paw140

Has anyone changed their coolant temp sensor without removing the UIM? The FSM says to remove the UIM, but I don't think it's necessary. What did you use to get on the sensor? A 19mm open end wrench? I'm having a little bit of a problem getting something on it.

Also, on the back of the filler neck, the top sensor with two wires is the coolant temp sensor, right? There is another sensor buried below it, with one wire going to it. Is that one for the tempature gauge?

KZ1

with one wire is thermo switch it is black, greeen one is thermosensor. you don;t need to remove UIM at all. It si trinky to get too,. I used some needle nose, and wrench.

paw140

What does the thermo switch do?

paw140

Ok, so I'm guessing that the thermo switch tells the fans to turn on at 226F... at least that's what I've found from my search on this forum. Does the green coolant temp sensor control the fans at all? Doesn't it tell the ECU when to turn on the fans?

I'm a little confused about how the whole fan system works. Does the ECU control it, or is it just a thermo switch?

wonder1and

Semi...jack

Can anyone tell me where the Air Temp Sensor is?
TIA
-R

DamonB

I beleive that is the one screwed into the underneath of the UIM; all by itself about midway along the tract.

DamonB

oops

DamonB

The thermoswitch is the sensor the ecu uses to determine water temperature and then enable the fans. The ecu actually controls the fans through the relays behind the passenger headlight.

The thermosensor is what drives the temperature gauge inside the car; it has nothing to do with turning the fans on. There is also the coolant level sensor in the front of the thermostat housing which trips the coolant buzzer and light.

The Miata thermoswitch is a common mod. It is a drop in for the stock RX-7 switch and turns the fans on sooner. The electrical connector is different however so you must splice that onto your harness. I find it a much more elegant solution than the "fan mod" many use.

paw140

The top green sensor, the one with two wires, is the thermosensor, right (the one that the ECU reads)? And bottom black one with one wire is for the temp gauge?

KZ1

no no, there are three

Thermoswitch, turns on at 226F, that si it, it just turn on and off.

Thermosensor, just tell ECU what teh temperature is.

the other single wire which if I rememebr is on front of thermostat housing is for the gauge.

spurvo

OK, I'm looking in the manual (93) wiring section. Page Z-32 (section B1-C) shows a water thermosensor with two wires, green with a white stripe, and brown with a black stripe. This goes to the ECU input 3E, which looks for temp either being 20C or 60C (or 68F to 140F). The sensor is located in the water pump backside. As KZ mentions, this tells the ECU what the engine temp is.

Next, page Z-42 (section B-2) shows the fan circuits. It shows a water thermoswitch with a single connector to a black with red strip wire. This switch is kinda "dumb" in that it turns on relay #3 at 226 F (108 C) independently of the ECU control. This is a last resort kind of mechanism. This is why replacing it with the Miata switch helps.

The wire on the front of the housing is the low coolant sensor. It controls the idiot light on the water temp gauge that lights and sets an alarm when the coolant is low. The water temp gauge sender is under the oil filter. Does this help?

paw140

Thanks everyone! That clears it up a lot. So the 226F switch is the one that turns the fans on high, but the ECU still decides when to turn them on for low and medium settings... is that right?

Basically, my fans run all the time, no matter what temp the engine is. And my engine is also sometimes difficult to start when its cold. I was wondering if the two might be related, like the temp sensor is telling the ECU that my engine is always hot. I'm going to pull the sensor, hopefully tonight, and test it out.

paw140

Pulled it. Checked it. Works fine. Dammit!