alve, check vacum line first (on heater valve)
In many cases leaves can cause smoke to blow through the defrost of a 1990 Ford F 150
If it's the front passenger side ,I would say your heater core is leaking .
ihave a 1986 chevette that when i turn defrost on it blows out heat and when i put heater on it blows out only cold air? ihave a 1986 chevette that when i turn defrost on it blows out heat and when i put heater on it blows out only cold air?
Blown Heater Core - The heater fan blows past the heater core and if it has a hole in it your antifreeze ios being blown onto your windshield.
The compressor is coming on because you have the heater set to defrost. Take it off defrost and the compressor will not come on. I would suspect the compressor is locked up.
Heater core leaking.
If we are talking rain water not engine coolant I would suspect the drains under the cowl are plugged up with debris (leaves and such). If it is engine coolant then I would suspect the heater core is leaking.
No...
I don't know why the vent would not open for the defrost, but it may be related to the fact that your heater core is blown. If your getting nothing but cold air out of the vents when the heat is on, your core is finished and if the core is finished then it is likely related to your vent not opening/switching. Something''s wrong with your heating either way.
The front expansion valve is clogged.
The most common cause for the defrost not properly working is a malfunctioning blending door. The blending door might be stuck halfway open. This would cause the defrost not to work unless the fan is on high.
You could always take it to the shop You could always take it to the shop