Question about 1999 Chevrolet Silverado 1500

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Heating/AC mode door does not move2004 Silverado 1500

On a 2004 Silverado 1500 4x4 the air only flows out of the defrost and foot area simulataneously, regardless of the position of the distribution knob on the climate control panel. The blower works fine in all positions. Is there a simple way to check the mode door motor or the control panel wwithout a class 2 scan tool? Where is the mode door motor located?

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If it is a ATC automatic control temperature , you will have to find the code. if it is vacuum controlled you will have to check the vacuum hoses that control the mode door. maybe something fell into the defrost vents and it got stuck. the only solution for that is to try fishing out the object through the vents..

Posted on Nov 22, 2008

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98 carolla heater blows cold ,I flushed system replace t stat check flow switch its working. when defrost, heat lever in heat position air blows out ac vent.


The mode selector is bad. IF you look at the plenum (where the fan motor is ) you should see two levers one is hot cold the other is mode. Check them by moving the controls and see if they move.
If the mode lever moves then the door on the inside is broken or something could have fallen in the vents and is blocking it.

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If the air flow cannot be controlled from vent/defrost/floor you may have a blend door issue. This is an electric motor that operates a door in the heating system to control airflow. There are usually 3 motors controlling airflow direction,heat and recirculation. The Ac not blowing cold may also be another problem. If the AC is operational, the compressor will be engaged and the Ac low pressure line under the hood will bw cold. If this is all ok then a blend door issue should be suspected

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Common failure the mode door motor is broken there are 2 mode door motors on the passenger side 1 for fresh / recir. air and the other position of heat/cold on drivers side has 2 also blend air and defrost/ floor . these are fun to remove usually held in with 2 5.5mm screws.

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I have a 2007 dodge 2500 diesel and the heater will only heat up to a very light heat.


Mode Door 1.
a.This door regulates air flow to either the dash vents or defrost/floor. If you have air flowing only through the vents with no defrost or floor, this door is failing. From our experience, this door is usually the first domino to fail. Not sure why, just is.
<!--[if !supportLists]-->4.Mode Door 2.
a.When mode door 1 is set to divert air away from the vents and into the second stage of the system, door #2 chooses whether air goes to the defrost vents to the windshield or out through the floor vents in the passenger and driver's side footwells. Failure is indicated by an inability to choose between defrost and floor.
b.This door turns out to be the most difficult to replace. In order to replace the door, the dash panel has to be removed. It's still better than the dealer fix because you don't have to evacuate the AC system, drain the radiator, and remove the plenum box. But it is more work than the other fixes. When this door breaks, it falls in the down position, blocking the floor vents and diverting all air through the defrost vents. From a safety standpoint, this is good since you'd most likely rather have a clear windshield than warm feet. We suspect that a large percentage of customers will just leave this door unrepaired. You will have heat through the dash vents and to the windshield, just nothing to the floor. It's inconvenient, but you'll have to make the determination if warm feet are worth the extra work to fix the door. An extra pair of socks is easier.

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There is a blend door problem.

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prev.gif next.gif HEATING AND AIR CONDITIONING Refer to Section 1 for discharging and charging of the air conditioning system. The heating system provides heating, ventilation and defrosting for the windshield and side windows. The heater core is a heat exchanger supplied with coolant from the engine cooling system. Temperature is controlled by the temperature valve which moves an air door that directs air flow through the heater core for more heat or bypasses the heater core for less heat. Vacuum actuators control the mode doors which direct air flow to the outlet ducts. The mode selector on the control panel directs engine vacuum to the actuators. The position of the mode doors determines whether air flows from the floor, panel, defrost or panel and defrost ducts (bi-level mode).

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Chanses are its either the blend door motor quit working or the controls ( temp., selector, blower) in dash are bad, my 95 did the same thing i chg. the blend door motor thinking it was that, but it turned out to be the controls in the dash, 5 minutes work and 140.00 later it was fixed.

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Is the correct name for the part that controls the air flow from a/c to defrost called the A/C Heating Servo Unit, the defrost does not completely defrost the front windshield but the a/c works just f


Well it would be the "mode door GM pt#88986538" or the "mode control cam actuator GM pt#89023358" on the side of the heater case. The door would control where the air comes out and the actuator tells the door where the setting is at on the control head.

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There are what they call mode door's these days they have a electrical motor that turnes the door to different posistions...sounds like yours is become broken. It would depend on your expertise in the ac & heating system area to replace the part. Sometimes easy and most of the times the dash has to be removed.

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