through the vents was ice cold.

The truck has factory A/C with the original compressor that Ford put on there in 1985.

Playing around with it today, I discovered that, with the setting on Defrost or MIX, the A/C clutch engages; it dis-engages on Floor and Vent.

So, I un-plugged the wires at the A/C clutch to see if things may get a bit warmer and the windows might quit fogging over.

If this cures my problem, I intend to put a switch in that circuit.

I didn't used to have this fogging and icing problem; in fact, the heater would run you out of there.

Any ideas ? Thanks.

 

Is the floor wet under the passenger side carpet or rubber mat ?

Can you verify the blend door is moving when you select hot air ?

 

https://www.ford-trucks.com/forums/1...oor-hinge.html

That may help too.

 

Not always as mine was not wet. I dont think it was leaking enough to do that but would fog the inside glass at times but the big one was the smell of hot coolant.

To the OP when the controls are on defrost or mix next to defrost the AC will run.
That is to condense the moisture that is in the cab to keep the glass from fogging up. Most of the time it will be raining or snowing and why it would be wet in the cab.

Now I would say the fogging is a leaking heater core. Hot coolant gets into the air and goes right for glass LOL
I just changed out my heater core on a factory AC truck and was pretty easy.
Coming out 1 of the hoses did not want to come off and I did not want to cut it off as it was not that old and I had none on hand.

The biggest issue I had was the shape of cores as there are 2 different ones.
Well I think our trucks only come with 1 shape but the replacement is different.
I believe our truck have a rounded top and bottom but the replacement is square.
I was able to put the square in my HVAC box but I may buy a round one so it fits better.

Look for a post "square peg in round hole (heater core)" as it has the right part number and has pictures of the cores.
https://www.ford-trucks.com/forums/1...ater-core.html

Maybe it is this one that has pictures and more information on the right part number.
https://www.ford-trucks.com/forums/1...questions.html

As for no heat have you checked the level in the radiator?
I bet it is low where it will not push thru the heater core but still keep the motor cool.
You can feel both hoses to the core and see if they are hot if not then I would say the coolant is low.

Now after I changed the core I also had no heat inside but both hoses where hot that you could not hold for a long time?
I mashed the throttle to the floor a few times and it then got hot. I think the core was air locked and coolant was going in and right back out.
I have not used the truck since the change but I do today and it is cold so I will be using / testing the heat.
Dave ----

 

If your heater core is leaking, you should be able to smell the coolant, and your windshield will not only have fog on it, it will have a greasy slime from the coolant leak.

If it doesn't smell like coolant, then the carpet may be wet.

Like was said, the A/C is supposed to run when in defrost. If you can't get any hot air in defrost, then you may have a air door problem in the dash, though you should have that problem all the time, even in regular heat mode. The heater has enough output to overcome the A/C being on, like was said the A/C comes on to dry the air out quicker.

 

I did a little more looking and between the other posts and what I see at Oreilly Auto Parts the core that would match mine, 81 F100 with factory AC,
is listed as NON-AC high output heat?
But if you use the filters to say "AC" it lists the NON-AC high output one that matches and the one that I installed with a lot of force.
https://www.oreillyauto.com/shop/b/a..._10078%2F10271

I also took the truck out for the weekly trash run and the heat worked but nothing like the old leaking core.
Later I will hit the parts store and order the one that looks like it will fit. I also need to pickup oil change supplies as truck is due for a change.
Dave ----

ps: looking at Rock Auto I could not tell by pictures good enough to trust going with rock.

 

All are very good suspects; thanks.

I have experienced many leaking heater cores in various vehicles over the years; in this case, there are none of the usual signs of a leaking core.

I won't rule it completely out; however, the floor and carpet are dry as powder, the glass is clean, and I don't wipe anti-freeze off my lips as I drive.

Very good call and would be a good explanation of my problem; I have not yet investigated that but I will today.

From what I gather, the temperature control is cable-operated --- correct ? ; and, I should be able to actually see this connection and watch the movement without removing anything --- correct ?

I kinda hope this is where my problem lies as I think it would be an easier fix than most anything else.


If the blend door is functioning, my next move is to investigate whether there is circulation through the heater core; if there is blockage in the core, the hoses should not be getting warm --- correct ?

