HEATER/DEFROST SWITCH 


Q: 

I have a problem with the heater/defrost switch on my 1957 Corvette. I am trying to find out what the wattage and resistance is for the resistor that is connected to the connecting pins mounted on the switch shaft for the heater/defrost switch. It is basically burned into now and I am trying to replace it. I can run the heater on low speed and the resistor seems to be normal, but when on high, the resistor will get extremely hot and I would think dangerous. It has already melted a hole in the plastic housing it is mounted to. I need help! Thanks.



A:

I am not sure exactly what is happening with your heater motor fan switch as the description is not quite as I would expect it to be.


The fan motor is designed to operate in one of two speeds at the request of the operator. It is a simple 12 volt electric motor, and if operated at 12 volts would operate at high speed.

In order to provide a low speed, the Corvette electrical engineers added a simple resistor to the circuit. This required a switch with three positions. Those positions are off, low speed and high speed.


In the off position there would be no voltage applied to the motor. In the high speed mode there would be a full 12 volts applied to the motor through the switch. In the low speed position, the switch would route the electrical voltage through the resistor, and then to the heater motor. Because the resistor is located in the circuit before the fan motor, and because the resistor creates heat, that drops the voltage in the circuit. That lower voltage being applied to the fan motor causes the motor to run at a lower speed. The motor is now running at a supplied voltage of about 9 to 9.5 volts, and the motor runs slower because of the lower voltage.


You write that you can run the heater at low speed and the resistor seems to be normal. When you say “normal”, do you mean that it is hot to the touch, or do you mean that it is at approximately room temperature? It should be very hot to the touch when the fan motor is running at low speed. You will burn your fingers if you touch the resistor when the motor is running in the low speed position.

You also mention that the resistor has already melted a hole in the plastic housing. This is probably because the resistor was placed up against the plastic housing at some point. The resistor is positioned slightly away from the plastic housing, and covered with a white ceramic coating to dissipate heat. It shouldn’t melt the plastic unless it has been pressed up against the housing. Sometimes the ceramic coating has been broken away, and the heating coils are pressed up against the plastic housing. If this happens, the housing will melt.


Another thing that can affect the operation of your heater switch is corrosion on the contacts inside the switch. That added resistance and heat build up can also burn a hole in the plastic housing from the inside.


If you are careful, you can bend open the metal tangs that hold the switch together, and open the switch to check the contacts inside. You may be able to clean them with sand paper or a wire brush. Similar to the drop in voltage due to the resistor, corrosion in the contacts adds resistance and therefore generates heat. That increased heat, caused by added resistance due to corrosion in the points inside the switch, can cause a drop in voltage which can lead to problems similar to the ones you mention. The added heat can also damage the switch itself and cause the blower motor to run at erratic speeds.