electric fans ?

it would really help to know what type of fans they are, to learn what kind of juice they are going to draw.
i will lay out the easiest way i can think of to do it. im sure others will disagree, but based on the limited info, this is how i would (at least initially) wire it.

get two Bosch style SPDT relays. stereo shops should have them, or online sources.
wire up the first relay:
30 fused power from battery
86: 12 volts from a key on source if you want the fan to kill with the key off, or from terminal 30 if you want it to always be able to be turned on.
85 connect the feed or "accessory" wire from your switch.
87 wire to the fan positive

inside the car, connect the other end of 86's wire to the feed or accessory prong on the switch. connect the other switch's terminal to ground.
when the switch is flipped, ground is sent to the relay, thus energizing the relay and fan.

use the second relay if you want to have the fan come on with the a/c or if you want to have the fans powered seperately. that might not be a bad idea, as the relay may be close to its limit with both fans running through it, depending upon the draw of the fans. also if one relay should fail, the other fan would keep going. to do that, just wire up the second relay the same as the first one. feed each fan (87) with each relay. make sense?


you could also get a controller to turn the fan(s) on and off at set temps. you could also have your toggle for manual intervention. it is a little more complicated to do it this way, but i like the idea, personally. i would run the fans sequentially, so that one comes on early, and then should the a/c be turned on, or the car continues to get hotter, the second fan kicks on (progressively).

but that is a little more involved. for just a toggle switch, this should work. it is simple. i hope that helps (it should atleast start another debate thread. LOL). there are other relays that could be used also, but they are harder to find. this is the easiest cheap way to do it that i can think of, assuming the fans dont draw too much power. upgrades later are easy. good luck.

oh, and about the diode- i think you are referring to the freewheeling diode for the time that the fan goes from a juice sucker to a generator. i would not worry about it on this set up. i dont think? it is necessary - i have never used one. on a million amp fan like a mark 8, it might matter but for what you are rigging together, i dont think so.

for wiring gauge (AWG), i dont know for sure, so i tend to overkill. the wire from the battery to the relay should be fat. it would depend upon draw. i would say go with somthing like 8 gauge (i know it is likely overkill, guys. playing it safe). you need to run a fuse on this wire, but you may have to guess. i would start smaller and if it blows, go up. just my thinking.
also run a fat wire from the relay to the fan positive. use a fat wire for the relay ground too.
the rest of the ancillary wires to the relay and at the switch, you can go smaller. they are just triggering the relay. a typical Bosch style relay is normally rated for 30 amps, though you dont want to run it near that for too long, and they tend to draw about 125 mA or so (0.125 amps). nothing at all. a paperclip could support that.
switch can be of your choice. it does not have to support current (just ground, and marginal at that). that is why the relay is nice (plus you dont have a wire with 30 amps or whatever running through the firewall - a short waiting to happen).

that is what i can think of (again). good luck.

10g is what is recomended for use with the Mark 8 fan. 12g should be fine for the fans that he has. 8g is much to thick, and will just make wiring more difficult since it's very think and harder to manuever

Actually, fine strand 8 gauge is very flexible/easy to route. It's also very expensive. Wouldn't you know it.

CPB - it'll take a switch designed to stand up to a lot of current to simply switch the fan on and off. As suggested above, a better approach is to use relays to switch it. If you're not familiar with how relays work, either get someone to help that knows them, or using some jumpers, a battery and a test light, you can hook up the terminals to see how they function. The terminals are numbered usually in a consistent way - that's what Hissin's referring to above. 16 or 18 gauge wire will be fine for your switched circuit. I'd use 10 gauge wire for the longer run from the battery to the power terminal on the relays. Then 10 or 12 gauge ought to be fine for the relay/fan grounds, and the hot lead out of the relay connecting to the fans. And, unless it's purely a race car, I wouldn't put it on a toggle. It's too easy to get distracted and overheat one. I'd put a temp. switch in the system somewhere and let the temp switch turn the relays on and off at preset temps. Also, the start up current to get the fans spinning will spike at something as much as 2X the steady state current draw for a few milliseconds when you energize the fans. So you really need a slow-blow fuse for your power lead.

I had a set up that triggered twin fans separately using relays and 2 different temp. switches. While it worked well, I found a better mousetrap. I just replaced it with a really slick variable current control device that can infinitely vary fan speed to hold to an adjustable temperature - good for control within +/- 3F. Here's the site: http://www.dccontrol.com No more relays, no more 'full throttle/full brake' fan control, no more HUGE start up current spikes. It's all digital control in one little unit that's about the size of a deck of cards. And easier to wire up than relays/switches, etc. Highly recommended.