Defrosting issue - 1993 NAS 110

[the_eaze]

I picked up a 1993 NAS 110 about a year ago and have been working on it ever since. Recently had the HVAC system worked on and fixed as the AC wasn't working. Everything works well now except every time I drive the truck now the front windshield fogs up. Doesn't matter if its warm or cold out it fogs. I do notice that if I have the heat or air blowing out the front vents or at the feet there is small amount of air always flowing out of the defroster vents. Even when the system is completely off there is a small amount of airflow coming out of the defroster vents. I've tried every combination of hot/cold, ac on or off, recirculating or outside air....all of it creates fogging. Finally applied some anti-fog spray to no effect. When it happens I try putting on the heated windscreen and it doesn't seem to help. I have to carry a hand towel in the truck to wipe the screen as I drive.

I never had this issue with my 1997 NAS 90 which was my daily for 22 years but I know this system is totally different. My question is...is this normal for the 1993 NAS truck? Or is something still wrong with it? I do live in a place that is humid all year round (Monterey, CA).

 

[Grnrvrs]

Is the fog pure water or slimy like coolant? I had a small weap/leak in a heater core on a SIIA that fogged the window everytime the fan was on.

 

[the_eaze]

Its very small misty droplets. no discernable bigger droplets. interesting you say that because when I wipe it, it seems to not wipe off and dry like just plan water. Almost smears and a small amount stays on it.

Is it normal with that system to always have air flowing through the defroster vents no matter if the system is on or off?

 

[erover82]

Heater box seals are shot or the cable is maladjusted if you can't shut off airflow. The lower dash/heater duct always directs air to the demisters. The flap inside only shuts off air to the lower vents. Definitely sounds like a heater core leak since it smears and occurs under all conditions.

 

[Mirrajumper]

See if system will hold 15psi at the coolant bottle overnight.