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Thread: 94 gmc heater problems

  1. #1
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    94 gmc heater problems

    I have a 94 gmc sierra k1500 5.7l. The heater doesnt get hot. I found the ac is not charged and is empty. Could this be the problem or are the two systems completly seperate. The truck has sat for about two years. Thanks

  2. #2
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    leolkfrm's Avatar
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    is the engine up to temp?
    4 wheels move the body
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    Snow is a lot like religion and a lot like sex....you never know when you are going to get it!....or how much you will get

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    yes it is. I tried driving on freeway for a little bit and then turned on the heater and still no heat.

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    Senior Member Oldmech's Avatar
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    the blower is pushing air? i will assume it is.. the next thing to check would be to make sure the flapper doors are working. you can also check the hoses when the truck is warm to see if they are BOTH hot.. if they are not, then your core has a blockage.. if they are, then the doors are not working.........

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    Senior Member Saugeen Drifter's Avatar
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    The AC Core and Heater Core share the same housing for heating/cooling in the truck however use separate paths thru the use of internal flappers/doors; I agree with the above suggestions, here are a few I have found to be problems in the past;
    - I apologize if this sounds too simplified, but have you checked your fuses? My 91 has a 25A "HTR/AC" fuse that has blown in the past, this panel is found to the left of the steering wheel below the headlamp switches
    - is your coolant topped up?
    - if the truck sat for 2 years did you replace the thermostat? If you did is it in correctly? (I once installed one upside down, little embarrassing)
    - let the truck run for 5 minutes, then feel the hoses entering and exiting the heater core which will be found inside the engine compartment on the passenger side. Both hoses should be hot to touch, if not then you have internal blockage of the heater core. There are other threads on this site informing of how to properly flush and what to use for the core. Use caution when fumbling around in there, the manifolds are cookin. Also take care when attempting to remove the hoses off the Heater core, I replaced one 2 months ago after breaking the inlet off by trying to pull the hose off, probably a godsend.
    - If both hoses are hot, then you have an internal problem. As stated in the above thread, the fan may not be turning, if the vehicle sat then the fan may be bunged up, if you turn your fan on high and alternate from defrost to vent to floor and feel no flow at any location, or can not hear the fan, it needs changed or freed up.
    - If the fan is running and you have no air in the vehicle you have one of 3 internal errors to trouble shoot, (i) debris build up on the face of the heater core (ii)the blender doors are stuck in place due to restriction/dust/doghair etc (iii) your electric blender door actuators are unplugged/cooked/disconnected.
    (i) I had to remove a heater core once that was not pushing any heat, it was coated in dog hair and cedar buds, looked like a mat had formed on the surface, cleaned it up and it was good to go.
    (ii) at times people drop things like change/tools or spill coffee/tea or have a large dog that sheds alot, and over time one or combinations of these factors cause the doors to jam. Read one article a mans son had put 63cents in pennies down the duct stopping the blender to actuate. You may be looking in to ripping the entirety of the dash out to perform this nightmare. So leave this to last.
    (iiia) behind the glove box heading towards the steering wheel is the blender door louver, by switching from ac to heater you should hear it working, it may be burnt/disconnected.
    (iiib) behind the stereo/htr/ac panels are a set of 2 electric louvers, one above the other sometimes they burn out, sometimes they become disconnected if someone was tinkering with stereo installations, sometimes they fall off the shafts. Simply remove the stereo/ac panel, and if you look directly behind it you will see one actuator, the other is visible from the floor of the drivers side. If looking from the floor you may have to remove a silver box directly inline with the actuators to see them, not to worry, the male end plugs that run in to the silver box only fit in one female end on the box. Ensure that when cycling thru heater settings that they are moving. On one of my trucks I had no dash defrost, found one of the actuator fell off the shaft, put some gasket maker on the shaft and put it back on, works like a charm.

    This should give an overall of where to start.
    Cheers

  6. #6
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    Thanks for your help. I got it working. I went ahead and changed the heater core, thermastat, and coolant. I flushed the system hooked all back up and now I have heat.

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