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Hi Walter - thanks a million for the input - you've given me LOTS to think
about. I'll start with pointing something out though:
My intention is to produce plans for a fan sequencer which anyone au fait with a soldering iron can build - and save quite a lot on the price of a Fanzilla. I'm building this for myself first and foremost, but I don't care if anyone else wants to copy it - the design is that simple. I won't be offering to build it for anyone else because my time's worth a bit too much and the Fanzilla's your best bet because of the guarantee. Yes the wiring for the fans still has stacks of inherant flaws, but this eliminates the two largest ones by a long way. I also prefer the idea of keeping as much of the car as original as possible because I don't want some future owner trying to work out what on earth I've done! Personally I'll probably tinker with my setup to provide an inhibit pulled off the full throttle microswitch which means the fans can never come on if I'm flooring the accellerator. Your point about the fans kicking in when the engine is off is definitely a bug.... now without my diagram to hand my immediate thought is to borrow the signal from the RPM relay to the fuel pump.... but I'd prefer an earth that is lifted when the engine runs..... any ideas? What you say about the melted bit immediately makes me think the Fanzilla may use resistors or diodes to detect a failing fan. This is one of the approaches I'm trying to avoid. The design I have in my head would make sequential shut-off very easy to achieve, though I think it's guilding the lilly a bit! The circuits are identical - but one has a longer timer in it - and that timer works both ways round - on startup and shutdown. BUT - (and I'm thinking therefore the fanzilla uses the same system) you can't have it both ways - at least, in this simple system - the fans will either shut off sequentially OR shut off immediately when the engine's turned off. The signal "given" to the module is the same either way - unless I pull a signal off somewhere else, which complicates things. I'm trying to keep this elegantly simple. If you or anyone else wants to go into more advanced circuit theory, I love playing with control logic - I'm using it extensively in my remote start/door launcher plans. And the beauty of logic is if you can wrap your head around its operation, you can build some wonderfully complex systems for a few quid/bucks. I don't agree that you need two fan fail lights to be truly "safe". All the light tells you is "there's something wrong". The system MUST not detect a difference between the two fans, but rather that there's a change in the load on one of the relays. I think that the stock sockets are up to the job of running the fans personally - as long as connectors are kept clean, a little warmth in this environment is okay IMO Anyway, I'm enjoying designing these solutions - it's a welcome return to "real electronics" given I've done none in the 3 years since finishing my degree :-) Martin
Walter Coe wrote: 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 them this 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 on mine 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 would also need separate fan fail lights. One other bug to fix that the FanZilla has 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 a few 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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