1999 siverado 1500. heater fan works on all five speeds but i have low air flow.having trobble keeping windows defrosted. could i be a blendgate.

Do you have heat? Does the fan turn on? are you getting air from the defroster vents close to the windshield? If you have heat make sure the defrost control is working. I'd open the outside air vent and turn the blower on high speed. If there is no heat, the heater control valve is not working. Double check the radiator fluid tank, make sure you have enough fluid (antifreeze) Low fluid levels can reduce the hot water flow to the heater coil inside the car. If there
is no air flow through the defroster vents you may have a problem with the fan control, or the fan motor. Good luck.

check the actuator motor on the heater sounds like the motor is not directing the air as it should. The motor is in the center of the truck inside attached to the heater and has arms on it to move the heater and ac doors.

direction flap in the heater

Sounds to me that you might have a leak in the line. Those heaters are all Vacume actuated.

Replace the fan resistance.

All cars are designed with a blower fan motor that pushes air through the ventilation system. This air is processed by a series of vents to direct the air to various positions. Example: heater, air conditioner and defrost modes. Most blower motors are power by a basic electrical circuit that utilizes a multi resistor or a solid-state resistor to control different fan speed modes (low, medium low, medium and high). If the resistor malfunctions it may lose one or more of the lower fan speed settings.
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it sounds like you have vacuum leak,

low 134a coolant, you may have a pin hole leak, or just need topped up on 134a. wallmart or most auto parts stores sell 134a kits. also check your rad coolant.

Your heating system is run by your cooling system. If your cooliant is low then you are not going to get adiquiate heat disapation to your heater core. So check the entire cooling systerm. Your water pump (low flow), hoses (leakage), you already checked the thermostat & heater core. So other than that I don't know.

i just went thru this problem with my 95 1500. on mine it was the controls just like you discribe on yours one for fan one for temp and one for settings.first i thought it was the heater blend door acturator. its located on left side of heater next to the gas peddel. pulled the wire plug out and test for power theres 3 wires to this plug one white one brown and one black. the brown wire was hot with key on the black is ground and the white is the one that sends a siginal to this electric motor witch redirects air flow from floor to dash vents to defrost. i had to order the controls thru autozone 135.00 with shipping. you might get lucky and find one at a bone yard the ones in my area dident have any. i almost bet you will find thats your problem too. good luck OLD SCHOOL.