Fan switch or controller?
 

Tonight I'll be diagnosing my "no fans when hot" condition. Before first start, I verified proper fan operation by grounding the wire that goes to the switch. I'll check again before deciding what to do next, but I think it's safe to assume that on a 25YO car the switch is the likely culprit.
So, I'm debating whether to stay with the old reliable OEM switch or go to a Hayden controller. I'm leary of going aftermarket due to stories of unreliability, plus I would only be needing the actual probe and controller as I already have a fused relay on each fan. Of course going aftermarket allows me to set a lower temp for the fans to turn on, although with so many Monsters using the stock switch, I wonder if that is even a concern.

One other thing. I want to wire up an over-ride switch inside the car. Is there already a wire going to the old ECU I can use. I know there is on the 1.8, mine's a 1.6.

Cheers. Brad

On the '90, the ECU did nothing (as far as I can tell) to the cooling system in terms of fan management. If the car has/had AC, the ECU did get in between the A/C on switch and the fan clutch (to manage the load maybe?). With A/C on the radiator fans (both the main fan and then the additional fan) would be forced on regardless of engine temp. The temp sensor on the engine merely grounds the solenoid that controls the main fan.

Instead of the Miata temp sensor I am using the old school "mechanical type" probe with the adjustment pot on it that grounds that wire. I can adjust it to exactly where I want it to come on.

Are you using the output of the original main fan power (connector with + & -) up by the driver side headlight?

No, I have thicker gauge wires and separate fuse and relay per fan, but all turned on via the fan switch. I just checked again tonight, my fans do come on if I ground out that wire.
Is your probe similar to the Hayden unit?