1982 Suburban Heater Problem

New to the forum, so hope this post is in the right spot. Having heater problem, started by blowing the 20 amp fuse when on high speed. Have replaced everything, switch, relay, resistor and blower motor. Will run on lower speeds, but switch to high and blows even a 30 amp fuse. The vehicle originally had air conditioning and was removed because no one could keep it working properly. The switch has the off position where the fan motor runs on low unless you disconnect it and then 3 more positions. I believe this the correct switch. Have changed relays twice. Would it be possible I am getting the wrong relay? Or could the relay plug in be bad? Any help would be greatly appreciated.

I’m willing to bet that your fat, red wire is shorting to ground somewhere. A continuity test would be helpful if you could rig something up, or you could attempt to visually trace the wire. Worst case scenario, you run a new heavy gauge wire to the controller.

Poor ground at the blower? If you unplug the blower, does it still blow the fuse when you switch it to High?

If its wired like my '75 truck, the high speed part of the HVAC system has a separate power path that does not go through the fuse box. Mine has a power wire running direct from the firewall junction over to the high speed relay on the heater/AC plenum under the hood, and it had a 30 amp inline barrel fuse in the wire. I replaced it with a newer blade fuse and holder because it kept melting the old fuse holder.

When you flip the blower speed switch to High there is an extra terminal on the back of the heater controls, and it applies 12 volts to the wire plugged in there. That wire runs across under the dash, out through the firewall, and serves as the activation wire for the blower relay high speed.

If the 20 amp fuse you are blowing is the one in the fuse block, it may be your HVAC controls or wires that are bad. When you flip the switch to high speed it powers the high speed terminal on the back of the controls, which then activates the blower relay. If the wire plugged into hat terminal is grounded somewhere, that may be the problem.

Bruce

Exactly. So if it still blows the fuse with the motor unplugged, then you know to look for a shorted wire.

Gentlemen, Thank you all for your replies. Yes, when the switch is turned to high, it blows the fuse when the fan motor wire is unplugged. So, can someone tell me if it is the orange wire on the switch that is the high speed wire? If it is the orange wire, can I splice in a new wire and run it to the relay? Then I could avoid having to try to trace the wire loom and avoid unwrapping 20 feet of black tape.

I just did this in my suburban. Deleted a/c and had only three speeds instead of four. Then I realized I had took a piece of the system out that I needed for proper function.

The blower motor relay. The orange wire is a sense wire for it. that triggers full 12v through the relay running your fan on high speed. I did find some diagrams from another post on here and used those.

There’s a harness coming out by the heater box, you will need all those wires except the green one, that can be snipped short and taped off.

Orange-sense wire
Black- ground
Red- 12v(from junction block by brake booster
Dark blue- other 3 speeds run through this(comes from resistor block)
Purple- goes to blower motor

The other wires go to blower motor resistor block