The diode clamps the spike caused when you open the circuit on an inductive load like a motor, relay coil or solenoid. As the magnetic field in the inductive load collapses, it creates a voltage spike opposite the polarity of the voltage that was first applied. This voltage spike has to go somewhere, or it can possibly damage the electrical system. The reverse diode clamps the spike and passes it back to ground, where it dissipates harmlessly.
If you don't have a high current alternator, you can forget about using the fan. The stock 65 amp alternator isn't big enough to run the fan and the rest of the car.
Switching a fan on and off manually is a bad idea. Too many guys have been distracted (hot girl kissing on their neck, too much to drink, dog tired and not thinking clearly) and cooked things because they forgot to flip the switch. An equal number have forgotten to turn the switch off for the same reasons and run down their battery.
The best fan controller available today is a DC Control unit.
www.dccontrol.com. Cost is about $110. Be prepared to wait 4 weeks or more to receive your controller once you have sent in your payment. The controllers are custom made in small lots and lead times can stretch out.
Next best is a SPAL controller - $70-$90 See
http://www.spalusa.com/fans/automated/accessories/fanpwm.html. Ebay will have the controllers for the $70: do a google search and see what you find.
At the bottom are the Hayden or Imperial controllers available through Advance Discount Auto Parts and AutoZone. The non adjustable one is about $30 ( Hayden 226206) and the adjustable one is about $60 (Hayden 226204). I recommend you do a google search on Hayden and the part number for more info.
If you are good with electrical stuff (90% of the people here aren't), build your own controller. The numbers on the diagram (#86, #87, etc) refer to the numbers on the bottom of a typical automotive relay.
If you are an experienced electronics tech or electrical engineer, email me and it will send you the prototype drawings of a fan controller that is probably as good as the DC Control unit. It is a build it, troubleshoot it yourself item. I will not build or troubleshoot units, so it is not suitable for anyone who isn't really good with electronics.
Alternate placement for a temp gauge sender or temp switch/temp sensor for an electric fan. Use the heater feed that comes off the intake manifold. Cut the rubber hose that connects the manifold water feed to the heater and splice in a tee adapter for the temp gauge sender. Be sure to use the same water feed line as the ECT sensor. That way you will get the most accurate temp readings.
Tee adapter info:
Make a pilgrimage to your local hardware or home supply center and get some copper pipe and a tee that fits the temp gauge sender. Solder two pieces of copper pipe onto a copper pipe tee with threads in the tee part. Find the correct brass fitting to match the temp sender threads to the tee fitting.
