I managed to smash that sensor that sits down at the bottom of the rad on the driver's side. It's the sensor that has two prongs out the back and there are two wires that connect to the prongs...

I poked around in the 88 FSM a little, and I think that the sensor is called the Radiator Switch Connector. The diagram (3-14) leaves me uncertain.

Is that what this sensor is (I have to replace it) and, also, what does it do? The car doesn't seem to be struggling without it. Not yet, anyway.

 

It turns the radiator on and off

Its your thermo sensor or maybe it isn't. Leave it to Mazda to move these around from year to year and call sensors switches and call switches sensors......

In case you run to the dealer for it.
And I hope this is what you are talking about:
from mtrix:
18-8400-FE85.....SENDER WATER THERMO...86-88 BOTTOM OF RAD....$ 44.54
And autozone.com says this:
Temp switch-- under hood, center, front engine area, mounted in lower driver side of radiator $15-$40 depending on year/auto/a/c/turbo/NA......

 

Thanx.

Turns out its a temp sensor for the idle. She starts fine (the water thermo sensor under the alt looks after start-up) but idles like crap (and stalls) when she's cold. The ECU doesn't know what temp the engine/coolant is at, and so switches to Limp mode. The A/F mix is set for Hot. So no remote starter for me when she's cold - she won't hold idle, and the R/S will just kick the starter motor's *** by re-starting the car over and over (4 times.)

I'm tracking down a used one. Should hopefully have one from a 7 mechanic friend 2morrow night. I'd be surprised if he asked for more than $10 (Cdn) for it.

 

Dude, don't confuse people. None of the temp switches and sensors moved at all from year to year.
The ECU's coolant thermosensor is in the back of the water pump housing, the electric fan thermoswitch is on top of the thermostat housing, the temp gauge sensor is underneath the oil filter the coolant level sensor is in the middle of the radiator's top tank, and the coolant temp switch is on the LHS of the radiator's bottom tank.
That temp switch is closed above ~17degC (63degF) and is part of the secondary air injection system. I believe it also controls the 3000rpm start-up idle too.

Amur, make sure your TB's thermowax system is working properly. This raises idle speed when the coolant's cold.

 

So what IS it?

 

The switch he's talking about is the water temp switch (bottom of the radiator).
It isn't necessary if your emissions are capped off/taken off what have you. I *think* the sensor at the bottm controls the ACV. The water thermo sensor is VERY important in all aspects though. Don't get the two confused . I don't know what the other guy was talking when he's saying Mazda moved temp sensors for different years. '86-89 temp sensors are in the same locations. Check your water thermo wax. Start the car and watch if the rod extends out all the way.
Not sure what you mean by the thermo sensor look ok after start up. You need to heat it up and check if it's within spec. (multi meter). It doesn't move.

 

Not 'looked' okay. It *was* okay. I smashed the damn thing last winter and the car went into Limp mode and so every start after that was a Hot start. And since it was the middle of winter, she didn't start at all. Took a few days to find a replacement.

The car started fine yesterday, so that let me know that the recently smashed water temp switch doesn't participate in start-up. The crappy idle let me know what it *was* supposed to be doing.

I had put up a bunch of questions in my original post, and was trying to answer them with what I'd learned in the time since. Seemed like good Forum behaviour.

 

Like the man said, the water temp switch located on the bottom left side of the radiator controls whether or not your car does the 3000rpm for 17 seconds start up and whether or not the Relief Solenoid is activated or not. If the plug is off the switch, the Relief Solenoid fails closed which in turn dumps the air from the air pump overboard resulting in the inability to pass emissions. Either wire off the sensor also wipes out the 3000rpm for 17 seconds. Repeated myself, didn't I.

In my case, the switch being disconnected never effected my startup or idle. Ever. Just me I reckon. But theres no reason I know of for its removal to effect the idle at all.

 

I understand. Regardless, this is how she is behaving... Maybe something else has failed that I don't know about yet.

 

Like I said before, check your thermowax.

 

OK, there's a bunch of confusion here. Here's the skinny.

The '86-88 radiators have a temp sender at the bottom of the radiator that 2 wires connect to - it's at the bottom corner, driver's side, of the radiator.

This sensor's only purpose in life is the 3000 RPM startup - THAT'S IT. Most '86-88 cars I work on I unplug the sucker - the car will rev up to about 2000 RPM then settle at 1500 RPM when you start it cold, no 3000 RPM startup.

The '89-91 cars don't have this sensor - I believe the ECU uses the main water temp sensor on the back of the water pump housing to determine if it needs to do the 3000 RPM routine.

