Cooling fan wiring and operation
 

About the only sensor left under the hood that's on the stock Miata harness now is the cooling fan thermoswitch. I've been looking into the fan wiring and wondering how I can eliminate the harness altogether to clean up under the hood. My understanding, based on the '92 wiring schematic, is that the thermoswitch grounds the switching side of the fan relay, closing the power circuit so that the fan turns on. Is this correct? I also see that there is a connection on the same lead to the ECU at pin 1R and the diagnostic connection, but I can't decipher how they interact with the relay switching.

Simply put: Can I just run a new lead from the relay to the thermoswitch and expect the fan to function normally? I'd like it to remain fused in the stock fusebox, and best I can tell this would accommodate that setup if it would work.

How do you guys with MS wire it up? My ecu does not have native control over the fan, and there's no input available that can reference a temp input, so I can't just program it internally to switch a ground.

with the ms3x i wire the NB fans independently to come on at different temps. the condenser fan also comes on with the a/c

Yes.

It's ground switched and those are just alternative means to ground it. In the diagnostic box if you jumper TFA and GND, the fan will (should) turn on. You would do this to test if you possibly had a bad fan relay or a bad thermoswitch and wanted to verify that the fan itself still works.

Yep. Just wire the thermoswitch in line between the relay and ground. Switch is normally open and will close at 207* to turn on the fan, and open to turn it off at 194*.

We basically do exactly what you've just said your neanderthal ECU can't do. :D

Great, thanks! And yeah, the next gen DSM ecu switched the fans (walking upright as it were lol) but I'm stuck with an external trigger. I've got the ability to monitor/switch based on other parameters, just nothing temp based.

And hmm, I thought the switch closed at 195...no wonder I can't keep temps below 206-210 when I'm beating on it. Will now look for lower temp switch.

(zack morris's phone)

Summit sells a switch like you are looking for based on temps

Thanks Brain. I knew I had seen one but I could not remember where.