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Heating system, air does not blow when set on heat setting

Tiny
RATPACKDAD
  • MEMBER
  • 1998 GMC JIMMY
  • 4.3L
  • V6
  • 4WD
  • AUTOMATIC
  • 200,000 MILES
Will blow air through the defrost setting when put on cold setting, but not when set to the heat setting. Is automatic control system.
Tuesday, December 4th, 2018 AT 9:47 AM

5 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,513 POSTS
Hello,

It sounds like you have a mode actuator that is not working due to a vacuum leak. Here is a guide to help you look for the broken line under the hood. If that is okay check the rotary valve at the control panel.

https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position

Check out the diagrams (below). Please let us know what you find.

Cheers, Ken
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Wednesday, December 5th, 2018 AT 11:55 AM
Tiny
RATPACKDAD
  • MEMBER
  • 3 POSTS
Thanks for replying. Cannot see or here any vacuum leaks. I just replaced the heater control module, didn't help. Just do not understand why the defrost flap works on the cold setting, as soon as set to heat it closes. Could it be the vacuum control unit. Any more help or ideas would be helpful. Also could it be the blend door actuator? Again thanks for the help.
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Friday, December 7th, 2018 AT 8:05 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,513 POSTS
I would pull the control head to see if you have vacuum present at the same time check the rotary vacuum valve.
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Friday, December 7th, 2018 AT 9:09 AM
Tiny
RATPACKDAD
  • MEMBER
  • 3 POSTS
Thanks for replying again. Will look at that stuff, but will not be until next week. Also thanks for the diagrams. Will let you know.
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Friday, December 7th, 2018 AT 2:10 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,513 POSTS
Sounds good please do.
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Saturday, December 8th, 2018 AT 10:26 AM

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