Water Tank &Fuel temperature Gauge

If you read my saga of the fuel temperature/vapor lock problem you will understand why I was looking for an accurate way of measuring the temperature of my fuel in my fuel tank.  I had also longed for a way of monitoring the water temperature in my water tank that feeds my air/water intercooler.  Believe it or not the steps taken to finally get to where I wanted to be (ability to monitor both accurately) was a very long journey with many wrong turns taken along the way.  In short if you have any desire to monitor any liquid in a low temperature range my design will not only provide you greater accuracy and range than any available product (that I could find) but will also cost you considerably less.

The first part of the equation was understanding what I wanted to monitor.  Water tank temperatures could range anywhere from 35 degrees (if ice was in the tank) to somewhere slightly above ambient.  Assuming the hottest temperature I would ever drive in was 110 I shouldn't see anything over 125 in the water tank.  For fuel I would see anywhere from ambient (at startup) to the point of vapor lock (180 with most fuel).  So this left a desired range of measurement from 35 degrees to 180 degrees.

The second step was to consider my requirements for gauge positioning.  I really don't want any more gauges installed in my car. As you can see in the photo near the bottom of the page I already have plenty of gauges.  I already have people telling me it looks like I am flying an airplane!  Ideally I would find a digital gauge or one with a sending unit compatible with one of my existing gauges.

The third step in the process was to find out what was already available.  My first check was to see what Autometer had available (since I already run Autometer gauges).  Also consider that I already have an Autometer Transmission temperature gauge that reads from 100 degrees to 250 degrees (higher range than I wanted). Autometer offers two gauges that could possibly work (ATM-4469 & ATM3531-M).  The first gauge (ATM-4469) offers reading from 60-210, the second (ATM3531-M) from 40-120.  The problem with the first gauge is it uses an electronic control box that would require a new gauge for each item to be measured (two new gauges, one for fuel temp, one for intercooler water temp).  Plus 60 degrees was a little higher for my starting point on the water temperature. It also is pretty expensive ($130).  The second gauge sounded great until I found that it uses a mercury filled capillary tube.  The problem with this is that not only would each item need its own gauge but the distance from the item to be measured to where the gauge was to be mounted was only 6'.  With my intercooler water tank in the trunk that would have resulted in a strange gauge mounting location. 

Further searching found several marine application gauges that could potentially work but the price typically started at $200.  I don't mind spending $200 on a gauge but I was still struggling to find a place to mount it.  A little more searching led me to the Saleen intercooler water temperature gauge (Product ID :  047-580)  that would work.  The problem I had with it was again I would need to find a place to put it and the range didn't look optimum.  Although it goes to 60 degrees the increments between 60 and 130 are awfully small.

So, I started thinking.  What if I could find a sending unit compatible with my transmission temperature gauge?  That would be great!  Since the gauge uses a variable resistance sending unit I could use a simple toggle switch to flip between all three (transmission temperature, fuel temperature, and intercooler water temperature).  The problem is I would need to find a sending unit that would read below 100 degrees (researching the internet found none).  It would also have to have a compatible resistance range as compared to the Autometer sending unit..  The first step was to find out what the resistance to temperature values were for the Autometer sending unit (ATM-2258).  A call to Autometer resulted in these numbers.

Great!  Now I had a place to work from.  Now to find a sending unit.  My thought was to find one from an older vehicle (before high temperatures were normal).  A trip to Napa and a little time in the "part book" let me to part number TS6713 designed for a (insert description).  The part book didn't show resistance values however the year and type of vehicle looked promising and the sending unit uses the same 1/8 NPT fittings as the Autometer gauge.

Wahoo!  I had done it!  First try.  You have no idea how ecstatic I was to find that the first temperature sending unit I found not only had the exact range I wanted but also a resistance value that mapped over my existing transmission temperature gauge values!  On top of that the sending unit responded all the way to 35 degrees!  How lucky can you get?

Now I had all I really needed but to verify my calculations I used a potentiometer to verify what resistance values mapped to the increments on my gauge. I used the potentiometer to move the gauge to line up exactly on each increment on the gauge and measured the resistance (3 times).  Anyway - here is what I found.

Wow - right on with the numbers provided from Autometer!  Looks like everything is coming together.  Now lets map the new sending unit temperatures to the existing Autometer gauge.

Amazing!  Not only do the values all closely relate to usable numbers but the whole gauge is used for the full spread of values I wanted to read.  I can't believe I got this lucky.  Now time for proof of concept (reality check).  I installed the sending unit in the fuel line (see photos above) to see what the gauge would show.  The temperature today had reached about 85 and my garage is a little hotter than that.  It was later in the day and the temperature had dropped to around 75.  If my calculations were all correct the gauge should read just above the 150 mark (70 on my new sending unit).  How did it do?  Exactly where I wanted it - WOW!

Now that the gauge was in and installed it was time to give it a try for a day or two to see how well it responded.  Immediately I noticed that I was always looking at a chart I had printed to see what temperature each each line represented and decided that I needed to mark the gauge.  It was frustrating to not have a quick reference of values like you had with the factory marks for transmission temperature.  I contemplated marking the gauge with a sharpie (actually finding someone with better penmanship) to using a p-touch labeler and finally decided to use transparency paper that was inkjet compatible to print the numbers.  I had decided already I wanted the color blue (water) to contrast with the black and started printing sheets until I got it to look right (or at least as good as I could make it).  Transparency paper is expensive so I kept moving the drawing around until I got it right.  It took a few cuts to get one that was clean enough to use. 

Another word of caution - taking apart the Autometer Transmission Temperature gauge is not easy!  I spent a lot of time with a small screwdriver prying the front cover off.  Once that comes off you have to take the electronics out and remove 2 small screws on the faceplate - but be careful!  Behind those screws are two small spacers that, if you drop, can be very hard to find.  I would guess I spent 3 hours installing the new faceplate cover and I don't think it could be done any faster.

So, not that it is installed how does it work?  I like having the reference numbers but one problem I did find is that with the 3 middle numbers (most common range) on the inside of the dial you can't read whatever number the dial is over.  Not a huge problem as you can read 2 of the 3 numbers and can most likely figure out the other one but still something to consider if you decide to do the same thing.

I did save the file I made (using Microsoft Photo Draw) in both jpeg and native photo draw format.  Fell free to use these to start with if you like:

I mentioned before the toggle switch that I use to control which sensor I am reading from.  I thought it would be fun to show the toggle switches with a legend for what each switch does.  The middle position on each switch is off.

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