fan speed

I have a high amp 12 volt fan built into the amp rack in the back of my truck. It is a single speed unit. Is there a way i can use relays to make it into a 2 speed unit? I don't need it running full tilt all the time ... but i want to be able to blast it on full when needed .... can anyone help or point me in the right direction ?

instead of using relays you may be able to use a low high switch with resistors in it. that is what controlls your heater fan speed in you car, you might be able to find a cheap one at an auto parts store.

I just want to have 2 speeds ... one being full blast and one being maybe 50% to 75% of full blast ...

Do you have only 1 fan?  It would be a very simple task using 2 relays to run them in series for half speed and then switch them to parallel for full speed.   This is if you have 2 fans.

No room for a second fan

my suggestion to do this was to just go to an auto parts store and pick up a simple 3 terminal, two speed blower motor resistor (RU-40 will work great). obviously the resistor has 3 terminals, one is the out to the motor and  one is an input that lets the motor run at full speed and the other is an input the lets it only run at about half. using a standard relay, term 30 goes to 12v, term 87a goes to the pin that provides 1/2 power to the motor, term 87 goes to the other terminal on the resistor that lets the motor run at full speed. connect the 85 and 86 terminals accordingly to whatever type of switch you are going to use to trigger the relay. then connect your fan to the other terminal on the resistor.  the fan will run at half speed all the time until you triger the relay (however it is you plan on triggering the relay).

Thanks for the idea. I check with the local auto parts store and they have no clue what a RU-40 is ... i cant seem to find it on the net either. Does this resistor reduce the voltage to the fan or the amperage ? I have all the relays planed out ... just need to find the proper resistor

The resistor will drop both the current and the voltage.  As the current goes through the resistor, a specified amount of voltage will be dropped across the resistor.  With the resistor in series with the fan, this will increase the resistance of the entire circuit, which will reduce the current.

an RU-40 is a  part number from standard. they are sold through Carquest. don't go to an autojoke or checker  parts store. go to a real Carquest or NAPA independent parts store that hires real parts people and not minimum wage chimps! any real parts store can take the number i gave you and interchange it to their own, arrggghhhh!!