Automotive Heater Fan Switches

Could be any number of reasons. For example:

• The low setting contacts are used more often. And you have to pass through the low connections getting to the high and then back again to off.
• The switch may be poorly rated for the DC voltage / current it must carry.
• There are other electronics involved in reducing the fan speed and those have failed.

And so on. That's the marvelous thing about all things electrical! So many unexpected ways to fail. It's like a candy-store full of exciting possibilities! And new flavors come along almost daily.

With ultra-high... no dropping resistor at all. Thus if the resistor pack fell out, you still had fan availability. I once had a Honda Accord, and half the fan speeds did not work so I acquired some Nichrome wire and made my own dropping resistors. This worked for many years until I was in better financial condition and bought the real thing from the Honda people. Interesting thing... they used the airflow of the fan to heatsink the heat from the resistors. One of my nichrome resistors got red hot without air moving over it... but it worked.

Bill

I would imagine that you are asking about an older vehicle heater, since there is a big difference between a heater that uses a blower motor resistor, than a system that uses a computer controlled automatic heater system. In an older one with a blower resistor, most of them are wired so that the current must travel through the resistor on the low fan settings. On the high fan setting, the current will flow through a high speed fan relay, so that the blower motor receives full system voltage.

Now if the system is an automatic climate control system, the HVAC computer will most times have control over the motor. Most of these systems with control the blower speed by pulsing the current to the motor.

If you want to know how a specific system works, print out a wiring diagram of the system and study the diagram.

Either way they are changing the voltage to a DC motor which changes the speed. The computer pulses the voltage so fast that the motor is noticing voltage change. You can graph the voltage change with a scope meter. I think it could be noticed with a DVOM but am not sure as I haven't tried. These units still have a fan controller to tell the computer when to change voltage.

Ford is using this idea to control fuel pump motors instead of using a pressure regulator to bypass so much fuel.

http://www.alldata.com/ This web site sells a years subscription for your vehicle. The $29.95 fee is cheaper than a dealer visit. They have good one line electrical diagrams for your vehicle.

It's a resistor thing in most cases, they just seem to go bad after your vehicle goes over a 100,000 miles. It's happened to me on my Chrysler and Chevy.

Temperature sensors are "Themistors" normally called RTD whose resistance varies with temerature of the surface [ or the air if floating in environment].

The motor of fan is simply switched ON /OFF.

But you should should remember that all such controls has some HYSTERISIS ie the device if switched OFF @ say 30 deg C will not switch ON when the temp reaches 30 deg but will somewhat 2 or 3 deg below. And the same way in reverse.

On hi settings you can't feel the difference while on lo it is prominent.

Older fans used a stepped resister arrangement with a zener diode as the last resister. The resistor is cooled by the air stream supplied by the fan motor. On high, the voltage for the motor comes off the top of the string. As you drop the speed, you keep dropping lower on the stack. The likely-hood of failure is higher the lower the speed because you keep adding more resisters into the circuit. More parts more failures.

Hello All!!!

I currently have the opposite thing happening with the blower fan speed control in my wife's 2004 Kia van with approximately 90,000 miles on it. It works perfectly well on settings #1, 2 & 3, but not on setting #4 which is the highest speed. If I play with it and get the knob to rest in between settings #3 & #4, then I can get the highest speed on the fan going. Of course this has become a real pain in the arse to say the least!!!

It appears to me that the contacts are worn rather than having a blown resister. Am I correct in my assumptions? I may have to replace it myself. Even though it's still covered under our 10 year 100,000 mile bumper-to-bumper warranty I am resisting sending down to the nearest Kia dealership 70 miles away for repair, and hence having the vehicle tied-up for a few days! The local Kia dealership where we bought it 5 years ago folded a few years ago, which is what we didn't bargain for, so we're stuck in the muck!!!!

Question: Is this sort of switch an OEM product and only available from the dealer, or can I find a suitable aftermarket one to replace the malfunctioning one I have here?

TIA guys and gals!!!

bwrench,

Many thanks for the good response!!!

====CaptMoosie