Hi Forum,
i am looking to hook up a 2nd evaporator in my truck, the Dash A/C is all automated and works ok bu this will be an additional evaporator in the rear cab.
I would like ti hook this up in a fairly simple method which is manual operation from the SPDT rocker type switch from the centre console for those ultra hot days...
i am seeking the switch to have a low / off / hi operation (12v DC fan ), with High being full fan capacity and lo ~50%, I am not looking for variable control just a 2 speed and off- how best to approach this??

 

Depends. What is the normal running current at 12 V?

ak

 

The usual method used to get multi speed fan motors was to fit a resistor block in series with the motor.
High speed bypassed the resistor block and fed the motor with full voltage from the switch. The lower speeds fed the motor through different parts of the resistor block.
The resistors were positioned in the air flow to cool them down.
Try a search on eBay Motors for "heater resistor".

 

A Google of Blower Motor Resistor should get you started. As K1W1 mentioned they come in assorted designs and flavors but all do about the same thing. Since you only want a High and Low it is just a matter of High the blower is directly to 12 Volts and any slower speed just involves placing a resistance in line, the value of which will depend on your blower motor's current draw. Usually it will be a low resistance around 10 to maybe 20 Ohms but a high wattage like maybe around 25 watts. If you look at some blower motor resistor blocks you will see many just use several turns on nichrome wire for resistors. You also need a switch capable of handling the current (or a relay).

Ron

 

The fans I've messed with typically used less than 10 amps. Radiator fans (dual) are usually operated on a 20 amp fuse, so I wouldn't expect a single fan to require anything more than 10 amps. So a switch rated for 10 amps should do nicely. As you said, you're looking for a SPDT Center off switch. They're easy to find. One of the two legs is wired directly to your fan while the other leg is wired to your resistor others have told you to use. I think you have all the information you need. I'm just wondering how you're going to have a second evaporator and not have any serious effect on your factory original evaporator. But I'm sure you have that part figured out.