How-To: Linear Water Temp Gauge
 

Ok, so maybe you heard but you can have a very accurate Water Temperature gauge in your car and all at the cost of a couple of resistors. Here is how to do it:


Tools:
Phillips head screw driver (use it to get the gauge out)
Soldering Iron
Solder

Resistors (½ Watt):
137-139 ohm (It is very likely you will need to series or parallel two resistors for this)
6.8 ohm
1k ohm

Recommended:
De-solder wick
Flux

I don't know who did the original fix on the temp gauge, but I used KiwiNB's pics. and text file. I have attached the text, but here is the picture:
https://www.miataforumz.com/attachme...ine=1313451772

Since the picture is pretty descriptive, I will provide you with some key points and how to get there.

Here is the water temp gauge out of the car. The diodes were color coded on my car, you want to mod the one with the black line, not the blue. This may not be the same in every car.
https://www.miataforumz.com/attachme...ine=1313451772

Here are all the solders done. Its pretty hard to say which one you should do first, but start from the outside and do the middle big f'in solder last. I numbered them in the order I found to be best:
-Solder 1: Make sure the relay is in the right orientation (the red thing with blue and black bands). This is the 1k ohm and relay in parallel.
-Solder 2 & 3: are the two series resistors.
-Solder 4: DO NOT gob too much solder on it like in the picture (see ahead).
-Solder 5: Have fun!
https://www.miataforumz.com/attachme...ine=1313451772

Now here is the THIRD solder in more detail. You don't really need to do this, but it may make your life easier. I decided to do a series pair. I suggest using a "hook" method of holding them together. It can be hard to solder two pieces hovering in the air as they may not exactly touch. Avoid that hassle by hooking them like this:
https://www.miataforumz.com/attachme...ine=1313451772

Another pitfall, but this time at solder #4. If you glob solder on it, then the gauge won't be able to screw on very easily. Here is what NOT to do:
https://www.miataforumz.com/attachme...ine=1313451772

Flux and/or desoldering wick are great here also. Here is what you want it to look like:
https://www.miataforumz.com/attachme...ine=1313451772

Do I need to get a how-to remove the gauge cluster going also? I have that one written up, but I figured its simple enough...

I'm guessing not, I wouldn't want someone who didn't know how to remove the gauge cluster to be performing this mod.

Do you know if this works for all models? I notice you have an NB

Well, some people are a bit afraid of pulling on that hood!

But to the important question, I have no idea if it will work on an NA or not. I don't see any reason why it wouldn't. Would you like to try it? Its not permanent and any nastiness left over wouldn't be seen anyway. plus, I get the feeling that once you peek at the gauge it could be obvious from the circuit elements.

Yeah, I'll try it. Pending motivation and time of course, thanks for the write up. I'll report back if/when it happens.

One picture I forgot to post:
https://www.miataforumz.com/attachme...ine=1313462156

Note: I have had some question over the legitimacy of this mod (damn m.netters and their naysaying!! jk). I have a scangauge and I can confirm the needle travel up to 200F. I haven't been able to get it any hotter with my new Rad. so that will have to suffice for now.

Note 2: Please please, realize the prespective is centered on the gauge and not from the driver's position. That may seem obvious...but not everyone connects the dots ;)

Awesome writeup, adding this to my to do list.

OHHHH, I love this, I'm doing it tonight!!

Since you have an NA, please post up your results. preferably with a scangauge reading just to confirm accuracy on the NA cars.

For sure, I had overheating issues last week, I plug megatune to see the temperature et realize the gauge was steady until it went up 210 or so, not really accurate. (at 230F, the needle was at the 3/4)

I'll test it with the megatune gauge and I'll take picture of it whent it will be done, maybe for next week.

I found the willyman writeup for the 90-93, I'm gonna use this as a guideline.

Share a link with us, that way any 90-93 guys will be able to use it also!

http://www.tyrion.net/miata/tempgauge/90-93-MTGLP.mht

;-)

this is awesome, im gonna have to see if i can figure out how to do this in a non miata.... a linear fuel gauge would be even better ....lol...

Me being one of them! I dont want to break those stupid clips...

