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

asked by on

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
  1. Home
  2. Questions
  3. Why does the a/c blower slower down when the rpms go up

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

Q: Engine misfires and hesitates while accelerating

If the issues only began to occur after the vehicle was fitted with a new MAF and different intake, then it may be possible that the issue is related to the recent work. The mass airflow sensor is one of...

Q: The A/C works only when the fuse has been replaced in order for it to continue to work. What causes this?

It is important to know which fuse you are replacing. Is it the fuse to the compressor clutch or to the blower motor to be precise. Either way, I can give you some insight that you can use to figure...

Q: Intermittent jerking when shifting

Since you said the problem only happens between 3rd and 4th gear, you likely have an internal transmission problem. An engine performance problem would likely be noticeable in all gears. Also, since the problem only happens when shifting from 3rd...

Related articles

P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.