EZ Heating System
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Fan Speed Control:

I decided to go the EZ and cheap way.  A resistor network.   I made my own ($0) after finding Toyota wanted $19 for one and a week to order.  I happened to have some nichrome wire sitting around.  Just hooked some up the fan  (12v->nichrome->fan->ground) with alligator clips.  By changing the length (moving the alligator clip) of nichrome wire exposed, the fan speed will change (more or less resistance). 


When a comfortable fan speed is found, cut the wire off, and wrap it around a screw driver to make a coil.  It took about 4” for slow speed, and 1.5” for med speed.  You might even try SS safety wire as I have used it for resistor bridges before for hot wire cutting.  Crimp the ends in some ¼” push on connectors and flox the network into the discharge of the fan to help keep the resistors cool otherwise they will quickly get hot and burn out.  Wire them up according to the diagram.  Overall I found the bilge fan only takes about 4 amps max, so you probably don’t even need the relays (cars don’t have them).  If I did it again, I would just direct wire them to a heavy duty toggle switch, or go to the junkyard to recycle a switch from a car.


Wiring Diagram



Resistance bridge


Bridge installed in fan discharge manifold



  Finally ready for wiring