Blower motor (AC/Heat) only works on High

blower on heat or AC fails dead.
but must work on HIGH, does blower high work?
there is blower high relay that most work on high.
the relay closed for high , on high and mode not set to off.(0)
if HIGH works and lower speed do not work the blower resistor is blown out.
causes,
fuse f10 blown.
to test just the blower 1 of 2 pins, ground wire LG/WT light green /white blower must blow or blower is dead. (or key not ON)


the wiring is here.
https://eautorepair.net/Marketing/Default.asp

I looked at 05 avalanche 1500--automatic hvac. The blower fuse is hot all the time. The fuse has an input leg and output leg, even if the fuse is blown, the input leg would still have voltage. If no voltage, you could use jumper wire connected to voltage on the output side of fuse cavity to see if blower would work. If you use jumper, use an inline fuse.
Yes, it could be something wrong with fuse box or wiring circuit?

Could be a number of thing's ! Testing the electrical circuit is the way to find out what's wrong . Using a wiring diagram and a DVOM - digital volt ohm meter . Videos on youtube showing how to test it ! Without testing i would only be guessing . Does it work in high speed ? In high speed it by-passes the resistor . If it doesn't work in any speed more then likely the blower motor is the problem !
Air Speed
The HVAC control assembly applies ground to the blower motor control circuit that corresponds to the selected blower speed. The resistors and the blower motor are in a series circuit. The following list represents the number of resistors in series with the blower motor per particular speed request:
• Low speed--3 resistors
• Medium 1 speed--2 resistors
• Medium 2 speed--1 resistor
When the operator requests High speed, the HVAC control assembly applies ground directly to the blower motor through the high blower motor control circuit. The resistor card has a fusible link which, if blown, will require that the card be replaced.

Blower Motor and Resistor Diagnostics by Wells

Sounds like high blower relay is bad

resistor blower is defect
hope it help

Maxima.org will have it.

you probably need new control switch.

You have a bad blower motor resisitor. The rssistor is normally under the dash on the passenger side. If you find the wires to the blower motor and follow them it should lead you right to it.

There are two components between the heater/AC contol panel/module and the blower motor; a blower resistor and a High blower relay.The problem sounds like a connection issue(a wire is loose) or the relay is going bad.Have the voltage tested at the blower itself,at the relay and at the control panel.Do this when the blower is not working.Wherever there is no voltage when you turn on the blower,that component is the problem

Check the blower resistor pack - usually located in the heater air ducting in the blower fan airflow. Check for loose connections and open circuit resistors on the unit.
If speed 4 works always, its unlikely to be the blower motor ground connection.