Why do my thermostatic electric fan switches keep failing?

The subject of coil suppression is interesting and quite complex. There are 2 broad areas of concern when using a mechanical relay. From the standpoint of electronics, suppression is used to protect whatever electronic part (transistor, FET, switch, etc) is used to turn the relay on and off. From the standpoint of the relay contacts, coil suppression can encourage arcing, burning, and welding. Those two design goals are at odds with each other. Then there is the problem of switching an inductive load (fan motor) which adds inductive kick as the contacts open. It might be that the contacts themselves would benefit from a reverse diode when fan switching is involved. Here is some extra exciting reading:

www.cbxelectronics.com/pdf/app-notes/app-note1.pdf

-L

Nice read! Thanks.

I've been using the mini relays found in newer cars to switch my fan. However it seems like it draws a lot of current, I may upgrade wiring and relay to diode type and see if that helps any. The voltage drops to 12.0v even, when it comes on and I have 100amp Denso alternator that has been rebuilt a couple years ago. The relay I'm using looks like this and has 40a stamped on it.

lummert said:

Hasn't anyone thought of using a fan switch meant for VW/Audi? They come in 2 and 3 connector varieties.

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Tell me more, what temps do they come on/off? Or maybe suggest a year and model of vw/audi and ill search. I need to find a better replacement for mine. Po had saab t with the stock "oh ****" sensor that comes on at 210 so i quickly replaced that with a mech fan sans shroud and ambient temps are rising enough that dont cut it anymore in traffic

Canadan said:

Tell me more, what temps do they come on/off? Or maybe suggest a year and model of vw/audi and ill search. I need to find a better replacement for mine. Po had saab t with the stock "oh ****" sensor that comes on at 210 so i quickly replaced that with a mech fan sans shroud and ambient temps are rising enough that dont cut it anymore in traffic

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O Reillys has the Saab switch for the T at 89C about 192F

Awesome! Thanks a lot.

It's been a while since my fan switch has worked. The Ron Francis switch eventually failed like the others. It lasted longer but suffered the same fate, suppression relay and all. I think the bimetallic switches just can't handle the heat. Factory electric fans use potentiometers, no? Maybe they tolerate heat better. I put in one of those adjustable switches with the probe that goes in the radiator fins, we will see how this works out. It looks kind of hokey, but the last switch of this type I had worked very well.

I've used a stock BMW switch in a tee that I put in the lower radiator hose. Has worked flawlessly for me so far.