I seem to have successfully replaced the clutch on my AC compressor, recharged the system and now everything seems to work fine, except that the AC compressor engages when I turn just the blower motor on, namely when I set the switch in the bi-level position (marked with arrows going to legs and face). Can this happen because I put too much R-134a in the system? Does anybody know how to properly check the pressure in the system by using that AC manifold pressure gauge (with the red, blue and yellow hoses) that one can rent at Autozone?
Thanks in advance!

I may be wrong, but its my understanding that anytime you turn on the blower for whatever use, the AC clutch engages, even when you are using the heat. I would think to that Autozone would have a user manual for the pressure gauges, the specs should be in a repair manual such as Haynes or CHilton's. Good luck.

The A/C clutch won't engage any time the blower is turned on. You may be thinking of the radiator cooling fan, which should engage any time the A/C clutch is engaged. The A/C clutch should engage though any time A/C or defrost is selected.

-Rod

From the Factory Service Manual:

When PANEL is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The defrost door and floor console duct door (if equipped) vacuum control motors are at full vacuum and the panel/floor door vacuum control motor is at no vacuum, directing airflow to the instrument panel A/C registers and the center console registers (if equipped). A small amount of airflow from the side window demisters will be present.
The temperature can be adjusted to heat the air, but the air cannot be cooled below the outside temperature.
The A/C compressor will not operate.
The blower motor is on.

When PANEL/FLOOR is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The defrost door and floor console duct door (if equipped) vacuum control motors are at full vacuum and the panel/floor door vacuum control motor is at partial vacuum, directing airflow to the floor duct, the instrument panel A/C registers and the center console registers (if equipped). A small amount of airflow from the side window demisters will be present.
The A/C compressor will operate if the outside temperature is above approximately 6°C (43°F).
The blower motor is on.

When FLOOR is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The panel/floor door vacuum control motor is at full vacuum and the defrost door and floor console duct door (if equipped) vacuum control motors are at no vacuum, directing airflow to the floor duct.
The temperature can be adjusted to heat the air, but the air cannot be cooled below the outside temperature.
The A/C compressor will not operate.
The blower motor is on.

When FLOOR/DEFROST is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The panel/floor door vacuum control motor is at partial vacuum and the defrost door and floor console duct door (if equipped) vacuum control motors are at no vacuum, directing airflow to the floor duct, the defroster duct and the side window demisters.
The temperature can be adjusted to heat or cool the air below the outside temperature.
The A/C compressor will operate if the outside temperature is above approximately 6°C (43°F).
The blower motor is on.

When DEFROST is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The panel/floor door, defrost door and floor console duct door (if equipped) vacuum control motors are at no vacuum, directing airflow to the defroster duct and the side window demisters.
The temperature can be adjusted to heat or cool the air below the outside temperature.
The A/C compressor will operate if the outside temperature is above approximately 6°C (43°F).
The blower motor is on.

-Rod

From the Factory Service Manual:

When PANEL is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The defrost door and floor console duct door (if equipped) vacuum control motors are at full vacuum and the panel/floor door vacuum control motor is at no vacuum, directing airflow to the instrument panel A/C registers and the center console registers (if equipped). A small amount of airflow from the side window demisters will be present.
The temperature can be adjusted to heat the air, but the air cannot be cooled below the outside temperature.
The A/C compressor will not operate.
The blower motor is on.

When PANEL/FLOOR is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The defrost door and floor console duct door (if equipped) vacuum control motors are at full vacuum and the panel/floor door vacuum control motor is at partial vacuum, directing airflow to the floor duct, the instrument panel A/C registers and the center console registers (if equipped). A small amount of airflow from the side window demisters will be present.
The A/C compressor will operate if the outside temperature is above approximately 6°C (43°F).
The blower motor is on.

When FLOOR is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The panel/floor door vacuum control motor is at full vacuum and the defrost door and floor console duct door (if equipped) vacuum control motors are at no vacuum, directing airflow to the floor duct.
The temperature can be adjusted to heat the air, but the air cannot be cooled below the outside temperature.
The A/C compressor will not operate.
The blower motor is on.

When FLOOR/DEFROST is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The panel/floor door vacuum control motor is at partial vacuum and the defrost door and floor console duct door (if equipped) vacuum control motors are at no vacuum, directing airflow to the floor duct, the defroster duct and the side window demisters.
The temperature can be adjusted to heat or cool the air below the outside temperature.
The A/C compressor will operate if the outside temperature is above approximately 6°C (43°F).
The blower motor is on.

When DEFROST is selected:


The air inlet door vacuum control motor is at no vacuum, admitting only outside air into the passenger compartment.
The panel/floor door, defrost door and floor console duct door (if equipped) vacuum control motors are at no vacuum, directing airflow to the defroster duct and the side window demisters.
The temperature can be adjusted to heat or cool the air below the outside temperature.
The A/C compressor will operate if the outside temperature is above approximately 6°C (43°F).
The blower motor is on.

-Rod

Thanks very much, Rod!
This is exactly how my AC compressor and blower motor behave!
Thanks to you I will sleep with piece in mind tonight!