Heater Blower Resistor

[Lar4t9r]

My 1969 F 100 heater motor resistor is shot (motor too for that matter). Tested it and no power coming thru it. Can't seem to find one at local parts stores or online searches so far. Anyone know a source? Or can I go to another year that would be interchangable? I can get the motor at NPD locally but they don't carry the resistor. How about going to a later model motor with readily available resistors, would that work? Thanks for any help.

[Dragon]

3 ceramic resistors would work. I will you get the wattage and values later.

[fordman]

i've got a good used resistor and a motor too if you want it.

[Lar4t9r]

Fordman hooked me up with a used blower and resistor, Thanks Fordman .

But I still want to understand the science behind this. Therefore, if anyone can further discuss what one needs to know to be able to make resistors if originals are not readily available, it would be appreciated.

The way I understand this, for a 3 speed heater blower fan, the switch sends 12 volt power to the blower at all three positions, but routes two of them thru different resistors that govern different speeds. Is this correct? I know you can buy resistors at places like Radio Shack, but are these also available at auto related outlets? How does one determine what ohm resistors to use? I'm no electric genius, but have a basic understanding and always looking to further my knowledge.

Thanks for any help on this.

[Lar4t9r]

3 ceramic resistors would work. I will you get the wattage and values later.

Hey Dragon:
Sounds like you know about this stuff (see my previous post). Hope you have time to provide this info.

Thanks.

[Dragon]

The way I understand this, for a 3 speed heater blower fan, the switch sends 12 volt power to the blower at all three positions, but routes two of them thru different resistors that govern different speeds. Is this correct? I know you can buy resistors at places like Radio Shack, but are these also available at auto related outlets? How does one determine what ohm resistors to use? I'm no electric genius, but have a basic understanding and always looking to further my knowledge.
:

You have the routing correct. Electronic supply houses and maybe radio shack would have the resistors.

If the motor was 20 amps (that is the fuse size for F Series) at 12 Volts the correct resistor for 1/2 speed would be this: V/A=R Volts 6 divided by Amps 20 is .3 of an ohm at 120 Watts so the resistor must be rated for 120 watts.
Watts is Voltage times Amps. so 6 * 20 = 120

1/4 speed: 3 volts divided by 20 Amps is .15 Ohms and needs a 60 Watt resistor. You would never want to touch those factory resistors while the truck is running the heater. It would be like laying your hand on a electric burner on low.

[fordman]

the resistors are really only coils of wire when you look at them. they have different thicknesses of wires that give the power to the motor. i find it hard to read that stuff dragon wrote. i can follow it but it just seems wierd to me.

[Lar4t9r]

The way I understand this, for a 3 speed heater blower fan, the switch sends 12 volt power to the blower at all three positions, but routes two of them thru different resistors that govern different speeds. Is this correct? I know you can buy resistors at places like Radio Shack, but are these also available at auto related outlets? How does one determine what ohm resistors to use? I'm no electric genius, but have a basic understanding and always looking to further my knowledge.
:

You have the routing correct. Electronic supply houses and maybe radio shack would have the resistors.

If the motor was 20 amps (that is the fuse size for F Series) at 12 Volts the correct resistor for 1/2 speed would be this: V/A=R Volts 6 divided by Amps 20 is .3 of an ohm at 120 Watts so the resistor must be rated for 120 watts.
Watts is Voltage times Amps. so 6 * 20 = 120

1/4 speed: 3 volts divided by 20 Amps is .15 Ohms and needs a 60 Watt resistor. You would never want to touch those factory resistors while the truck is running the heater. It would be like laying your hand on a electric burner on low.

Thanks for the formula. Now, is the 1/4 speed and 1/2 speed the normal way these heater blowers were set up? I confirmed the 20 amp fuse set up for the heater in my fuse block and my fuse block is the correct original according to the pics in my manual. So, it seems you have done the math for me and I should be able to buy the .3 amp/120 watt and .15 Amp/120 watt resistors and assemble (solder) them to my plug from old resistor assembly and it should do the trick. Right? I understand I need to remove the old coiled wires first.

[kaptnkaos]

Where exactly is this resitor??

I have one speed (high) on my heater and have been told to check the resitor...

KaptnKA S

[fordman]

its on the back side of the heater box. i dont think you can get to it from the cab without pulling the heater box. but you might be able to. it takes 5/16 socket.

[kaptnkaos]

I figured as much... I just put it back in the truck after it being in storage for a year or so....

[Lar4t9r]

Where exactly is this resitor??

I have one speed (high) on my heater and have been told to check the resitor...

KaptnKA S

Here is a link to the '65-'72 Heater Parts technical drawing on this site.

http://www.fordification.com/images/sch ... sh-air.jpg

The resistor is shown on the right side just above center and is part # 18591. I tried to remove mine with the heater box installed, but seemed like an impossible task.

Remove the box by disconecting the hoses and nuts and control cable on the engine compartment side. (Note: drain down the radiator first). Under the dash, disconnect the cables at the control assembly and unplug the wiring. There is ground wire from the motor connected to a metal part of the heater box or the dash. Disconnect the defroster hoses from the heater box that go up to the vents in the top of the dash. Remove the rubber boot from the heater box to the incoming air vent on the right hand side. Pull the box out. I don't know if you have to remove the glove box, because mine was already out when I did this.

Hope this helps.

[Thunderfoot]

I figured as much... I just put it back in the truck after it being in storage for a year or so....

I wonder if you just didn't plug the wires into it You might think back when you installed it as to if you hooked them up, if not you may be able to feel/reach back there and hook them up.

It will work on High speed only, if this is not plugged in or if it is plugged in wrong... or if they are bad...

FYI, the reason these resistor are mounted into the heater box is because of the heat they generate when in use. By being in the box the air flow over them helps cool them. also, if they get wet in there (from leaky heater core) the water on them will kill them from thermal shock, when they get hot and then get water on them cooling them off to fast...

[Dragon]

Ford manuals say remove glove box, Remove upper retaining screw then use long 1/4 extension and remove lower screw. Pull the unit out by the wires. Plug the wires onto the new unit and the put in upper screw then lower.I just found the specs. Page 36-10-02 says 8.0 Amps max for F-Series for the motor @ 12.8 Volts. So back to the math.
102 Watts max
1/2 speed .8 ohms and the resistor must be 54.4 watts
1/4 speed 1.2 ohms and the resistor must be 76.8 watts
Round the resistor wattage rating to the next higher standard value. So a 76.8 should need a 100W ceramic sand resistor.

If you can get them with those wattage ratings you can put them in parallel that requires more math but can get you what you need,

Say you can only buy 10 Watt resistors that means 8 resistors in parallel. Ouch the required area to mount them gets big.

[kaptnkaos]

Thanks Thunderfoot..

I'm pretty sure I got the wires plugged in... but not sure if they're in right or wrong...
Didn't know they could be hooked up wrong.
I've got another complete heater set up that I'm gonna hook up in the bus.
I'd like to be able to heat up the cabin area without having to run a propane heater while driving.
I'm probably gonna pull everything out of the cab next spring so the can wait 'til then...
At least now I have heat and defrosters... and it's normally runnin' on high anyway...

KaptnKA S