Defroster engagement with a groan!
 

Hi all,

I went to run an errand this afternoon with my Blazer (see below) after not driving it for at least a week. After I got it started I heard a strange groan type of sound (duration ~2 Mississippi's) when the air delivery rotary switch was turned to the the Defrost or Defrost+Heat position. I would hear it again when switching to something other than the Defrost or Defrost+Heat setting.

After a couple minutes of me trying to find the origin of the sound that seemed to come from under the dash and/or the engine compartment, including a trip under the hood all while it was running to try to track down the noise, I could not get it to come back.

It seems like there is a vacuum leak perhaps or a bad vacuum actuator. During my trip around town, about 15-25 minutes of driving with two stops, the symptoms did not come back.

Am I on the right track with my diagnosis?

The other thing I noticed that might or might not be related is that at the input/output to the AC compressor the tubing on the passenger side was getting hot whereas the one on the driver side was getting cold during the warm up in my driveway. The compressor clutch was not engaged so I thought this was weird. I do have a leaky AC system at the compressor which I'll be dealing with in the spring. When I got home I felt the tubing again and they were now both warm, neither hot nor cold.

Thoughts?

Thanks for your help!

Christine

P.S. I have replaced all of the under-hood vacuum hoses that connect to the HVAC and 4WD system.

To me if there was a vacuum leak, then turning the controls would not do anything....whereas you are hearing the vacuum actuator moving. Although it does sound like something is binding or already broken and that could be the defrost door. A real PITA to change out.
Better for you to do a search for defrost door to read about "fixing" it than me trying to type it all out. To give you a hint though; door is in middle of dash within the HVAC air duct....get the picture??

Good luck.

So I think I figured out the problem. I found that the HVAC/4WD vacuum circuit had a small leak. I tested it using my Mityvac vacuum tool. The leak was where the hose to the HVAC controls connected to a tee. When I put a zip-tie on it to serve as a hose-clamp, it no longer leaked and the groaning sound went away.

What prompted me to look at the vacuum system was that when driving up a long hill (1,700' in 6 miles) I noticed that the air flow system would change the flow from defrost to vent and back again as it shifted with varying load on the engine.