Hi this is Dan. I have a 1979 Ford F150 300 i6. My switch for my heater does not work on High. I'm guessing it is probably the switch of the fan. It works on low, medium and on high it kicks down to a medium/low. So i looked through a post on on the bullnose 1980-1986. I'm not sure if this will work. Here is what was said:

"I made a simple folded aluminum bracket that is held to the blower-motor flange by the two top-most screws.
This bracket is approximately 8" long and I drilled seven equally spaced holes for future relay installations; I only used two holes for this project.
Just above the blower is a good copper GROUND-strap that grounds the hood to the firewall.
I used the firewall end of that strap for an attachment point for my 12AWG GROUND-wire.
I spliced both little black GROUND-wires from relay-terminal #85 into the 12AWG GROUND-wire, thus providing GROUND for the switch-circuit of both relays.
This GROUND-wire attaches to relay-terminal #30, or "battery", of the GROUND relay.
Another length of 12AWG wire attaches to relay-terminal #87 and from there it splices into the existing GROUND-wire near the blower-motor plug.
The relay-terminal #86 "switch" wires of both relays are spliced into one wire that runs through the firewall, into the cab, to a switch mounted in the dash.
For HOT, I started with a 12AWG wire connected to the HOT stud of the starter-relay, which is also the main power junction.
This wire goes to relay-terminal #30.
Then, from relay-terminal #87, a 12AWG wire splices into the existing HOT wire at the blower-motor plug.
Turning the fan-speed switch to HIGH, then flipping the dash-mounted switch to ON, thus energizing the relays, switches direct full voltage to the blower-motor.
The single dash-mounted switch controls both relay. Both the 12AWG HOT that feeds the relay and the switch-wire are properly fused,

Can our fan take this voltage jump? I have had continual fuses blow between the battery and and resistors a 40 amp fuse when I add power to it. Looking at the diagram on the relay I may have wired it wrong.



Let me know what you think.

 

is there any reason why you wouldn't just test the existing factory switch and replace it if its bad? or inspect the current factory wiring from the switch HI connector position to the blower motor?

 

Sounds like you're on to it with the switch. It could be the wiring for the HIGH position though, and not just the switch. However the switch is usually what wears out, so is a good first culprit.
But why not just change the switch?
Don't get me wrong, I'm a big fan of adding relays to circuits like this. Getting the maximum available voltage to a component (especially a motor) is a good thing. We add them to our headlight and horn circuits all the time, so why not a heater motor too. Right?
But swapping out a switch sounds pretty simple.

Yes, it can handle all the voltage that the system normally delivers. You're not "multiplying" anything here. Only allowing the full system voltage to make it all the way to the motor. Just like it was designed to do.
That voltage value is 12 or so (12.6) from the battery when the engine is off, and approx. 14.5 when the alternator is charging the battery normally (and running the rest of the system). Nowhere near hurting the motor.
Blowing

In a typical automotive application, voltage does not blow fuses. Amperage does. So running 17 volts through a 12v system doesn't hurt the wires, or the fuses.
But creating too much flow, such as through a short-circuit to ground will do it. As will creating too much load being pulled through the system by a faulty component such as a tired old motor. So a jammed up heater blower motor could blow a fuse. As could a shorted out one.

But what resistors are you speaking of? This truck? Or just using an example of another vehicle you had?
Frankly though, blowing a 40 amp fuse usually takes a full short circuit. Very little, if anything on our trucks is capable of pulling more than 40 amps when it's working properly. Even a big-assed heater motor probably only uses 30 amps on a bad day. Likely more like 15-20 I would think.
But I could be wrong on that. Some of these trucks had pretty big motors spinning a big fan. Might take some current to get it spinning. Still, I doubt it would be much more than 20a.
Resistors do create heat, but they do it to and by reducing voltage. Not increasing current flow. That's only done at the other end (the business end, so to speak) by whatever load is "pulling" the current and voltage through the wires and resistors.

Paul

 

I put a new switch in and it may be giving the high setting just a tiny bit more power. It is hard to tell. When on high the low and medium settings are receiving less power then before but they still have power going through. Do I need to get a new resistor that is connected to the air flow housing? Could the blower motor have just two settings?

 

Not surprising that a new switch would get more power to the motor. Where the old one had been adding resistance through old contacts, so slowing the motor down. So that part sounds right.
Some Ford vehicles (such as the old Broncos) had only two fan speeds. But most of the other vehicles I've played with have had three speeds, as yours seems to. The motor itself is not what dictates this though. It's the switch and the circuit routed through the resistor(s) that dictates the speeds simply by reducing voltage that reaches the motor. On high, the resistor is bypassed and full voltage reaches the motor. On lower speeds the resistor lowers the voltage seen at the motor. Not sure what the voltage values are though. Never measured them myself.

The heater resistor is usually pretty robust and reliable. Though some do fail, most will last longer than the vehicle. I suppose you could need a new one, but it seems like it's ok for now since it was working before, right? Now with the new switch the two lower speeds are just a little less? Or a lot less? Like so much that the first two speeds are kind of useless?
Could be something in the connections I suppose. Where is your resistor? Is it easy to access? Or awkward as heck, like most?

Paul

 

[QUOTE=1TonBasecamp;18198895]Not surprising that a new switch would get more power to the motor. Where the old one had been adding resistance through old contacts, so slowing the motor down. So that part sounds right.
Some Ford vehicles (such as the old Broncos) had only two fan speeds. But most of the other vehicles I've played with have had three speeds, as yours seems to. The motor itself is not what dictates this though. It's the switch and the circuit routed through the resistor(s) that dictates the speeds simply by reducing voltage that reaches the motor. On high, the resistor is bypassed and full voltage reaches the motor. On lower speeds the resistor lowers the voltage seen at the motor. Not sure what the voltage values are though. Never measured them myself.

The heater resistor is usually pretty robust and reliable. Though some do fail, most will last longer than the vehicle. I suppose you could need a new one, but it seems like it's ok for now since it was working before, right? Now with the new switch the two lower speeds are just a little less?

The lower speeds are the same as they were before. The higher speed seems almost equal to the medium speed.

Or a lot less? Like so much that the first two speeds are kind of useless?
Could be something in the connections I suppose. Where is your resistor? Is it easy to access? Or awkward as heck, like most?

It is pretty easy to access. It has two resistor coils that don't seem to be in bad shape. I've cleaned the connection contacts off of the resistor, which hasn't made a difference.