I have a 75 F-250. I'm toying with the idea of using relays to operate my headlights, heater, wipers, etc. to keep my wiring inside the cab to a minimum and have the bulk of the wiring by the battery/solenoid. My question concerns the heater. If I use a potentiometer to control the heater motor, will the motor be able to run from 0-12v or is the motor only able to run at 12v, period?

 

The factory resistor pack essentially drops the voltage to the motor down by creating a divider between the blower motor and the series resistance. If the motor only ran with 12 volts across it, there would be no way to slow it down. The factory speed switch effectively swaps in series resistors in between the 12-volt source (in this case the battery) and the blower motor windings. As the speed selection is decreased, the voltage across the motor drops while the drop across the series resistance increases - but the sum across the motor and the resistor remains at 12 volts. Obviously there is some minimum voltage that the motor needs to start, but it's less than 12 volts for sure.

What kind of potentiometer are you planning on using? I don't think this is a very practical idea. The run-of-the-mill potentiometers you see in most cases are just for trimming and will not handle the current of the blower motor. High-current potentiometers aren't really practical for a panel-mount application like this for a number of reasons. The factory resistor pack was put isolated in the heater box (where it would have large open space around it) for a reason - so that the power dissipation is kept away from the driver or nearby objects. Even if you found something "in between" that I haven't seen before, you'd still need to do some calculations and use some engineering judgement to make a call - but I don't think this is a worthwhile avenue.

Why do you want to use relays for everything? This just adds more work and more wiring. In most cases (in fact, for all the loads you mentioned) the factory switches are designed to run the load current right through the switch contacts. If you go with a custom relay setup, you still have all the wiring going into the cab (for the control coils) - but now you've got all the extra contact wiring to route under the hood - not to mention the added cost and space of relays for everything. You would effectively double the amount of wiring by doing this.

The factory wiring setup was designed to get the job done as simple as possible so that it could be mass produced in the millions. If there were any other ways to reduce the amount of wiring (and save more money), Ford would have done that. That's true of any mass manufacturing.