I'm sure others' have had this problem - 1973 Blazer, all fan speeds work, but when switching to the highest speed, it shuts down. Replaced the switch about a year ago. Thoughts/solutions? Thanks.

 

i believe there is an online fuse that runs to the wiring for the motor . My chevelle was a 73 and worked that way . It was just below the top of the firewall .

 

There is a blower motor relay that energizes when you put the fan in high speed. Sounds like the relay might be bad to me. My repair manual wont go that far back. But on a vehicle that old it should be easy to find. Should look kinda like this.



I dont know the wiring diagram for that vehicle . But 1 of the wires should be power only when the blower switch is in high. 1 wire will be bigger and have all the time battery power to it. And 1 will be bigger and go to the blower motor or the blower motor resistor block. If you jumper the 2 big wires the one from the battery to the one going to the blower the fan should run on high speed.

 

if none of those fix it i recently had a similar problem on a old bonniville. it was just a bad connection. it would work on the bottom three. i found a connector in the middle of the wires and just touched it and it started working. so cleaned it up and was done