1987 International 600 series HVAC fan help

Hi, newbie here.
I thank you in advance.
A friend has this 1987 600 series International and the fan for the heater doesn't work. I applied 12v to the fan motor and it works fine.
I pulled the + wire off the motor and connected it to my multimeter. When I operate the fan switch on the dash, I get zero when in the bottom most position (this makes sense, position is off), .43 volts in the second position, 2.4 volts on the next highest position and 8 volts in the top position.

What do you experts think? Is the switch bad? Are the resistors bad? Where are the resistors? In the switch or somewhere else?
And why are there 5 wires connected to the back of that switch? Are the resistors not inside the switch?
There's also another connector mounted in the bottom of the plenum near the blower motor. The wire feeding the blower motor comes from there. Is that the resistors?
Thanks,
Joe

 

Sure, it's a 32 year old truck, but there are still a lot of them on the road. And it is an International. It has a 7.3 rebuilt by Cat that runs great. It has the two speed differential which is cool.
So, the fan motor resistors are not in the switch. I'm guessing they are in the plenum under the passenger side dash.
I once owned an F250, an early 80's model that had the resistors in the cowl under the windshield wiper grill. Back then the grill was slotted, now they are holes. Anyway, leaves would get in there and catch fire from the hot resistors. Hey mister! Your truck is smoking! I heard that a couple times.

 

Thanks for the response.
I did find the resistors in the air plenum under the blower motor. And that in fact was the problem. Pull the nylon connector off, and two screws to remove the resistor device. Easy.

The resistor device has one of those thermal protection fuses on it. Those pesky things that you find in hair dryers, food dehydrators, etc. They are a solid silver thing that kind of looks like a diode or electronic resistor.

Well, there wasn't any continuity across that thing, so I temporarily jump it out and the fan worked on all speeds.

I was going to solder in a new thermal fuse, but we got a whole new resistor device for $20 on Amazon.

I hope this experience helps others with heater fan motors not working.