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The resistor is designed to get hot. Three coils of resistance wire of varying length, hence each coil having a greater resistance. Using Ohm’s Law, (E=IR), the longer the resistance wire, the greater the voltage drop, heating the coil. The coil assembly is installed in the blower flow path to dissipate this heat. Also, notice in the video, the maximum speed contacts were the only contacts damaged. This switch setting bypasses the coil assembly and full motor current passes through these contacts. In a series circuit, this voltage drop reduces the voltage applied to the motor thereby slowing it. I’ve witnessed the coils glowing red and suffering no damage other than a slight increase in resistance.
Replacing the switch only would result in the customer returning with the original complaint.
(E=IR, E=voltage, I=current, R=resistance)
This explanation help?
Dan
The resistor is designed to get hot. Three coils of resistance wire of varying length, hence each coil having a greater resistance. Using Ohm’s Law, (E=IR), the longer the resistance wire, the greater the voltage drop, heating the coil. The coil assembly is installed in the blower flow path to dissipate this heat. Also, notice in the video, the maximum speed contacts were the only contacts damaged. This switch setting bypasses the coil assembly and full motor current passes through these contacts. In a series circuit, this voltage drop reduces the voltage applied to the motor thereby slowing it. I’ve witnessed the coils glowing red and suffering no damage other than a slight increase in resistance.
Replacing the switch only would result in the customer returning with the original complaint.
(E=IR, E=voltage, I=current, R=resistance)
This explanation help?
Dan
Good job Eric! Been there, done that on a 1976 LTD 30 years ago. The circuit is basically the same; Ford just added a relay to the circuit, probably for safety (different story, different day). The motor IS the problem, no doubt, the bearing grease is gone. As the motor runs, the bearings start to heat up, start to seize and slow the motor armature. The armature slows, the motor’s internal resistance decreases causing an increase in current as the motor attempts to run at its designed speed (E=IR).
The speed control switch contact area is very small. As the current draw of the motor increases, a point is reached where the contacts start to arc. Pitting occurs and the resistance increases between the contacts. This increases the voltage drop across the contacts and it all starts to heat up. Eventually, the plastic melts and the switch consumes itself.
In this case I would have changed the relay too. The entire circuit was stressed and the relay was subjected to high current loads also.
Always dirty,
Dan
Good job Eric! Been there, done that on a 1976 LTD 30 years ago. The circuit is basically the same; Ford just added a relay to the circuit, probably for safety (different story, different day). The motor IS the problem, no doubt, the bearing grease is gone. As the motor runs, the bearings start to heat up, start to seize and slow the motor armature. The armature slows, the motor’s internal resistance decreases causing an increase in current as the motor attempts to run at its designed speed (E=IR).
The speed control switch contact area is very small. As the current draw of the motor increases, a point is reached where the contacts start to arc. Pitting occurs and the resistance increases between the contacts. This increases the voltage drop across the contacts and it all starts to heat up. Eventually, the plastic melts and the switch consumes itself.
In this case I would have changed the relay too. The entire circuit was stressed and the relay was subjected to high current loads also.
Always dirty,
Dan