defrost and vents trouble

[88workcar]

01 truck, the selector switch does work, I can move the air from defrost to face vents to floor etc.… it will swap around. BUT... the defrost never fully closes off and fogs the windshield. Have to put the wipers on every morning just for this. What's the fix?

 

[franklin2]

Why is the defrost fogging the windshield? Even if it did leak it should not do that. Sounds like your heater core has a small leak. They can have a small leak and create steam, and you won't see anything on the floorboard.

 

[88workcar]

That is fully disconnected at this time, for the last 4 months

 

[88workcar]

It is 90* at 5am here and 600% humidity, with the AC blowing to the windshield it condensates.

 

[Curious Hound]

It is 90* at 5am here and 600% humidity, with the AC blowing to the windshield it condensates.

I know that routine. Had to run a little heat this morning to get rid of condensation on the outside if the windshield.

Sounds like you have a problem with the damper actuator - sometimes called the blend door actuator.

 

[cbxer55]

The blend doors are known to break, resulting in exactly that. Expensive fix, or el cheapo fix. See link below.

https://www.therangerstation.com/tech/broken-heater-air-conditioning-blend-door/

 

[RonD]

After the fog has cleared try this test
At idle
Select Panel or Floor
Feel if any air is coming from Defrost vent, it shouldn't
REV engine while feeling for air at Defrost vent
If you get some air flow as Vacuum drops in engine(REVing) then you have a leak in the Vent Vacuum system

This usually occurs on the vacuum line that runs from firewall to Vacuum Reservoir "ball" in engine bay
Its a black hard plastic vacuum line and it can crack or melt, near exhaust pipes

Could also be the Reservoir is cracked and leaking


The Vent doors use vacuum "motors" to hold vents in selected position, Defrost is the Default position, the No Vacuum position
The Engine is the vacuum source, vacuum is high at idle, low while accelerating(REVing)
The vacuum reservoir is used to store vacuum, like power brake booster, to hold vent doors in position regardless of engine vacuum
If there is a leak then vacuum level at the vents follow vacuum in the engine, so if vacuum in the engine drops Defrost vent gets air flow because vacuum is low

The Blend door is the temperature control door, it is electric from 1995 and up, not vacuum operated

 

[cbxer55]

But it still breaks. Mine is a 98, and all I get is hot air, except in the Max Air position. Breaks between the motor and the door. I've not bothered to fix it because it doesn't bother me. I drive with the windows down anyways, except in the winter, when I want hot air, especially on the windshield.

 

[RonD]

But it still breaks. Mine is a 98, and all I get is hot air, except in the Max Air position. Breaks between the motor and the door. I've not bothered to fix it because it doesn't bother me. I drive with the windows down anyways, except in the winter, when I want hot air, especially on the windshield.

This would be a blend door issue for sure
In MAX AC the hot coolant flow is diverted around the heater core by the by-pass valve in the engine bay, it has a Grey plastic vacuum line that runs thru firewall to the selector switch

In MAX AC vacuum is applied to the by-pass valve and coolant flow thru the cabs heater core is stopped
(in MAX AC the fresh air vent is also closed and air for the fan is pulled in from the cab, under glove box)

The electric Blend door diverts air flow thru the heater core or around the heater core or anywhere in between, that's what the TEMP knob operates, yours is stuck diverting air thru the heater core

 

[88workcar]

Now we talkin LOL, thanks fellas, I'll get on it

 

[sgtsandman]

I think it's a fault in the system design. My 2019 does it too when the humidity is high.

 

[RonD]

The AC unit is on all the time to "dry" the air, but when you first start up the vehicle any residual moisture left in the evaporator housing will get blown into the cab vents
And until the evaporator gets cold enough to condense any moisture in the incoming air it will pass thru into the cab and condense on surfaces, like windshield

Check the evaporator drain to make sure the condensed water can drain out easily, these can be clogged up easily and its just a gravity drain so there is no pressure to push the water out of the evaporator housing

 

[Dirtman]

Leave an open can of clam chowdah under the heater core. The chowdah will absorb the excess moisture to keep your windshield from fogging. Clams are extremely hygroscopic.