High speed fan and other fan issues

[egorny]

Just purchased a 76 GMC short bed. Runs great, no rust.
The high speed on the blower fan doesn't work. I did have the other fan speeds but yesterday  it started to blow the fuse as I started the engine.
Prior to that, I changed the blower relay, then the resistor unit and checked the fan switched, all are good. Also checked the fuse on the firewall as well as checking that the junction box got power. Still no high speed.
I did check the fan and ran a wire from the battery to the fan and got the high speed.
Any other suggestions? Any clue  to look into the fuse blowing now?
Tha miss
Ed

[hatzie]

Power for blower HI is run through the SPDT relay switch contacts from the Junction stud on the firewall.
 
With the relay at rest, meaning when the blower switch is set in any position other than HI, the relay feeds the fan power from the resistor pack output wire. 
When you move the blower switch to the Hi position the relay should trip.  When the relay is tripped it feeds the blower motor directly from the firewall Junction block.

You probably have a fault in the HI wire from the blower switch.

Does the fuse blow no matter what position the blower switch is set to or just when the blower switch is set on HI?

[egorny]

Forgot to mention it has factory air that I think I will have repaired.
Yes the fuse blows in and speed.
Tried it with the speed relay disconnected and it still blew.
I was getting a low speed fan in the high speed setting before the issue with blown fuses.
Ed

[hatzie]

The blower relay told me it was an AC truck... 

The HVAC sub-harness is fairly compact.  You probably have a short in the wires between the blower switch and the HVAC harness firewall pass-through.   Download the 1976 GM Factory wiring diagrams and look at the RPO C60 sub harness wiring diagram.

Look over the wires from the control head to the firewall for chafed jackets.
Since the fuse is blowing in all positions check the wire from the fuse panel to the mode lever (AC, VENT, HEAT, DEF) on the control head.
LO in the HI position tells me there may be a "modification". 

[75gmck25]

In my '75 I had issues with the high speed fan blowing the fuse, even after I replaced the blower motor.  I finally took out the old fuse holder and fuse (which is mounted in the engine compartment up high on the firewall) and spliced in a brand new blade fuse holder  and modern fuse (fuse holder is rated for up to 30 amps).  I put in a blade fuse that was the same amperage as the old glass cartridge fuse. I have not had any issues since.

It seemed like the current draw on high speed was fine for the wiring (no visible damage), but the fuse holder itself could not deal with the heat that was generated.  As it generated and trapped heat it finally blew the fuse.  Mine was not an immediate short, and it always ran for a while before it blew the fuse.  In my case, my guess is that corrosion inside the old fuse holder was creating a bad connection to the fuse and that was generating the heat.

Bruce

[egorny]

Pulled part of the dash, cleaned the fan contacts etc. Got the fan working fine in all speeds but high.
Nothing blows in the high position. I thought low speed did but I was wrong.
Having the ac looked at on Tues anyway. Would like to get that running again,
Ed