I have a 2000 Olds Bravada. AC system is new. It worked very well on recent 3,000 plus mile trip. Temps were into the low one hundreds a few days. Question is when the control is set for the interior to recirculate mode it cools the best. But when I pull a grade the mode goes to letting exterior air in the mix. All air still comes out where it is supposed to. It just lets the air temp change to slightly warmer, yet still very cool. As soon as flat ground is reached all goes back as it was. If I shift from 4, overdrive, to third gear, it will go back to the way it was, all good and cool. My Bravada ran excellent on this trip. I was getting over 19 mpg and as high as 21.08 mpg highway.
Any ideas are appreciated.
Thanks,
RTJ

 

The actuators for the ducts in the HVAC system are controlled by vacuum from the engine. There is a reservoir and a one-way check-valve that is supposed to isolate the vacuum that supplies the actuators when the engine is under load (e.g. going up a hill) and cannot supply the regular amount of vacuum. (When under load, the pressure inside the intake manifold is only a little less than atmospheric pressure as opposed to when it is not under load when the pressure is much lower than atmosphere; i.e greater vacuum.)

So, it could be that you have a leak in the vacuum hoses that supply the vacuum to the HVAC actuators. These hoses in the engine compartment are known to rot over time. There are several threads on this repair including a link to one in my signature file. You should look at those and your vacuum hoses.

Because you have a Bravada, you will not have any hoses that go to the front axle or transfer case so that your vacuum system will be much simpler with only the check valve and the reservoir.

The vacuum system can be checked with a hand vacuum pump but it is worth doing a visual inspection first.

 

Thanks Christine_208.

RTJ

 

No problem and Good Luck! Be sure to report pack what you learn or what works.