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DIY Auto Transmission Oil Temperature Gauge

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  • DIY Auto Transmission Oil Temperature Gauge


    PradoGXLV6 kindly shared his excellent auto transmission temperature gauge. I come from an electrical engineering background so I have a different approach.

    Recently, I was able to purchase the Prado workshop manual on CD and from this I was able to glean that the D4D auto Prado has two temperature sensors in the 750 gearbox. Access to either (or both) of these is straight-forward at the connector to the transmission computer under the dashboard.

    I designed a simple circuit using a dual op amp, a LED panel volt meter from Jaycar ($30) and a 5V to 5V DC-DC converter ($20), also from Jaycar. Total cost was $70 plus a bit of time to assemble it on some vero-board.

    The input impedance from the temperature sensor to the op amp is extremely high so that there is absolutely no impact on the vehicle's transmission computer.

    The advantage of using one of the Prado's existing temperature sensors is that you don't have to cut any oil hoses or have to remove the drain plug to fit a sensor. (I hate mess!) It's also cheaper to use the perfectly good Yota device.

    If anyone is interested in the circuit, please email me and I will sketch it out a bit more tidily than what it is at present.



  • #2
    Very cool!

    I like it!

    Will it work in a A340F auto trans for the Hilux?


    73 de 9W6VX


    • #3
      Nice useful setup. IMHO the unit is too big for my liking. Have you tried a smaller unit? Is the brightness adjustable for night driving?
      SSSHHHH be very very quiet were huntin wabbits.


      • #4
        Hi D4D-GXL,

        You are very observant and your comments are insightful. In fact I am working on mark II of the unit. You are 100% correct in saying the unit is too big. It simply is.

        Mark II will use a LCD panel meter sourced from Altronics in Perth at a cost of $25 plus postage. The size of the digits are 13mm and it comes in a package 48H x 24H x 16.2D (mm).

        Version one has a LED display with 20.3mm digits in a package 85W x 42H x 25D (mm). Unfortunately the Jaycar LED display does not have adjustable brightness not that I find it a problem where it is mounted. In any case it can be turned off at night because I certainly do not tow at night. (I only need the unit to monitor tx oil temperature when I am towing.)

        Hi 9W6VX,

        Because of the circuitry employed, the unit would probably work with any vehicle's temperature sensor, both oil and coolant, but the two pre-set potentiometers used in calibration would have different settings. Measurements of the voltages on the sensor when the engine is cold and when at operating temperature would be needed to be carried out first.



        • #5
          Looks good. What have you used as a basis for calibration to confirm accuracy?


          • #6
            Hi Piggy,

            For the first calibration point, I assume that after the vehicle has been sitting for 15 hours, the temperature of the transmission fluid is the same as the ambient.

            The second point I use for calibration is when the vehicle is at normal operating temperature. I have a digital temperature gauge with a remote sensor which I tie to the metal oil line going to the cooler. Basically the oil is at or close to 80 degrees C after a half hour's driving.

            The change in temperature when towing is actually more important than absolute accuracy. After all you need to know whether the torque converter has locked up. If it hasn't, the temperature of the oil will build up quickly due to slippage.



            • #7
              Cannot wait for MKII Sacred Cow - Should be a real dazzler. Great Stuff.
              SSSHHHH be very very quiet were huntin wabbits.


              • #8
                Hello Sacred Cow,

                Looks great and I reckon your mk2 kit will look even better

                I've been looking on the jaycar website and found this - ... BCATID=347
                Do you know what probe I would need to use this kit for an EGT gauge?? This would be a cheap option against a VDO unit which costs about $300.

                Sorry if I am hijacking your thread

                Cheers Ben
                2008 Holden Calais 6ltr


                • #9
                  Hi Dieselman,

                  The temperature range of exhaust gases in a turbo diesel is usually in the range of 500 - 800 degrees C. Therefore the probe that comes with the kit is probably satisfactory but probably it can not be mounted satisfactorily. Usually the exhaust manifold is drilled and tapped so that a sensor can be screwed in. I don't like the idea of iron filings getting into the turbo.



                  • #10
                    try wholesale automatics.


                    • #11
                      sorry: google wholesale automatics I have just been in contact with them and found them very helpful they have many modifications such as temp guage kits, torque convertor lock up kits and shift kit valve bodies and trans coolers


                      • #12
                        Hello Glenn, Back in 2008, you posted about a DIY Auto Trans. digital temp. guage that you had made[for 120 series Prado]. The photos and/or links don't seem to be available anymore. I have some electronics skills,so was wondering if you still had a copy of the circuit .If so I would be most appreciative if you would be able to forward it to me. Thanks for your time. Kind regards-Ian


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