You MUST use a relay for each fan winding (speed) you wish to control. The simplist and easiest to wire would be all fans either on high or off using a 30A relay (PM me if you want to control speeds separately - much more complicated). For this you put a 30A fuse between +12V battery and common relay contact (#30) using 10ga wire. Other relay contact (#87) goes to all the + wires of the fans (10ga wire). All the - wires of the fans go to ground (10ga wire). Relay coil pin #85 goes to ground (20ga wire). Relay coil pin #86 goes to the switch (20ga wire). Other side of switch (20ga wire) goes to ignition switch power (or a "+12V only when ignition switch is on" wire). Pin #87A is unused. The relay symbol and pinout is shown here: Single Pole Double Throw (SPDT), Single Pole Single Throw (SPST) Automotive Relays.

A flyback protection diode is strongly recommended if relay doesn't have one internally (so relay doesn't get burned out when fans turn off) between relay pins 85 and 86 (stripe end to pin 86). [Diodes are most often used across the coil to provide a path for current when the current path to the relay is interrupted (i.e. switched off, coil no longer energized). This allows the coil field to collapse without the voltage spike that would otherwise be generated. The diode protects switch or relay contacts and other circuits that may be sensitive to voltage spikes.]

You can tell if a relay has an internal protection diode by measuring the relay coil resistance - it will be less one way than the other. You must use an analog ohmmeter, however, as a digital one does not output a high enough voltage to forward bias the diode (so diode will be an open circuit and you will get same reading each way). Also note that most analog multimeters use reverse polarity on the ohmmeter scales. See here for another explanation plus a diode picture: Installing Relays or Solenoids(read this) - DSM Forums.

Alright just for perfect clarification
run 10ga wire with a 30a fuse from battery to #30 on relay
#87 goes to all "+" wires on the fans
all "-" on fans go straight to ground
#85 goes to ground
#86 goes to a switch---what is this one? what type of switch? Just an on/off switch like a toggle switch? and from there to a accesory wire?


Where does 87A go? or is it used?

spooln4fun said:

Alright just for perfect clarification
run 10ga wire with a 30a fuse from battery to #30 on relay
#87 goes to all "+" wires on the fans
all "-" on fans go straight to ground
#85 goes to ground
#86 goes to a switch---what is this one? what type of switch? Just an on/off switch like a toggle switch? and from there to a accesory wire?


Where does 87A go? or is it used?

Click to expand...

Yup.

Wires to fans + and - must also be 10ga [each bundle of + or - fan wires should connect to a 10ga wire: eg. the + wires on 1 fan connect through one 10ga wire to the + wires on other fan which connect through one 10ga wire to #87. And similiar for - fan wires to ground].

The switch is any type of on/off switch (eg. spst toggle) you mount in your dash so you can control the fan operation. The other side of the switch goes to a wire that only has power when the key is ON. That way the fans automatically turn off when you turn the key off regardless of the switch position (and when key is ON fans turn on when switch is ON).

87A is not used.

Good luck.
(Note also that I changed the relay link in post 2)

For future reference (people who come here on a search) there is a much easier way to add a fan override switch using the existant relays and wiring. See: http://www.dsmtuners.com/forums/showthread.php?p=151311484#post151319397