My 1996 Chevy s10 pickup is blowing cold air when the heat and defrost is on. I already checked for leakage and there is none. Any ideas on what else it might be?

Possible blend air door malfunction.

the heater vacuum motor and lines needs to be checked.Also get the blend door checked.

NO HEAT ON FLOOR VENTS

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As per your problem mentioned. There are few solved help links to troubleshoot these problems. Go through the list and Click the link below:----

2000 Toyota Avalon: No heat on Passenger Side?

http://whatisbyme.blogspot.in/2012/05/2000-toyota-avalon-no-heat-on-passenger.html

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No heat on Oldsmobile Silhouette?

http://technoanswers.blogspot.in/2012/02/no-heat-on-oldsmobile-silhouette.html

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Heater blows cold?

http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

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Heat blows cold on drivers side and hot on passenger side?

http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html

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Hot air on the driver's side vents only?

http://repairhelpcenter.blogspot.com/2012/01/hot-air-on-drivers-side-vents-only.html

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Temperature gauge does not move and no heat when heater is ON?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html

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1992 Toyota Camry: A/C Control light Flashing and A/C not Blowing Cold?

http://technoanswers.blogspot.in/2012/04/1992-toyota-camry-ac-control-light.html

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No heat, blower blows only cold?

http://technoanswers.blogspot.in/2012/02/no-heatblower-blows-only-cold.html

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Ford F150 Ac only blows through defrost vents?

http://technoanswers.blogspot.com/2012/01/forf-f150-ac-only-blows-through-defrost.html

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Dodge A/C come out of defrost vents only?

http://technoanswers.blogspot.com/2012/01/dodge-ac-come-out-of-defrost-vents-only.html

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Ac vacuum line on dodge cummins?

http://technoanswers.blogspot.com/2012/01/ac-vacuum-line-on-dodge-cummins.html

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Ford ranger AC blows cold through defrost vents instead on cabin vents?

http://technoanswers.blogspot.com/2012/01/ford-ranger-ac-blows-cold-through.html

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2005 Chrysler PT Cruiser: A/C Blows Warm?

http://whoisbyme.blogspot.in/2012/06/2005-chrysler-p8t-cruiser-ac-blows-warm.html

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2009 Mercury Grand Marquis: A/C blows Cold From Defrost Vent Only?

http://whoisbyme.blogspot.in/2012/06/2009-mercury-grand-marquis-ac-blows.html

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AC not blowing through Vents on 2007 Chevy Trailblazer?

http://schematicsdiagram.blogspot.in/2012/04/ac-not-blowing-through-vents-on-2007.html

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Passenger side temperature is warm air out of dash vents?

http://schematicsdiagram.blogspot.in/2012/04/passenger-side-temperature-is-warm-air.html

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These will help.

Thanks.

The plenum may be cracked, check for air blowing out where it should not, or the blender door is not opening, a cable sheath may have popped loose, or the heater core itself is clogged with leaf debris blocking air flow..

Sounds like ur cold start fuel injector went bad

Could be the control switch itself. Or possibly the actuator that moves the door from defrost to heater. With the heater on, try switching from defrost to heat and try to hear if you can hear anything in the dash operating.

If it is blowing hot air out of the defroster and not out of the heater vents then the door that directs the heat from the defroster isn't working properly it is blocking the heat from going to the heater vents. Also check the cold air door. It sounds like it isn't closing all the way to shut off the cold air and not letting the hot air pass.

First be sure that you do not have the controls set to defrost the windows since on just about all vehicles nowadays with the controls set that way the air conditioner will be turned on...even if it is not so warm outside. So...be sure that you do not have any of the controls set to defrost and that the heater is turned all the way on.

It sounds like though that the control for the vents is not operating properly. This is a combination of mechanical movements of the vent flaps and small "air motors" that operate the flaps. These air units are controlled by air vacuum taken from the engine compartment through tiny hoses. If the hoses get cracks, holes or come loose from their fittings then your vents will not move correctly so it is a matter of looking and tracing each one of the hoses to make sure they are fastened correctly, have not come loose and are not cracked or leaking. The problem can even be in the engine compartment where the hose(s) connect to the engine vacuum system...so it will take lots of looking carefully with flashlights etc....good luck.

You need to check your coolant level first. If it is low you have a coolant leak. No hot coolant=No heat

the temp door is located in a box behind the glovebox, the actuator is either under or above the box. if you remove the glove box you should be able to see it.

heater core or leak on cooling system