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I'm going to start from the beginning. The blower motor gets power from the high blower relay and it is grounded thru the fan switch and the resistor. So you should have power on the pink white blower wire coming from the high relay to the blower motor. The orange black wire from the motor is ground going to the resistor and fan switch. The blue wires on the control panel are for the ac compressor. The high blower relay gets power from fuse number 5 in the box. The blower motor is grounded thru the resistor and then thru the fan switch. Photo courtesy of AutoZone.com
Have you replaced the fan switch? Or if you have a voltage meter you can check for voltage at the motor, and at the switch then in between where you can to isolate the short if there is one. Also since you mention running a separate 12 positive wire into the controls be sure that you connected it correctly. Also check and see if you have voltage at the wire you added. If you have voltage at the wire you added, then you need to check if the switch is letting power through it. So use your volt meter to test for voltage, check all the fan speeds also. I'm guessing you had problems with this before and that is why you have ran your own wire. So you probably kept blowing the fan fuse, this would most likely be from a bad switch, a short, or a bare wire.
First check fuse 103 in the battery relay box and swap the blower relay with another similar relay, such as the horn relay. If this isn't the problem, you should hot wire the blower motor to the battery to make sure it works. If all of that is good, next check the switch/motor controller. Take the connector off the blower motor and check the voltage in the connector. With the key on, the voltage should be 12 volts with the fan setting on high. You can also use a 12 volt test light for this test. If there is no voltage, replace the switch/controller at the control panel. If you need any instructions or have any questions, please get back to me.
Hi. You will need to acquire a volt meter, and test the blower motor for proper voltage(12v). If the motor is registering a 12 volt current(at contacts with the switch activated), simply replace the fan motor. If not, replace the fan relay, or resistor switch.
There are two fuses for the fan. One is probably a 20 Amp in the fuse panel in the cab and marked "Heater". or "Heater/AC" The second is under the hood may be marked Heater, Blower, or Fan and should be 30 Amp. The one in the cab supplies switched 12 volts to the fan speed switch. The one under the hood supplies the 12 volts to the relay. The relay is normally energized by the 12 volts through the 20 Amp fuse in the cab and through the switch. I think you find that the fan has the normal speeds except for missing HI with the underhood fuse removed, and will turn off with the ignition that way. There is no hazard in operating it that way but you may want to replace the resistor/relay assembly before it gets too cold. The relay is a pretty trouble free device in most cases, maybe you can find a deal on a good one at a junkyard.
The whole idea with the relay under the hood started back in the Sixties believe it or not. There is considerable voltage drop in the wiring between the fan switch and the blower fan motor. By adding the relay under the hood right next to the motor, the voltage drop was reduced, both by the shorter wire, and by using heavier wire to boot. The motor gets really close to the full system voltage when the relay is closed. In your case where the relay keeps the fan running, it can drain a battery rather quickly too!