Why does blower fan speed switch lock-out off and low speed positions on 1998 chevy silverado? It stopped working altogether then I replaced a 10 amp fuse and now only medium high and high will work but, switch will not turn back to medium low and off positions.

The blower motor relay is energized when the Blower Motor Control is set to the High position. This removes the blower motor resistors from the circuit and battery voltage is then applied directly from the HVAC BLO fuse (RH IP Accessory Wiring Junction Block) to the blower motor though the relay switch contacts. The blower motor then runs at high speed.

HVAC Blower Controls Circuit Description
The blower motor is a variable speed motor. The motor operates at a higher rate when voltage is increased to the maximum level.
The blower motor resistors reduce the voltage supplied from the A/C FAN fuse (LH IP Accessory Wiring Junction Block) when the blower control is set at low and medium speeds (speeds 1-4). The blower motor relay is energized when the Blower Motor Control is set to the High position. This removes the blower motor resistors from the circuit and battery voltage is then applied directly from the HVAC BLO fuse (RH IP Accessory Wiring Junction Block) to the blower motor though the relay switch contacts. The blower motor then runs at high speed.

It sounds as if the blower motor relay is stuck, on your vehicle the relay is part of the blower motor resistor, the resistor is behind the glove box.I hope this helps. Take care. If you have more questions for me about this vehicle or if you just want to say Hi! use testimonial. Thanks

switch has failed and needs replaced would be my suggestion....good luck...

there is a relay in heater box go to autozone they can get you one and let you know where it is located

replace the blower switch you control by hand on the dash

there could be partial contact in the blower controls which has resulted in total loss of contact, in blower speed affected. replacement of the entire blower control switch should resolve it

THE FAN BLOWER RELAY IN STUCK CLOSE POSITION.YOU HAVE TO REPLACE FAN BLOWER RELAY.

The most common part that will cause this symptom is the resister module. Its simple to replace.

I had a truck about the same year with the same problem, it was the wiring harness, but id check the resistor and the fan first before replacing anythings,

Check the relais module located inside under the rh side of the heather box you can see the connector most of them need to be replaced as they burn out you may need to replace the blower motor as well,high amp draw caused by faulty bearings will overload the blower motor controller