Q: Why does the a/c blower slower down when the rpms go up

asked by on July 14, 2016

When the truck sits and idles the blower works fine but when your driving and start to pull a hill the blower slows down when the truck is at about 3000 rpms nothing really is coming out of the vents as soon as you let off the accelerator it picks back up to normal

My car has 161000 miles.
My car has an automatic transmission.

The blower motor is not slowing down. What is actually happening is the mode control for your AC and heat is losing its vacuum source. When you switch the knob for defrost/vent/floor etc, engine vacuum is what moves the mode doors to achieve the airflow from the blower motor to where you want it. The default position is defrost/floor when there is no vacuum. Engines do not produce intake vacuum when under load.

To overcome this, there is a vacuum reservoir and a check valve to maintain vacuum in the HVAC system so it still functions when the engine is under load. The reservoir is just a plastic ball. It could be cracked, the hose is off, or the check valve could be bad.

If your S10 is 4WD, there is a lot more testing involved. The transfer case and the front axle are both controlled by vacuum. The vacuum switch on the transfer case is known to fail. When it fails, engine vacuum pulls transfer case oil into the system.

In summary, this could be an easy fix, a vacuum line off, or a cracked ball. It could also mean checking and repairing a lot of components. The fact that your HVAC works at idle points towards an easy fix. If you’d like to get this fixed, consider YourMechanic, as a certified technician can come diagnose your A/C issue and fix it accordingly. Good luck.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Car won't start

Hello. I usually start by checking the battery. If it is low enough these other systems will work but there will not be enough power to engage the starter. If the battery is fine then most of the time this...

The Mazda shut off trying to pull into parking lot and won't turn back on

You may have a fuel pump that stopped operating. You can check the fuse F1-6 located in the underhood fuse box. You can also listen to hear if the fuel pump is running. Remove the gas cap and put your...

How much would this cost to repair? 2001 Chevrolet Impala

Hi - first task would be to check the transmission fluid level. Also, is the fluid red, or closer to black? If dark in color, it may be a good idea for change the transmission fluid and filter. Check also...

Related articles

Symptoms of a Bad or Failing Canister Purge Solenoid
Common signs of a problem with a car EVAP canister purge solenoid include rough idle, difficulty starting, and the Check Engine Light turning on.
How to Diagnose Your Car's Air Conditioning System
There There is never a good time for your air conditioning in your car to stop working, but it typically seems to happen in the dead heat of summer. If your AC system has either quit working or has stopped...
The Best Used Cars to Buy That Can Be Towed Behind an RV
Summer Summer is a great time for a road trip, but often travelers like bringing all the conveniences of home along, in the form of an RV. Then they tow their car behind it – sometimes this practice is called...