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

1998 GMC JIMMY

200,000 MILES • 4.3L • V6 • 4WD • AUTOMATIC

RATPACKDAD

MEMBER
3 POSTS

Will blow air through the defrost setting when put on cold setting, but not when set to the heat setting. Is automatic control system.

Dec 4, 2018 at 9:47 AM

STRAILER

✓ASE Certified Mechanic
54,832 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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Dec 5, 2018 at 11:55 AM

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.

Dec 7, 2018 at 8:05 AM

STRAILER

✓ASE Certified Mechanic
54,832 POSTS

I would pull the control head to see if you have vacuum present at the same time check the rotary vacuum valve.

Dec 7, 2018 at 9:09 AM

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.

Dec 7, 2018 at 2:10 PM

STRAILER

✓ASE Certified Mechanic
54,832 POSTS

Sounds good please do.

Dec 8, 2018 at 10:26 AM

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