I do have a big ball-valve in the inlet line that closes off coolant flow during summer months and makes for super-cool A/C; I have considered the possibility that several months of no flow may be cause for the core to develop blockage.

 

https://www.garysgaragemahal.com/blend-door-repair.html

There will be other info you can get to from that site.

No sure you'd have blockage, if you have good coolant in the system. Maybe in the future, open the valve a couple of times during the non-use period.

 

Without removing that panel under the glove-box, I could not see anything; however, as stone-deaf as I am, I can easily hear the temperature door opening and closing and it slams shut very noticeably --- KER-THUNKKK!.

Coolant is plumb up in the neck of the radiator.

I will try to find time to run the engine today and feel the hoses.

One thing I do wonder about = consider this is a 6BT Cummins in this truck --- ever since the engine has been in the truck - 2006 - the heater hoses are routed above that stiff A/C condenser hose on the right side of the engine (the truck's right, not my right); this routing makes about a foot or so of those two hoses the highest point in the coolant system.

Anytime I have reason to drain and then replace the coolant, I have a "vent plug" at the ball cut-off valve that I remove to allow trapped air to escape; elsewise, it won't take half the coolant that was drained.

This vent opening is higher than the engine, but not as high as where those two hoses pass over that A/C line.

This leads me to wonder if possibly I have acquired an air-lock at the high point of those hoses.

I sort of dread removing the hoses and routing them below that A/C line, as they haven't been messed with in ages; I might should wait until I am sure that I have sufficient new hose at hand just in case; there are plenty of trucks around here that use that same size hose.

 

Smart move, waiting to have new hose in hand...and coolant, just in case.

 

What is the air temperature from the floor, dash, and defrost vents? When you cycle the lower control lever to the different positions, is airflow directed to the correct location(s)?

If the airflow is being routed correctly, my best guess is the air is not being heated properly. To check the heater core, set the hot/cold lever to the far right and the fan to medium. With the engine fully warmed up, measure the temperature of the two coolant lines at the heater. The supply hose should be close to the operating temperature of the cooling system (180ish?). The return line should be about 30F cooler. A proper temperature drop like that indicates heat is being transferred from the cooling system to the HVAC plenum.

If you don't see a drop across the heater core, next step is to figure out why. Poor coolant flow could be one culprit, such as the shutoff valve not fully open. If coolant flow is good, crud on the outside of the heater core could be restrict airflow and thus heat transfer.

 

No need to add such a switch. Fix the problem versus putting a band-aid on it.

The heater core supplies a massive volume of heat, essentially limitless for our purposes. The AC system, which removes moisture, can only supply a much smaller and finite volume of cooling. Mix the two together and what do you get? Air only slightly cooler but much drier than if just from the heater. Sure seems like the heating side of the equation is coming up short.


Did you catch this major clue in what you said? Past tense, you said the heater action was great. Not so much any more?

 

While I am waiting on this wet rainy period to pass so I can get back to my investigating, I have a question about the various "doors" in the duct-work.
To my understanding, there are three doors; one that diverts the air to the dash vents --- the one that the hinge breaks, one that channels the air up to the defrost vents or down toward the floor, and the cable-operated one that "blends" the Hot and Cold air and somewhat controls the temperature.

Am I correct in this three door business ?

Okay, I would call the cable-operated door the "blend door"; the door that breaks is NOT the blend door --- correct ?

My understanding is that the vacuum-operated door that breaks is an all or nothing affair; it is either open or closed --- right --- so it can't "blend" anything, right ?

Thanks.

 

It has been a while since I installed the new heater core and I can't completely remember; if I remove that plastic cover underneath the glove-box, can I see and if need be clean the core ?

It is possible that a mouse (or many mice) may have built a housing complex in there that is blocking air flow.

I know that on all of our "1st Gen" Dodge trucks, I have cut access windows with removable covers inside the cab just for such purposes and it was unreal how much trash I dug out of them on the first cleaning.

 

The A/C coil is the one that gets blocked most commonly. It sweats during use, and then dirt enters and it turns to mud.

Here is a diagram of the HVAC system under the dash.