1999 Volvo S70 heater and defroster blowing "fog"

Look inside the glove compartment for the vent control they are probably run by vacuum. See if they are leaking.

What make an model an year vehicle do you have ? The air blows out of the floor vents an center vents but not defrost ? Possible mode actuator , stuck mode door ! There is a door inside the heater / A/C assembly that directs the air to the desired position . These doors can be moved by a vacuum operated actuator or an electric motor . Sometimes thing's put on the dash end up inside the heater box , fall through defroster vent ! Pens , pencils etc.. Depends on the make vehicle an system , manual , or automatic climate control . Some of the newer vehicles the HVAC system as it is called , the control panel is a electronic control module an can store DTC'S - diagnostic trouble codes !

is it the button type or the knob type. knob type is a cable they can snap the plastic clips that hold them on. button type there is a rest sequence you need to do to reset the system but i cannot remember it off hand. think its hold down both switch ( buttons ) at same time then the display goes to two dashes and resets.

To troubleshoot this types of problem,click the link below :----http://howtobyme.blogspot.in/2012/02/heater-blows-only-from-defrost.html
--------Go through the procedure to confirm the issue.This will help.Thanks.

So you have the vent settings to blow out of the windshield vents and still have fogging?

You can dry out the air more quickly if you use the air conditioner setting. It sounds backwards, but this is exactly what the manual in my older car said to do. (Some cars are electronic control and won't let you do this however - or they do it automatically with the defrost vent settings).

For the older car, I would set the heat to the hottest setting and press in the a/c button. The a/c dries out the air, but it does not have to be set to run cold.

Another thing that could help is to make sure that you do not have the vents set for "recycling" the air. The picture should look like a u shapped arrow. Make sure it is not pressed in.

On most cars vacuum hoses from the engine power diaphragms that open and close the doors that change where the air blows in the heater system. Check for vacuum leaks. sometimes the diaphragms will go bad. It’s hard to say without actually digging into the system. There might even be something blocking the doors from moving that fell into your defrosters. But at least this gives you a few places to check

Hi! Volvo have some problems with compressor you must put in new distans chims so you get right distans between beltwheel and compressor.
Go to an AC work shop.
They could help you.
We have this problem on our S70.
BR
Tomas

Your Heater Core is Bad= Coolant on Pass Floor
If A/C Does NOT blow Cold anymore the Evaperator Core is Leaking
First time I have seen them BOTH go Bad in 30 rs of Bein A Mech/Tech

Sure sounds like the heater core, the only other thing would be some time of leak that is raising the moisture in the car is the floor board on the passengers side wet? If not look toward the heater core.

It is your heater core. I fixed mine for $144. Ordered the new part on Ebay and followed these instructions http://www.volvospeed.com/vs_forum/index.php?showtopic=9717 It was very simple, I'm a middle age grandmother! The hardest part was cleaning up all the antifreeze. You will need a shopvac, a small one is $20 at WalMart.