Hey all,

Thinking about linearinzing my temp gauge, but after studying it out, I find out the following.

Once linearinzed the gauge indicates a much higher temperature at the cold tick.

The stock config:
1) Needle starts to move from middle at full open thermostat.
2) First hot mark is boiling point of pure water.
3) Second hot mark is boiling point of 50/50 antifreeze

If the gauge is working fine, does it tell me enough information to keep me out of trouble? People say flippantly the gauge is worthless, but I am not so sure. I see Mazda's reasoning. In my thinking, I should know that 3/4 reading is danger and requires a response. I have never seen the needle move above half yet. Can I be sure that this is ok? What is all of your experience?

TM

 

Not really. The gauge basically just has a "range" at which it sits in the middle (about 185-240). By the time it starts moving upward past the middle, the engine is already running extremely hot. Now, I'm guessing about the heat range, but I've heard of people hitting 250F and the stock temp gauge is just starting to move up from the center. That could be accurate or not depending on where their second temp gauge probe was installed.

I used to push the linearization mode a lot. However, I've changed my tune to aftermarket water temp gauge for mostly because installation is about 10x easier than linearizing the stock one

 

Just out of interest - for 2) did you mean the first hot mark is the boiling point of pure water at cooling system pressure? Because I've seen 210F (99C) on my aftermarket gauge and the stock gauge was still horizontal, nowhere near the hot zone.

 

I just installed an aftermarket gauge in the thermostat housing and that by far has been the best and cheapest "Mod" I have done.

It is very re-assuring to be able to see how warm the car really is operating. A solid number that changes as the car warms and cools.

If I were to rate "mods" on a scale of 1 to 10 I would give the new gauge a 9.

Mike

 

According to the Mazda literature that I have found, boiling point of pure water is 119 C at 0.9 bar (13 psi).

The hot mark (1st mark) is about 121 C.

What I am hearing is that this is just not reality.

What are the chance if there being a significant difference in temp from the various locations around the engine?

TM

 

i forgot who sells this, (i think its greddy but i cant find it anymore) but someone has an upper radiator hose w/ a bung for water temp sensor. when i saw this i regretted drilling through my filler neck. if i can find this product ill post it up for ya or if anyone else knows what im talking about can post it.

 

https://www.rx7club.com/showpost.php...0&postcount=36

 

>The stock config:
>1) Needle starts to move from middle at full open thermostat.

nope. OEM 180F T-sta wide open (and inernal radiator bypass loop closed) at about 200F. Gage doesn't budge from normal 'tll about 240+F. Stock fan trips are 221F, and 226F. Mazda allowed at least 10F rise above the 226F trip before gage starts to jump.

Stock gage is not linear ... stays at "normal" horizontal position from about 160F-240F

Linearized oem or added gages are best. But my simple mod (to oem sensor wire) gets stock gage moving up off normal at 225-230F, for proper early warning:

https://www.rx7club.com/forum/showth...light=resistor

 

I have a greddy upper radiator hose adapter. Bought it because I didn't want to drill the t-stat housing. It works just the same. You should be able to order one from any greddy dealer for around $30.

 

I linearized the guage and calibrated it so...the second (from bottom) cold mark...marks the point when the t-stat opens. Best mod that can be done, without adding a gauge.

 

I linearized mine, and calibrated it so that when the pointer is horizontal, that's when the Miata/FC thermoswitch turns on the fans. Thermostat temperature is about 3/16-in above the cold mark. Anything above horizontal is time to start paying close attention to the temperature.

 

I use a PFC and and just look at the temp readings on the commander.

 

According to the 1994 Body Electrical Troubleshooting Manual, the coolant temperature gauge needle should sit on the "C" mark when connected through a 178.4 ohm resistance to ground (240 ohm resistance in a "cold" area). And it should indicate "H" when the resistance is at 16.9 ohms (21.1 ohms in a "cold" area).

Mazda evidently doesn't want to give away much about the resistance curve of the Water Temperature Sensor. All they say in the (same) manual is that at a water temperature of 122°F, its resistance should be 190 - 260 ohms. That is the only pass/fail criterion!!

 

That's the easiest!

 

here:

 

That was the graph I was talking about. According to every other post I have seen, this stock curve is totally bogus. it might be nice if it worked that way, but it doesn't as near as I can tell. I have never seen the needle move and that has me concerned.

When the fans have come on, they stay on for just a few seconds and then off again, so my guess is that it has never gotten too hot, however, I am definately not comfortable at this point.

I am going to try the parallel resistor first and calirate the gauge readind against a thermometer.

TM

 

I use my PFC for a real reading, my gauge as an indicator .. normal is a hair below parallel, horizontal about 195 deg... anything above horizontal Pay attention. The gauge has some value IMO; above lower white tick engine warming up, middle to below middle ... Great, middle to above ... Warning, moving up more ... Danger.
I just accept that small movements indicate a trend, who cares what the real temp is .. small movements mean a lot. The gauge may not be linear but it is repeatable .. the non linear scale does provide for use of the small scale by removing the middle normal op temp spread. I also have a idiot warning light, Auto Meter mini pro light on steering column that is to wake me up at 225 deg (I believe, do not remember which themoswitch I have in now). But after my Rad, ducting switch out I have not hit 100C, even in 1 hour racing in 90 F plus air.

On the other hand, my oil temp gauge moves all over the dial, and up and down when all is normal ... just a function of such things as which gear I am in, etc. .. who cares.. I need to know three things from it: 1) needle off peg ..Ok, oil warming to Op temp. 2) needle above 190F .. Warning, end of normal range, 3) needle to 220F .. Danger, something not right really the gyrations from 140 to 185 don't give me ANY information taht is useful at a quick glance. Mazda did this with the WT gauge in a 90 deg sweep rather than a big 270 sweep gauge. but also failed to give us piece of mind with true meaning of the needle postions

 

well in stalled the simple upgrade with a 250 ohm resistance in parallel. Gauge comes up quicker, and starts to move about the timing of the first fan. I am not sure yet, but it may need a little less resistor, since resistance drops as temp goes up. Ideally, I would want to think that the gauge should start to move at the same time the system moves above "normal". That is most important.

I think I will warm the engine and use a pot to find the 'knee' of the response. On the high side of the pot > 250 ohms, the needle shouldn't fluxuate much as you adjust it. There should be a point, however, specific to each car where the needle starts to move. I am guessing around 200 ohms. My thought is to calibrate the horizontal position with the maximum normal warm temp..

Thoughts?

 

What are the recommended setting people have used for their linearized gauge?
What resistances coincide with specific points on the gauge are advisable to use?
Anyone have some good recommendations for where the needle should be at what temperature to maximize its usefulness?

Once my car is up to temperature, it does not deviate very much from its set points.
Would a good suggested place to put the needle be perfectly horizontal at operational temperature?
Or setting the max allowable temperature to the white/red hash?

Any thoughts on placement of needle verses temperature?