I copied this from the NPM article I did awhile back, thought it would be more accessible and usefull if it was here as well.
Many people like to be able to run their radiator fans all the time while at the track or drag strip. Fortunately this is a lot easier than it might seem and can be accomplished very inexpensively and doesn’t require running wiring to the engine compartment or connecting new circuits to the battery. These instructions include wiring colors that apply to all trim levels of ’93 Sentra and NX’s and has info for both automatic and manual transmission cars. If you have installed aftermarket fans that use a separate controller, depending on how you installed the controller would determine how you would need to override that controller to operate the fans at your whim. Please feel free to e-mail me if you’d like me to help you override your aftermarket fans. Also provided are instructions on how to determine how to do this to any other year B or P series car if you have access to wiring diagrams, a Factory Service Manual, or are generally handy with a multi-meter. As a general safety precaution please remove your negative battery cable while doing any electrical work on your car.
The B13 Sentra/NX has different fan configurations. Manual transmission SR20DE and GA16DE cars came with single coil fans which operate at only one speed, and only one relay is used to operate these fans through a single circuit. Automatic transmission SR20DE powered cars came with 3 coil fans which operate at two speeds and three relays are used, using two circuits, while GA16DE powered automatics have 2 relays utilizing two circuits. I would guess that this is due to a transmission cooler coil that is either incorporated into the radiator or as an independent coil that is placed up against the radiator. In both cases, we want the fans to operate at their highest speed on demand via a dash operated switch as well as when the ECU calls for it.
Things you’ll need are:
1 – SPST (single pole, single throw) switch. You can use an illuminated one if you like, and you can wire it so that the switch is always lit, only when the switch is thrown or only when the fans are running, be it by ECU trigger or the switch. You can also power the switch light by using the interior lighting circuit. See diagrams and info below.
~ 15’ of 18 gauge wire.
3 – 22-18 gauge (red) ¼” fully insulated female quick disconnects.
2 – 22-18 gauge (red) ¼” fully insulated male quick disconnects.
1 – 22-18 gauge (red) ring terminal (you’ll have to decide what size the ring needs to be depending on where you’re going to ground it).
~ 10 – 6”-8” cable ties.
3 – 22-18 gauge (red) 3M wire tap.
1 – 10-12 gauge (yellow) 3M wire tap.
1 - IN4XXX series diode (Any one of these will do: IN4001, IN4002, IN4003, IN4004, IN4005)
Wire stripper.
Wiring crimp tool.
Notes: You could get two different colors of wire if it makes wiring things up easier, or use different colored electrical tape to wrap the ends of the wires to keep them straight. A third option, which is a good habit to get into when doing wiring projects in your car, is to buy some numbered bands that you put on the end of wires, then you can write up a legend as to what each wire does. 18 gauge wire is small but sufficient since the current draw you’re dealing with will be marginal, around 1.5 amps. I rarely use wire this small, except when operating small load, short time use circuits like relays.
Normally I would recommend soldering the wiring tap splices, however since the wires you will be tapping into are so small and current is so low, the 3M taps will work fine and will be much easier to uninstall at a later date if needed.
You can refer to the diagrams while reading the following so that things make sense. The GA16DE ECU is the same as the SR20DE ECU in regards to the fan relay trigger wires that we are looking for.
First you should get to your ECU wire harness behind the center console on the floor under the dash. If you refer to the ECU diagram you can get an idea where the wire you’re looking for is located. The wire you want to tap for a manual transmission car is the solid blue wire in terminal position #9. If you have an automatic, then you want the solid blue at #9 as well as the blue with a black stripe wire in terminal position #10. Make sure you get the wires at the correct terminal, there's more than one wire of the same color in the harness, and getting the wrong one could damage your ECU among other things. Use red 3M wire taps to tap into these wires. Then connect these wires to your switch.
If you’re doing this to a car other than a ’93 Sentra/NX, then you’ll want to get wiring diagrams or a factory service manual to verify which wire from the ECU goes to the fan relay(s) in your car. This is the wire that the computer grounds to operate the fan relays, and is the wire you want to tap into to ground through your switch. This wire can be found by looking at the ECU foldout in the back of the FSM. If you opted for an illuminated switch, you’ll need to tap into a keyed power source so that the switch light only operates when the ignition switch is on. Although I would ordinarily suggest that you run a wire to the positive battery terminal to do this, since the light only draws a very small amount of current and will only be operating when the switch is thrown, tapping into an existing wire in the car should be relatively safe. The best wire I’ve found to do this is the white wire with a blue strip that goes through your ignition switch. Rather than tap into the wire near your ignition switch, I would do so behind the fuse block under the dash. There’s a clear/black two wire connector that you can release from the back of the fuse block on the bottom outside edge of the fuse block that has the white wire with a blue strip and a brown wire with a red stripe. It’s harness E46 if you want to find it in a factory service manual. Again, these colors are only certain for a ’93 Sentra/NX. Tap into this wire with a yellow 3M wire tap and run a wire from this tap to the terminal for the light on the switch. Then ground a wire to connect to the switch and you’re done. Use the cable ties to hold all your wires in place.
If you want to do this to another year B or P series car, then you need to determine how many fan relays it has and what those relays do. You want to trip the fan relays that will provide the most airflow. Since these relays are tripped by the ECU, you can use the wire color on the fan relay(s) and a meter to find out what wire at the ECU does this. Then by sending a ground to that wire, you can trip the relays in the same fashion that the ECU would, without drilling holes, hacking into engine bay wiring where it can corrode, or running unnecessary wiring. Relays always have at least 3 terminals, although most have 4 or more. There will always be and ground wire for the relay coil and power wire for the relay coil. The other two or more terminals will be where circuits are opened or closed. The coil is what makes the switch inside the relay operate. So knowing this you can pull a relay out and figure out which two terminals go to the relay coil, in most cases the terminals have number designations and a small diagram on the relay showing you the normally open and closed positions, as well as what number terminals are where. Switching the ground on things is the preferred method, hence why the ECU grounds the fan relays, rather than send them power.
This is a picture of the bottom edge of the fuse block. Circled in red is the two wire connector that I mentioned and you can barely see the white/blue wire on the right side of the connector. You can see the clear plastic on the right, which is what you need to depress to remove the connector. Again, make sure the negative battery cable is removed before taking this connector out.
This is a fuse block cut from a car and you can see the connector better as well as the white/blue wire and the tab you depress to remove the connector.
Here's the possible wiring configurations you can use and how that will affect an illuminated switch. The black thing with the gray band at one end is the diode you'll need for that configuration. You can also tap into your Illumination wire instead of the white/blue wire, in which case the light will still behave as mentioned in the drawings, but only when the dash lights are on. You'd use a red wire tap instead of a yellow one, and you'd tap the Red/blue wire at your dimmer switch. The switch light will not dim along with the rest of your gauge/switch lighting. The reason it will not is because the dimmer regulates the ground for the lighting system, which causes it to dim. Since illuminated switches ground through the circuit they are controlling they don't have a seperate ground for the bulb. Since we are using this switch to open and close a ground circuit, we cannot use this circuit to power the bulb and ground the bulb through the proper wire at the dimmer.
The reason two wires are shown tapped is just to depict how you'd tap into both fan triggers for an automatic.
As a courtesy to fellow enthusiasts, PM or E-mail me if you’d like me to put together a kit for you which will include a hard copy of these instructions. I can also help you figure out how to do this for any car you own.
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