Ac shuts off when turn to high

Discussion in 'Chevy Tahoe Forum (GMC Yukon, Cadillac Escalade)' started by 10ecman, Aug 9, 2014.

  1. 10ecman

    10ecman New Member

    I just got a 99 Tahoe 4x4 the ac blower cuts off when switched to high. Also when hit a bump dome lights flash on. No experience with Tahoe's any help is appreciated.:money:
  2. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    The blower problem is probably the speed resistor assembly. However, some trucks have a fuse for speeds 1 thru 4 and a separate fuse for speed 5, check fuses.

    The dome light sounds light a door problem (the light usually comes on when a door is opened). The first door I would check is the rear door(s), it may not be completely closed.
  3. 10ecman

    10ecman New Member

    Thanks RayVoy. The light was the tailgate. The blower I have already replaced the resistor and the problem didn't go away. I looked in fuse block inside drivers door but all fuses are good none blown at least.
  4. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Do you have the manual system (you move sliders, or knobs for temp selection and you change fan speeds), or the automatic (set the temp and the system changes fan speeds)?

    The manual, on some trucks (not sure about yours), has a fuse for the #5 (high) fan speed and a separate fuse for the other fan speeds. If the fuses are good, maybe a wire is broken; if not, the speed assembly module probably needs to be changed
    The auto system uses just one fuse and has a transistor (a FET) the changes speed control. This type of speed control will work opposite to your problem, when the FET fails, the motor will only run on the high position (it will also run even if the key is turned off). The cure for this is a new speed control module, or replacing the FET if you know how to solder.
  5. 10ecman

    10ecman New Member

    I have the manual system I have replaced the resistor assembly but the fan rattles and the hi setting just cuts off when switched to hi. I have checked all the fuses by the door and can't find one blown so now I am in a quandary as to what to try next.
  6. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    With out an owner's manual, I don't know where the fuses are; but, there is a main fuse panel under the hood. I suspect, that is where you will find the blower fuses.

    If it isn't a fuse, this might be the problem. Is the fan so lose and floppy, that it could be jamming on high speed?
  7. 10ecman

    10ecman New Member

    I will check that out it rattles sometimes on the lower speeds but even under the hood all fuses appear to be good no blown arcs. It still works on low through 3/4 speed but hi it's like it is in the off position
  8. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Ok, here is a wiring diagram


    Your truck has a relay for the high position.

    Speeds low, med1 amd med2 use the resistor pack. The blower get power through a relay contact (SPDT), a relay that is not operated.

    Speed high, is supposed to operate the relay and the blower power goes the same contact set, only they are now operated. When the relay is operated, the power comes from a fuse in the underhood fuse panel. It looks like fuse #3, it is labeled BLOWER
  9. 10ecman

    10ecman New Member

    Checked that already it is good swapped for new fuse still no joy sooo.... I am going to replace the blower???
  10. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    It's none of my business, but that method of trouble shooting gets to be expensive. Your blower works on the lower speeds, the blower is probably not the problem.

    You need to check for voltage at the blower motor when the knob is in the high position. I can tell you right now, you do not have voltage at the blower when in the high position.

    You need to start at the blower and work your way back to 12 volts.
    - no voltage at the blower.
    - does the relay operate?
    - if yes, are the contacts good?
    - if no, is the speed selection switch good.

    If I'm wrong and you do have voltage at the blower motor, pull it out and test it with a 12 volt source.

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