79 F100, w/factory AC, fuse blows on high setting only. I replaced the blower motor when it quit some time ago. The fuse would blow on high even when the old blower motor worked. This is the only electrical issue I have, but I would like to solve this problem considering how weak these motors blow on low and medium. Any suggestions on what to check?

 

Grounding.

It appears to me that the circuit is drawing too much juice.

either
1) Bad ground
2) the motor is comutator /brushes are drawing too much juice

 

I have the same problem on a 74. Maybe the right answer will show up for both of us.

 

Check the Blower switch, on a newer vehicle it uses relays but on our trucks all the amperage goes threw the switch itself. They have a tendency to 'burn.' They don't really catch fire but char up the connections.

 

Thanks, I meant to add that I replaced the switch, too. The wire going to the high connection shows signs it has been getting hot.

 

Thanks, I'll check the ground.

 

I also replaced the switch and could see where the wire for high speed has gotten warm.

 

What amperage of fuse are you running.
Also
Can you amp out the blower when its running on high speed?
Don't amp the negitive or ground side, but the power side?

 

30amp is what I'm using, can't find the 35amp it calls for.

 

Are you able to amp it out?

 

I'm sorry, can you explain "amp it out" ?

 

An amp meter will tell you how hi of a current draw you are pulling, if your blower is pulling lets say 35 amps on high speed, your 30 amp fuse will blow fairly quick, if you have a short in the motor it should trip right away, is your system blowing the fuse pretty fast?

I,ve seen two speed motors work on low speed and short out on high speed

 

May want to check your high speed wire to see if it is cut or pinched and/or possibly touching metal

 

It seems to take several minutes before it blows the fuse on high. I'll check the amps tomorrow. Thanks
I'll check those wires, too.

 

It runs for several minutes before it blows the fuse.
I'll check the amp output and the high speed wire. Thanks