Technical Musings: DIY Fan Module (long)
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Technical Musings: DIY Fan Module (long)



Hi all,

I thought I'd share with you my thoughts on the fan wiring system that I've been puzzling over for the past few days on and off. Here's what I've come up with (advanced apologies for those who already understand the system, but I thought it'd be an idea to explain things from first principles):

I caught myself wondering how the Fanzilla worked and how I could duplicate its function for my own car. I soon realised that the sequential engagement of the fans is really simple to achieve. So simple, in fact, that I reckon anyone with a bit of soldering experience should be able to build it, given the right components.

With the fan-fail bypass installed in your car, the fans are simply hard-wired to the relay that engages to turn them on. This relay is engaged under two conditions: when the engine reaches a certain temperature, and when the A/C compressor engages (which is does regularly when the A/C is on). It's the latter of these which cause the biggest headache. The reason for this is that the two fans draw a considerable amount of current on start-up (for maybe a second or so), and given the signal to the relay coincides with the A/C compressor engaging - let's just say I hope you're not drag-racing at the time!!!

There is a second problem with the system: The relay which powers the fans is a single 40A standard relay. The fans draw more than 20A each on startup. There are documented procedures for re-wiring the fan-fail-module's socket to let each fan have an independent relay. This is a good thing from an engineering point of view, but doesn't solve the "kick" problem, and doesn't give you your fan-fail light back.

The Fanzilla elegantly solves the "kick" problem by sequentially engaging the fans, with a couple of seconds pause between each, and initially a couple of seconds after the compressor engages. It is this functionality I wish to duplicate. It also replaces the fan-fail light function. Ironically, this function is the most difficult to achieve (I'm not trying to copy the Fanzilla here - I am merely reverse-engineering what it does to save myself some pennies!). I feel confident in providing a design for sequential fan engagement, but not to replace the fan-fail light function as yet (there are many many ways of doing this, but I'm not happy with any of them, because they are either not perfect from an engineering point of view, or too complex for a "hobbyist" to duplicate).

My design involves constructing two INDEPENDENT circuits - one for each fan, and each fan is driven by an independent relay (I believe the Fanzilla uses solid-state devices for driving the fans - this is a better system, if you can get it right, but I want to keep things simple and reliable for a "hobbyist" to build). Each one of these individual circuits uses only seven components - and only one has more than two legs! Add to this the relays themselves, and a protection diode, and voila!

Is this safe?

I work as a network engineer, and the words "single point of failure" exit my lips every day. I wanted to make sure that if something goes wrong with the board, you could only lose one fan (short of a fluke). You rely on the single wire to the original relay to turn the fans on, so from that point onwards, my design runs two seperate systems to control the fans.

My design will plug strait into the relay sockets for the fan relay and fan-fail module. You get two further relay sockets, and into these you plug a new relay for each fan (I found some neat relays in Halfords the other day which have a blade fuse socket as part of the case). The cost of the components (not including relays) is about $30 - and most of this is the cost of two new relay sockets! I got all my components from Farnell.

Would people be interested in this design? I want to build and test one myself before publishing the circuit, and I need to think further about how to achieve the fan-fail light. From a "pure" sense, the light must come on if one of the fan motors goes short or open circuit. The function of the light is not to check whether your relay has come on - which is all the "reverse" function of wiring the light to the fan output achieves.

Right - enough "blinding with science"! The circuit it simple, it's designed so you'd be hard pushed to bugger it up, it's easy to test, and should anything go wrong, you're far less likely to lose both fans than with the stock setup.

I'll have a crack at building mine next week.

Martin
#1458
DOC UK







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