The main ECU water temp sender is on the back of the water pump housing - it's a green sensor with a green fuel injector style plug going to it that has a metal clip. This is the main sensor for the fuel injection system to richen the mixture when the car is cold - if this is bad/broken, the car will have a very tough time holding a cold idle. All 2nd gens have the same ECU temp sender, and it's in the same location with the same plug going to it.

Anyhow, that's a little water temp 101.

Dale

 

I'll have a look 2nite. Maybe the ECU temp sensor is out. I'll try to check out the thermowax, too.

Thanx for all the input!

 

I'll bow down low at the waist to DCFC3S, and say *disregard anything I said about the Relief Solenoid* in regard to the water temp switch.

A trick question here. Is the water temp switch made or not made at running temps?? Serious trick question.

 

If you still need a sensor, I have one for $10.

 

I will sell mine for $9.99...

 

OK. Mines now $9.98 delivered.

 

$9.97.......lol!

 

You're both out of luck. I picked one up half an hour ago. He didn't want anything for it. I also dropped off some 7 411 and talked about the car while I was there. No time left to throw it in. I'll get it 2morrow and report on the results...

 

Glad you got one. Brand new they run about $50.

 

Answered above...

 

Ok NZ, since the switch is made when the engine is hot and an open circuit when cold, why is it when DCFC3S pulls the plug off the water temp sw the car does not do the cold start routine???? Its already a open circuit so pulling the plug should have no effect. I've done the same thing DCFC3S said he does all the time. The cold start will not happen if that is done.

While I just did eat my words in the my post above this one, I KNOW that that switch does effect the Relief solenoid because I just checked it last evening on a *cold* (its 95 degrees outside) car.I pulled the plugs off the water temp sw. The Relief solenoid shut off and let the acv dump air overboard. Later after the car was fully heated up I could pull the plugs off the water temp sw and and the Relief solenoid now would stay energized and of course air did not get dumped overboard.

 

All the water temp switch on the bottom of the radiator does is go closed circuit when the radiator is cold, and open circuit once the car warms up a bit - the temperature where it switches open is pretty low.

So, when you unplug it, the circuit is always open. This will effect the ACV at COLD start, since the ECU uses the input from the water temp switch for going to cold emissions mode - as in, start getting the cats warm as fast as you can by dumping air into the cat and stoking the fire.

If you're in a state with emissions testing, I seriously doubt you'd have a problem with the sensor being unplugged. They only test when the car is warm and at operating temperature. Mazda had to jump through some hoops to comply with federal cold start emissions - that's why we have pre-cats and the 3000 RPM startup. And it's proven that you can pass emissions without either, at least on the tailpipe side, not necessarily the visual.

Long story short, with the switch unplugged the ECU just thinks the radiator is above like 80-90 degrees or so all the time. That's it. It's a REALLY simple switch, not an input with fine resolution like the ECU's water temp sensor that changes resistance with temperature.

Unplug the sucker .

Dale

 

Looks like 2 conflicting answers:

NZ says it is closed at 17 degrees C (and up I assume)

dcfc3s says it is open when car warm

from FSM, looks like NZ is right. But who knows ??

so, what's the verdict ??

Hugues -

 

Ok DCFC3S... Theres my problem. Its just too damn simple a circuit. Open/closed. See, I went out to my fully warmed up hotter than a pistol car and took the connectors off and read .2 ohms across the switch. Now I see where you say it goes closed when cold.

In the past I've been tempted to answer those posts on how to get rid of the 3000 start up by answering *pull the one or both connectors off the water temp switch* but didn't because I've seen what I call a glitch with the Relief solenoid.

While its a simple circuit, just remember we're dealing with my simple mind! I'm gonna double check that switch tonite as to whether its open or closed when cold. I must be doing some thing wrong but I can't put my finger on it.

I took that switch out of the radiator last nite and put a meter across the pins. I read continuity. I put the switch on a ice cube. The reading went open. Ah HA says I. OPEN WHEN COLD! Heat it up with body temp and I get continuity. CLOSED CIRCUIT WHEN WARM!. See why the confusion?

 

Hmmm....now you got me thinkin'

Just looked at the shop manual (from fc3s.org) - according to the shop manual, the water temperature switch is open when cold, and closed when hot - just like you found out. That doesn't make logical sense to me why it would disable the 3000 RPM deal when unplugged.

During my lunch break I'll dig into my paper shop manual at home and look at the circuit diagram and report back. You got me curious now .

Dale