It's possible to do either 2 speeds or a variable speed thing. DC motors will run fine at lower voltages. It actually is easier on them than full voltage.
The air a fan is basically a squared function of the speed. This means that at 1/2 speed you would get about 1/4 of the airflow. So, you would probably want to drop the speed maybe 15 to 25%.
The cheap way would be to find a small value large wattage resistor. Start with something around 3 ohms and around 20 watts. Put this in series with the lower level switch to the fan. Get a ceramic one and mount it to the side and in front of the rad where it will get airflow since it will likely run hot. If you know the running current of the fan, I could give a better guess on the resistor to try.
If the fan runs too fast, get a higher value resistor. Too slow, get a lower value. You can get variable high power resistors, but I don't think the ceramic variable resistors should be used under the hood of a car. You will probably have trouble with the wiper corroding when water hits it. Look in electronic surplus type stores for resistors. You can probably find them really cheap if you scroung around.
Connect the second temp switch across the resistor and first temp switch to provide full voltage to the fan. This also has the second bonus of acting as a back-up temp. switch in case the first one fails.
Search for posts on electric fans. There was one a little while ago that mentioned an electronic controller that did variable speed control of the fan. The controller is expensive, but it will work.
You could also build a circuit with a transistor, thermistor and some other parts to make your own simple variable controller. The transistor would have to be high powered on a fair chunk of heatsink to work though.
Peter
The air a fan is basically a squared function of the speed. This means that at 1/2 speed you would get about 1/4 of the airflow. So, you would probably want to drop the speed maybe 15 to 25%.
The cheap way would be to find a small value large wattage resistor. Start with something around 3 ohms and around 20 watts. Put this in series with the lower level switch to the fan. Get a ceramic one and mount it to the side and in front of the rad where it will get airflow since it will likely run hot. If you know the running current of the fan, I could give a better guess on the resistor to try.
If the fan runs too fast, get a higher value resistor. Too slow, get a lower value. You can get variable high power resistors, but I don't think the ceramic variable resistors should be used under the hood of a car. You will probably have trouble with the wiper corroding when water hits it. Look in electronic surplus type stores for resistors. You can probably find them really cheap if you scroung around.
Connect the second temp switch across the resistor and first temp switch to provide full voltage to the fan. This also has the second bonus of acting as a back-up temp. switch in case the first one fails.
Search for posts on electric fans. There was one a little while ago that mentioned an electronic controller that did variable speed control of the fan. The controller is expensive, but it will work.
You could also build a circuit with a transistor, thermistor and some other parts to make your own simple variable controller. The transistor would have to be high powered on a fair chunk of heatsink to work though.
Peter
