Re: [doc] Technical Musings: DIY Fan Module (long)
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Re: [doc] Technical Musings: DIY Fan Module (long)



Martin,
 
Get ready to hear all the flack about rehashing a problem that has already been solved.  That aside, I always enjoy a good engineering challenge.  Switching the fans on sequentially is a good thing, but some will argue that it isn't necessary if the power to the fans is pulled from another source (such as directly from the battery.)  This is the way that John Hervey made his FanFix.  I'm not sure if he still makes themthis way, though.  Despite the simplicity of his design, I prefer the engineering expertise of the FanZilla.  For state-of-the-art fan sequencing, I suggest a circuit that will toggle between the fans, so that the first one to come on alternates between right & left.  That way both fans get used evenly and the heat load & cooling on the radiator is more balanced over time.  In addition to switching on sequentially, the FanZilla also switches them off sequentially.  I've never had the FanZilla apart, but I could unmistakably hear the relays clicking inside.  The case onmine has melted a bit too.  I'm not sure if this is from the module internally or if it is because it has been pushing against a warm wire from the a/c blower breaker.  The FanZilla's fuse holders are still underrated for the duty cycle they are put through.  I suggest something one size larger (in duty cycle, not amperage.)
 
You might say that my idea of alternating/sequential switching of the fans doesn't isolate the circuits like you want, but with some good engineering practices, this won't be a problem.  Besides, if you wanted it totally separate, then you wouldalso need separate fan fail lights.  One other bug to fix that the FanZillahas is to shut the fans off immediately with the ignition.  Otherwise the fans run for a few seconds after you shut the car off.  When people ask what this is about, I just say that the flux capacitor is winding down.  :-)  I would also wire the fans to not come on while the ignition switch is in the first position.  Otherwise when you are sitting in the car without the engine running but playing the radio, the low pressure in the a/c system will tell the radiator fans to come on.  Sure, you can stop this by shutting the a/c off, but if you are only going to be sitting for afew minutes, it is annoying for the fans to be running 100% duty cycle while the engine is off.  (This might also be the case if the otterstat activates the fans.)
 
Another problem with the FanZilla is that it is too much plug-n-play.  It uses the standard relay sockets to run everything from, but still these spade lug terminals are not very well suited for the duty cycle that the circuit requires.  It either needs larger spade lugs or (better yet) screw lug terminals.  But this would involve cutting wires & crimping connectors which is definitely not plug-n-play.  But what the hell, if you aren't technically minded enough to cut wires then you don't belong in the relay compartment anyway.
 
Walt    Tampa, Florida


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