Great writeup Track! Eventually I plan to pull the cluster and do something similar to this, I want to make it with gauges that have actual numbers for peace of mind though.

if you want a set of gauges like that, take a visit over to revlimiter's blog and either a buy a set of his pre-made gauges or make your own following his DIY.

I will be moving my LED gauge cluster+mylar how to sometime in the near future and possibly doing a how-to on the OPG linear mod and degreasing. I am hesitant to really do it at this time though as the OPG linear mod is not my work at all, its just there isn't a clear how to on it (although the degreasing is).

so if you are interested in the OPG, get some numbers on that gauge too!

Just thought I'd toss in some more data for consumption on this thread:

I've been playing with a spare cluster that I pulled from a '93 car and wanted to make the temp gauge linear. I followed the instructions created by Wallyman, but I came across two issues along the way.

1) My sensor seems to have a slightly different resistance/calibration curve. At 120F, my sensor has approx. 30 ohms less resistance than what is specified by Wallyman. However, the deviation in resistance gets smaller the closer it gets to 270F. I've attached a graph of the resistance curves from 120F-210F.

2) When I used the resistor values that he suggested, my gauge was skewed such that the range was approximately 1ooF @ C to 180F @ H. I ended up using a parallel value of 210-ohms and a series value of 18-ohms. This gives me a range of approximately 150F at C to an estimated 270F @ H. (I have no intention of getting my car that hot, so I can't verify exactly how accurate it is. :P )

Not sure if I've got an odd sensor AND an odd gauge, or if the calibration specs for the gauges and sensors are just very loose.

I'll try to get some video of the gauge in action later this week.

I have not tried this on a miata but the question I have is series or parallel. In the first post you can't do both .
Doing series circuit will make it work one way and doing a parallel circuit will affect it in a different way
Did you possibly mean a series parallel circuit whereas there is a series circuit and inside thhe circuit is a paralell circuit
Just wondering for clarification .
I may just be reading the first post wrong

if you are asking me...Let me explain a bit of circuitry. Series and parallel are just two different configurations to do the same thing. Following Ohms law

R=V/I

In otherwords, resistance is a ratio of voltage to current. So if you have a basic circuit were 5v is applied to a resistance of 10...you get:

I=V/R=5/10=.5A

So you have a half amp of current. Now if you have two resistors in parallel like this:
https://www.miataforumz.com/attachme...ine=1320117210

R1=R2=10
Ground is at the bottom. (sorry I am stealing this picture and too lazy to draw it in).

Now lets apply 5v to the white circle, you will get .5A down R1 and .5A down R2. Since that current has to come from somewhere you figure out that 1A is coming out of the voltage source (from white circle to black dot is 1A, .5A goes down R1 and .5 goes down R2).
Now, lets do the same thing another way, since we know 5/10 is .5A and when we put two parallel resistors together we ended up with double the current, then maybe we can do this by find an equivalent single resistor value to give us this.

So, there is two ways to go this now, you can either take the 5v and 1A and plug them into the ohms law giving you 5Ohms. Normally you won't know current (so you know when your car is running that the voltage will be 14.4v and your resistance value is given in the how-to, so current is the unknown here)...So we can use a good old formula:

Rtotal= (R1*R2)/(R1+R2)

Easy enough eh? To simplify things, parallel and series resistances are the same thing, with different values. In our case, its two different methods of coming to the same end. You need X resistance and you can get that by two formulas:

X= (R1*R2)/(R1+R2) - Parallel resistors

OR
X=R1+R2 - Series resistors

Track you did good. WAith serries or parrell there is a difference with voltage two 6 volt batteries in series gives you 24 volts in parrell it is still just 12 volts jump more amperage and longer draw.
By no means am I a electroics person was just think that if you did it parallel then it would have a different voltage and amperage on the circuit than with them in series
I guess my thought was if in parallel you would have more resistence than in series or maybe that is basakwards

did you read what I wrote? Series and parallel resistance is completely different than series and parallel voltage/current sources.

parallel voltage sources provide the same voltage at node, but can have a larger capacity for amperage. technically the power output at each power source will be half (same voltage at node, half current output). This is much more difficult to demonstrate in circuit diagrams and is irrelevant here. In fact, most circuit diagrams ignore such limits. For the most part KVLs and KCLs make quite a lot of assumptions (all circuits are in 1 point in space, so distance has no effect on power for one, leakage current?, current limits?, wire resistance?, etc. etc.), but its all not required.

