How Feasible is this Project? Automotive switching circuit...

Discussion in 'The Projects Forum' started by lost honda pro, Feb 22, 2009.

  1. lost honda pro

    Thread Starter New Member

    Feb 10, 2009
    I asked a question on here a few weeks ago, and received a tremendous amount of help. In the interest of saving everyone's time, I'll ask if it's feasible before asking how.

    My engine coolant temperature is monitored by a thermocouple, with resistances varying from around 300,000 ohms @ -40*F to around 400 ohms @ 250*F. This thermocouple has +5.1V on one side, and the other side goes to the engine control computer. I'm assuming the engine computer has another set resistance so it can tell how much the temperature varies, and thereby know the engine coolant temperature.

    I'd like to know how plausible it would be to make a circuit that compares the voltage from this circuit with a preset value to activate the coil of a simple mechanical relay, which will turn on an electric fan. I'd simply like to turn on the fan @ 210*F, and run it for some predetermined amount of time.

    How complicated is this, could anyone on here help me with it, and is it practical if there is an over-the-counter $60 option?

    Thanks for any help/opinions! :D
  2. DickCappels


    Aug 21, 2008
    If the resistance in the engine control computer is large enough compared to the 400 Ohms, it should not be difficult to buffer and compare the voltage on the engine computer side of the thermistor.

    How about measuring the voltage at the point you are interested in, while the car is idling after warming up to operating temperature? Of course, the next step would be to block airflow to the radiator to see what happens to the voltage when the engine starts to overheat.

    CAUTION: You risk destroying your engine control computer, and this could be very expensive! Every imaginable disclaimer applies.
  3. lost honda pro

    Thread Starter New Member

    Feb 10, 2009
    I'm sorry if I didn't make this clear in the original post, but I can unplug the two-wire pigtail on the thermocouple, and measure the resistance at any temperature. I also have a 6th order polynomial curve that describes the resistance as a function of temperature. So I know the resistance at the thermocouple at any temp. (at 210*F it's right around 680 ohms)

    I'd like to make a circuit that activates a relay based on any resistance less than 680 ohms, but don't know how to get that information from the system I have: I'd only be measuring the voltage after the thermocouple and before the engine computer. And I don't know the resistances of anything other than the thermocouple.

    Should I add a diagram? I'm probably not describing this too well...
  4. ascari

    New Member

    Feb 26, 2009
    Hi. I think you should post the diagram. But by now I would like to know how can you measure the voltage?

    How can you make a relation between the temp and the voltage?

    As I know.. the problem in this things is that a thermocouple changes its resistance depending of the temperature, right?
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    You are using the wrong term or have a bad sensor. A "thermocouple" is a variable voltage sensor. It produces a very small voltage that is a function of temperature. A "thermistor" or RTD is a variable resistive sensor.