Why does the fan on my AC switch into defrost mode when I'm on the interstate rung about 70-75 mph? Does not happen around town

Question. Is the flapping at a very fast pace that would indicate the speed of a fan blade hitting something? If so, you may be on to something there such as a misplaced gasket or something has dropped/fallen into the blower box via a vent.

If it is not that speed then see the following..

Hope I can give you some direction. Speaking from a generic sense all ac/heaters these days have a door or doors in the blower box that control air flow from the main part of the box to different positions when you ask for vent, heater, mix, defrost etc..

Most these days are operated with a small electric servo/motor and unfortunately the mostly have plastic gears that wear and can "thump-thump" as the motor cycles trying to open or close the flapper door.

Try to change the air direction from Max AC to regular ac to heater, defroster, bi-level etc all with the AC switch in the on position if you have a separate on/off switch.

Listen to the sound and see if it comes and goes with a different location of the selector switch.

One last question. Does it happen at idle or driving down the road at any particular speed?

Lets start with that and see what happens. This is interesting.

Good luck!

check that the oil level is correct as extra oil foams up and the oil pressure falls. Around town driving does not put a strain on the cooling system but continuous highway driving does and shows up as blocked cores and collapsing bottom radiator hoses. Ignition timing is critical at high speeds so ensure that that is correct. Check that the temperature gauge is accurate as the noise and missing is indicative of an overheating engine.

Hi. There happens to be a common problem with these trailblazers inside the ducting box located under the dashboard. The mode actuator door motor stops working due to a faulty motor itself and the default position is defrost only. As far as the Air Conditioning not working, there maybe another problem to diagnose there. Could be low refrigerant, or electrical problems to the compressor, or faulty fan clutch(another very common problem with these trucks). Type in mode actuator door and 2005 trailblazer in your search engine online i.e. Google or Yahoo and you will find many posts about this problem. I think there may even be a Youtube video about replacing it. Let me know you need more help. Thanks.

The defrost feature in most later vehicles utilizes the AC to speed the drying of the windshield inside. It is the mfr's choice as to whether to let you know that this is happening. ACs cool by extracting moisture from the air . . . or the inside of your windshield. BTW, in the defrost mode, you can use the heater while the AC is running this warms the air that the AC is drying- it works fine. Using the AC does increase fuel usage so some makers might not want you to know that it is operating while in the defrost mode.

It's normal for that to happen.It uses AC system to help remove condensation for defrost. You'll notice it doesn't happen on heat, only defrost. AC may be slightly low on freon causing it to cycle on & off more frequently.

read the owner manual to see how the auto works. try the defrost mode whcih is a fail safe mode and should always come on unless a major system issue.

the flashing light is to tell you something is wrong, have a/c system checked.

Your blower motor may be going or the fan.

ac fan fuse should be in engine comprtment in a labled box near firewall passenger side

This is normal. Four cylinder engines do not have a lot of horsepower and torque.

When your a/c compressor and cooling fan kick on, there is a load on your engine that casues rpm to drop. When it kicks off, the load goes away and the rpms go back up.

Your a/c system is the cycling type that increases fuel economy. It should cycle 4 to 7 times/minute at 70 degrees F.

Please rae as fixed if this answers your question.

PS. With the a/c (and defrost) off, it should not drop any rpms.