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2000 Tahoe window fogging problems

rossbrrossbr Posts: 3
edited April 2014 in Chevrolet
This is my first winter with a 2000 Tahoe LS. The cab has a problem with steaming up the side and rear windows. It will steam up with only myself in the vehicle and the only way I can get it to clear is to drive around with the windows open. The heating system is basic, however it is all electronic. The dealer has said operating per spec 3 times and has told me not to bring it in for that problem again. I have protested.. Does anyone have a similar problem. I drive around looking at other tahoes on the road and they do not have steamed up windows. One thing I have noticed is that the windows clear up if I stop and get in and out of the vehicle frequently. They also stayed fairly clear when I have the service techs in the truck and we are switching heating modes. One thing that puzzles me are the two buttons for recirc/circ. When the system is set to cold and circ that air is cool but not as cold as outside air, when the system is set to cold and recirc it is cold like the outside air. With these AC assised defrost units will the AC run when the outside air temp is less than 32.. In defrost mode the circ/recirc is electronically forced to circ.

I am lost when it comes to servicing these new vehicles..


  • alcanalcan Posts: 2,550
    When recirc is chosen the system takes warmer and more humid air from the passenger compartment and recirculated it through the system. Hence the higher discharge temp. And the fogging on the windows. When set to circ it brings in fresh and less humid, although colder, air. Result is colder discharge temp but less humidity and window fogging.
  • rossbrrossbr Posts: 3
    This is opposite of what is happening.
    "One thing that puzzles me are the two buttons for recirc/circ. When the system is set to cold and circ that air is cool but not as cold as outside air, when the system is set to cold and recirc it is cold like the outside air."
  • spokanespokane Posts: 514
    You are correct that the A/C compressor is inactivated at a minimum temperature; probably about 39*F. Below that temperature, the absolute humidity is sufficiently low for the heating system alone to accomplish defrosting if everything is functioning properly. I presume your problem occurs above this temperature.

    It does seem that something is wrong with your damper controls. As you indicate, with the A/C off and heat mode set to "cold", "recirc" should not significantly change the cabin temp and the "outside air" selection should provide air at outside temperature. Yours seems to be backwards. If this damper is malfunctioning, it could also explain the poor defrosting performance of the side and rear windows. If you are actually recirculating much of this air due to a defective damper, rather than flowing it through the car during normal driving, the side and rear windows are are in a stagnant area and will be slow to defrost. Normally, in the "outside air" mode, the air flows through the entire cabin and out through vents which are located (in most cars) in the lower portion of the rear door pillars. I believe this also fits your symptoms of "clearing up" when the doors are opened and shut a few times.

    The Tahoe may differ in some detail but this damper operation can be checked on most vehicles by operating the fan at high speed and verifying that the fan noise in the cabin increases when "recirc" is selected. Also, when you put your ear to the cowl outside the car at the base of the windshield (with another person inside to operate the selector), the fan noise should increase distinctly and quickly when "outside air" is selected. If this is not the case, you can complain to the dealer specifically about this damper operation.

    While you're at it, you may want to check the basic operation of the ventilation system components. On a warm day, suggest that you verify the A/C is working by simply operating it with the heat off and verify that the air is cooled substantially. Similarly, be sure the heater will fully warm the air as it should. You can then check the other dampers by operating the blower at high speed and verifying that the flows to the footwell, dash vents, and defroster vents are good strong flows and are according to the selector setting. Also stop and look at the compressor to see that it is running while you are experiencing the problem -- some sort of problem in the selector switch could cause a problem even though the A/C mode works OK.

    Good luck. Please let us know what you find.
  • rossbrrossbr Posts: 3
    I have looked closer at the operation of the dampers specifically the outside air / recirculation. Is there only one damper which controls this function?

    I can see the outside air / recirculation damper behind the glove box. It appears to be functioning properly unless the engine is running. When the engine is started it operates ok for 40 seconds then moves to the recirculation setting regardless of the setting of the heater controls. This is definitely my problem. Any idea what is causing this?
  • alcanalcan Posts: 2,550
    Ask here. Be specific re year, symptoms, etc:

  • spokanespokane Posts: 514
    You have certainly identified the problem. All of the vehicles I have encountered have a single damper for the OUTSIDE / RECIRC choice and feel sure the Tahoe is no different.

    On most cars, presumably including yours, the dampers are operated by vacuum diaphragms, often called "vacuum motors". These devices are similar to the vacuum advance devices that you may recall attached to the distributor of older cars to help control ignition timing. Failures in the vacuum-based system will often cause a damper to slowly drift to the wrong setting. A few cars use small electric motors to position the dampers but a failure in an electric system is unlikely to cause the 40-second drift which you described.

    Since engine manifold vacuum is required to operate the damper system, it is not surprising that the system operates differently with the engine off or running.

    For one condition (I don't know if it's OUTSIDE or RECIRC on your vehicle) there is no vacuum and the damper is held in place by a spring. The spring may be an integral part of the vacuum motor. For the other condition, the selector opens a small valve which applies vacuum to the
    damper motor - and the vacuum pull on the diaphragm is strong enough to overcome the force of the spring.

    Since I don't know which position is "vacuum on" and which is "vacuum off" I can't describe the problem exactly. I do believe, however, that it almost surely has to be one of the following:
    *The vacuum valve (possibly called a vacuum switch) is defective or out of adjustment. It could be failing to open properly or failing to close properly.
    *The vacuum motor is leaking - replacement is required.
    *The spring for this damper is broken or loose. (The spring may or may not be inside the vacuum motor assembly.)
    *Vacuum tubing is defective or perhaps simply not connected properly.

    If the air flow selections for footwell - dashboard vents - defroster vents all work OK, this shows the other dampers are functioning. That shows the vacuum supply to the control panel is OK, further verifying that your problem is limited to the OUTSIDE / RECIRC damper.

    I suggest that you tell the dealer service dept specifically that the OUTSIDE / RECIRC damper is malfunctioning. Describe how the damper defaults after 40 seconds to RECIRC setting regardless of the control setting. This is a key piece of information and they should know to check the items mentioned above. If you don't get a favorable response, or if their explanation makes no sense in view of my remarks, you my want to go to another dealer. A mechanic who has any experience with these damper systems, even if not on a Tahoe, should be able to correct this problem with the description you now have. Replacement of the vacuum motor is not difficult on most vehicles; removal of the glove box may be required. If access to the valve is necessary, a trim panel is easily removed from the dashboard and the heater/A-C control assembly slides out for servicing. In either case, it's not much labor on most vehicles. The hard part is the diagnosis - which you have done!
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