Lets not get off topic on boring electronics. Take it from an electrical Engineer. you can do either parallel resistance or series, depending on what strikes your fancy. just follow my two formulas for calculating overall resistance for the two resistors and you will be good to go.

If you have questions about what you figure out, post it on here and i will help. I will go so far as to offer doing this mod for free for anyone having trouble (who pays shipping and handling).

perhaps its my directions that are confusing?

You need a Zequivalent of 137-139. So you can do
A) Series: 100+37=137, 100+39=139, 80+59=139, etc, etc.
B) Parallel: (280*280)/(280+280)= 140, (560*184)/(560+184)=139, (1000*159)/(1000+159)=137, etc. etc.

Best to stick with series anyways, you can probably pick two easy resistors from radio shack and be done with it.

Yes I did read it and said "you did good," so don't get tacky about it,it was not meant to ruffle your feathers or to discredit you I had no idea you were experinced in the field ,I just did not know it would do the same thing.
I have read so many things that are not true or that is wrong I just ask for clarification
Did you know that you can take the six volt DC cell battery and rip it apart and use the AA batteries in it instead of buying a bunch of batteries and a lot cheaper. That statement is as false as it comes but a ton of people have fell for it. So had no idea if what you said was correct or you left something out.
I have made some small circuits and have burned some up .Really thought it was from the way the resistors were hooked up. Using LED'S they looked brighter hooked one way or the other and burn up or not burn at all

^lol, well I can't really tell you why your LED looked brighter without looking at the circuit you setup. but if you did series resistors then used the same two resistors to make a parallel circuit, then of course the LED will look brighter. two series resistors add together, while two parallel resistors will divide. note in all the parallel examples, I used two higher resistors to get a lower equivalent resistance!

Anyways, let me know if you need help with your circuit beyond this. I shall go hibernating again.

Thanks Track for the offer.That was just something I played around with a while back.
I made a small box with the idea of installing an led arrow design to put on a something. I get it to work after some hits and misses but was a fun learning experience
If I do decide to tackle another project I will give you a yell

I can't wait to make one of my friends do this for me!

Just did this to mine on Friday and it appears to be working. Great write up Joe! This wasn't very hard at all todo.

Thanks for the writeup and pics! I did this yesterday... car is still down for awhile, so I can't verify I didn't mess anything up yet! lol

https://www.miataforumz.com/attachme...ine=1334275704

This might simplify things for anyone who's hesitant to pull their NB gauge hood. I sorted it out on a fluke when pulling mine the first time, and it made the second time around a snap!

After removing the panel between the pedals and the steering column cover (2 screws) and then the steering column cover (three screws, then the cover splits top and bottom with a little tug), you can reach a finger behind where the hood meets the dash next to the column. There is a tab on each side that pokes into the dash from the hood, pointing toward the front of the car. If you curl your finger in there and lift that tab just a bit (not too far - it'll bend!) and grab a tool to pry the other side (see below - click for bigger), you can pop off the lower clip on each side, making the rest of the hood a lot easier! Use a proper tool to pry; the one I used was handy for its hook shape and wide surface at the end. The tan part is rather strong, but don't test it, OK? The second pic below should give you a better idea of what you're up against - the tabs you'll push up to pop each bottom side can be seen along the flat, toward the center, and the metal tabs toward the outside will come right out. That leaves the three around the top. Fingers under the hood, pull straight back. Simple!
https://www.miataforumz.com/attachme...ine=1336411718 https://www.miataforumz.com/attachme...ine=1336411718

If anyone needs a good reason to do this mod, here's one: After the mod, from a cold start I found the temp gauge slowly walking a ways past center and then suddenly dropping back to vertical. OBD-II verified this to be correct - it went to ~217 and then sank quickly to ~190. With the stock gauge setup, this would have been invisible. This is a cooling system problem, more than likely involving the hose that drops from the thermostat housing to the water pump inlet.

Thanks for the writeup! :)