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

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.