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How to test water temp gauge on 308 with gauge out of car?

Discussion in 'Technical Q&A' started by 308GTS, Mar 18, 2004.

  1. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    I have my coolant temp gauge out of my car and the stock sending unit. I put the sending unit in water with a thermometer and connected it to the gauge. It isn't showing a reading. I then put 12V on it and grounded it, still nothing. How do you make it work? Any info would be appreciated.
     
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  3. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,282
    Texas
    Full Name:
    Steve Magnusson
    What year/model/version F?

    You may also need to ground the gauge (as well as the body of the sensor) -- can you post a schematic/sketch of your "test" set-up?

    The "sensor" most likely is a thermister so you can first check for continuity (wire-to-body), and, if OK, then measure its DC resistance at maybe RT and immersed in boiling H20 to see if you can detect a change.
     
  4. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    It is off of an 85' 308 QV. What I did was make a wire from the gauge to the sending unit. Checked continuity between wire and unit, it is ok. I then checked the resist. of the sending unit to make sure it is functioning properly and from my previous resist. values I have in my log, it is acceptable.
    I then put 12V via a battery to the gauge and ran a ground wire from the gauge to the negative of the battery. The gauge would move but it would move all of the way to the left, before 140. (jump all of the way back)
    I ran 12V the same way to the light bulb in the gauge with the same ground and it lit up.
    The reverse side of the gauge has 3 poles, +, black ground wire, and +/Com which from the schematic is the gray wire from the sending unit, straight to the gauge/this connection.

    The only other idea that I can think of it to run the ground from the battery to the ground on the gauge then take another ground to the sending unit. Thanks.
     
  5. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Here is a pic of the rear of the gauge. The terminal above the stamp 25 is the Com. (sending Unit) connection. It is the gray wire.
     
  6. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
    572
    NW Rural Nevada
    Full Name:
    Mike Florio
    The sensor (brass piece that screws into the block) is a temperature-sensitive variable resistor. Make sure the case of the sensor is grounded to the same ground that the gauge is grounded to.
     
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  8. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,282
    Texas
    Full Name:
    Steve Magnusson
    Your gauge connections sound OK, but (like Mike and you said), the battery, the gauge, and the sensor body should all share the same ground simultaneously. If the sensor is still mounted in the engine you might want to confirm that the DC resistance is low from the body of the sensor to a good chassis ground.
     
  9. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Sensor is out of the engine. My gauges reads in the car. That is what I am testing for. I am testing for accuracy. I will run the ground to the sending unit and wire it up the same as I had it and let you know.
     

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