alternative 3-speed universal fan harness

[H2Ogbodies]

Wow....its been awhile since I been on the board....a lot of stuff has changed....! Well, if anybody is interested, I have been using a 3-speed relay harness for the Mark VIII fans and in reality this is overkill for the Motorcraft fans and will give any fan or dual fans the ability to run at 3 speeds.....consists of 4 relays in a bank, a resistor board and has only 6 total connections to be made. This is my way to provide a relay system that is automatic, parts can be easily replaced at any parts store (so no wait if you are on the road and something goes wrong.....just find a parts store.) This can even be used as a system backup...I'll post some pics here shortly....





This shows the basic layout-you only have 6 connections to be made....this relay bank allows current to pass through certain series of resistors to allow low, medium and high speed if AC is used. As you can tell, it's very similar to the way GM controlled blower motor speed settings only now I do it through a few relays to vary the voltage resistance. Included are 2 temp switches-212/202 and 228/218 for low and medium speed. When used wiht a Mark VIII fan, rarely will you need to go over medium speed but if for some reason medium speed via the 2nd temp switch isn't enough to keep it cool, you can connect two jumper leads I wired in so that when the 2nd temp switch goes to ground, it triggers high speed which bypasses all resistors in the board and runs 100% fan speed....this is nice if you have an AC delete application. If you are running AC, simply connect the AC main wire lead to your clutch coil or low pressure switch and you'll have AC bypass for 100% fan speed when the AC is activated. This harness uses GM hardware and 5-pin relays and operates in a manner similar to the factory Mark VIII vehicles where they had their own VSC for just the fan-varying the fan speed via different resistors which acted like switches altering how the fast the fan moved-not quite pwm but as close as you will get with a relay system and nobody else offers this. low speed comes on very quite and the fan eliminated spiking by gradually ramping up to 1 more resistor value fixed speed then a 100% bypass for high/AC use. I've been using them for quite awhile now so they've been battle tested-I've made a few changes over the last several months and they are pretty much bullet proof now. Let me know what you think!




Here's a close up of the relays...a 50a HD relay provides full speed power @ 100% fan speed while the other 40a 5-pin relays provide various switching operations triggered by the 2 temp switches. All have built in tamping diodes and resistors and are industrial grade-all wiring is also j1128 spec Delphi XLPE crosslink polyethylene and all connections are soldered and heatshrinked where possible.



Here's a close-up of the MIDI style 40a fuse I use with this kit-since the harness kit by design does not cause any inrush voltage spiking problems, a simple 40-amp fuse is all you need to maintain total system control. keep in mind if you are using an old fan or if you've bought a used one, if it has a lot of wear on it, operating current draw could exceed 40a very easily so keep that in mind too-a simple request for any fuse range up to 60a can be included-anything more than that and you have another issue going on....lol

[H2Ogbodies]

I forgot to mention-this kit can convert any fan or dual fan setups to operate @ 3 speeds-there are some fans that are wired a little different such as the Chrysler dual fans but the same thing can apply to those as well-just requires a slightly modified version of this harness. Sounds funny now but this harness is a culmination of 2 years worth of trial and error with both every type of controller on the market as well as various relay designs I've drawn up-this one is the most versatile, easy to install harnesses I've ever designed so far....and the best part imo is none of the parts are hard to get/find so no worries about future design change smaking this obsolete or company closures making them dis-continued either.

[GNon18s]

So the fans don't come on until 212 degrees via the temp switches?? I think the last time I wired up a pair of fans they were set to kick on at 185 and turn off at 170. Does that sound rite?

[H2Ogbodies]

That depends on what your engine requirements need to be-some like to run cool, others like to run 200 or shortly above-a lot of times emissions require running warm...tuning has pro/con affects as well.