Question about 1999 GMC Sonoma
I had the same thing happen on my 2000 Sonoma, last week. I quickly found it to be a broken vacuum line. Look between the brake master cylinder and the drivers side fender- up top. Easy to find, there are 3 vacuum lines connected to a tee. One goes towards/ and into the fender itself. On my truck this line was cut due to rubbing on the fender opening. Withthe engine running it was drawing a good deal of air. I put a screw (plug) in the open line and my vent controls now work. Unfortunately I cannot find a vacuum diagram to tell me where the line into the fender ends up.
Posted on Nov 30, 2008
My 99 sonoma heater contol blows air but on the floor. cant find anything wrong with the vacuum. got any ideas?
Posted on Apr 29, 2011
There are two components that alter airflow in these, and there's a good chance that you have issues with both. The first is mechanical, and you've indicated a problem with one of the levers. I have to 'V' Series 1988 Suburbans, and on one I was able to 'fix' that, but on the other it was off to the boneyard to take out 10 or so bad ones before I finally took out one that could be fixed.
The other thing is vacuum. Each body style plumbs these vacuum lines a little differently, but you likely have at least one (and that's all it takes) line that is either disconnected or leaking. A keen eye or one of the 'generic' shop manuals at the corner auto parts store should be all you need to track them down.
Now, there's one 'other' thing worthy of note. These vehicles run the supply air through the AIR CONDITIONING system in the 'defrost' mode (In case you're wondering, it's mostly because if you don't use the air conditioning the 'other' 6-10 months of the year, the seals dry up and the system fails), so if you've got an A/C problem, no matter how sweet you get the duct and damper operation going, you're still gonna have steamy windows and a cold nose...
Posted on Sep 19, 2008
SOURCE: When turned to defrost, no
The vent flap foam lining has come loose and needs to be removed or reglued . A spray on contact adhesive works best , don't glue the flap to the vent casing.
Posted on Nov 16, 2008
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