I have a 97 Blazer LS. Last summer I had ac problems where when on high the fan would shut off after awhile. Last fall I replaced the fan motor and the fan resistor to avoid winter problems. Everything was was working fine but now the weather is turning warmer I still the same problem with the ac.

The fan kicks off but only on ac and only on high. It has basic controls.....not the climate control.

Any suggestions?

 

Provided there are no other accessories that quit when the fan quits, the problem is with the high blower relay, or it's circuits. It is a 1" black plastic cube shape, with 5 wires attached to it, located on the firewall, near the blower resistor.


All of the following tests are done with the relay plugged into the vehicle harness:
The red wire must show battery voltage at all times.
The black wire must show less than 5 ohms resistance to ground at all times.


The remainder of tests require the HVAC mode switch to be in the HEAT position, and the ignition in the RUN position.


The purple wire goes directly to the blower motor. Voltage will vary depending on blower switch position.
The dark blue wire comes from the blower switch. Voltage will vary depending on blower switch position. (high speed fan should show zero volts).
The orange wire also comes from the blower switch. It should only show battery voltage with the fan speed set on HIGH.


Post your results.

 

thanks for the reply. I'll check it out and let you know the results.

 

after closer observation I figured out that the blower motor wasnt shutting off.....it was being redirected to the defrost vents. havent had a chance to tear into the dash yet but i assuming that the problem is with the blend door actuator.

 

Your issue could be a disconnected or broken vacuum line, or possibly a blocked vacuum check valve. Before you dig that deep into the dash.

The defrost position is the "home" position for the actuator. If the vacuum line is broken or disconnected, that's the position the vents go to.

 

Before you do too much fooling around.... The blend door regulates temperature only. It has nothing to do with where the air flow comes out. The HVAC fan is supposed to run in all mode switch positions except OFF. When you select OFF, air flow is drawn in through the cowl, and when the vehicle is in motion, air flow splits between the dash registers and the floor, and the temperature can be regulated by adjusting the temp dial. If the system loses vacuum, airflow will default to defrost, as Racer_x mentioned.