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Odd temp gauge behavior

Discussion in '456/550/575' started by Chuck Taylor, Apr 11, 2017.

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  1. Chuck Taylor

    Chuck Taylor Rookie

    Dec 9, 2005
    11
    #1 Chuck Taylor, Apr 11, 2017
    One of the cars I care for is a 1932 Ford Roadster with a 550 drivetrain. The owner complained that the temperature gauge was reading too low, like 120 when well warmed-up with an oil temp of 180. It's easy to get under the dash, so I did a quick check of the back of the gauge, and found .5 volts at a coolant temp of 60 degrees, and 5 volts with a hot engine, so the sensor is doing something.

    Then I observed the following. Shut the engine off, temp showing 120, fan continued to run. After a minute the fan shut off and the temp went instantly to 180. After some more observing, I concluded that as soon as the fan comes on, the temp drops quickly and rises quickly when it goes off.

    Any ideas?

    Thanks,

    CT
     
  2. Rifledriver

    Rifledriver Three Time F1 World Champ

    Apr 29, 2004
    36,882
    Cowboy Capitol of the World
    Full Name:
    Brian Crall
    #2 Rifledriver, Apr 11, 2017
    Not really enough information. Ignore for now the electric gauge and put a mechanical gauge or a known good electrical right in the coolant tank and take readings there. Before diagnosing the gauge you need to know what the temps really are.
     
  3. Chuck Taylor

    Chuck Taylor Rookie

    Dec 9, 2005
    11
    #3 Chuck Taylor, May 3, 2017
    Sorry to go dark on this. A couple of days after I posted this, the fuel cell sprung a leak and and had to come out. A new one is being fabricated and it will be at least a couple of weeks before the car is running again.

    There is no coolant tank. I did check the temperature with an infrared gun, and it was showing 180s at the top of the radiator.

    CT
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,632
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #4 Steve Magnusson, May 3, 2017
    This may be an indication that fan motor and/or gauge and/or sensor is not well grounded. I'd suggest that you measure the DC voltage (when the fan motor is running):

    1) from the fan motor minus terminal to the negative battery terminal,

    2) from the gauge minus terminal to the battery negative terminal, and

    3) from the sensor (thermistor) metal body (assuming it's a one-wire thermistor) to the battery negative terminal.

    All three of those should be close to 0V DC. If one isn't, try running a jumper wire from the offender to the negative battery terminal and see if the gauge behavior is more correct. Good hunting!
     
  5. Chuck Taylor

    Chuck Taylor Rookie

    Dec 9, 2005
    11
    #5 Chuck Taylor, Sep 24, 2017
    With the new fuel cell in place, was finally able to spend some time on this. The fan is controlled by a standard four-pin relay. When the temperature sensor supplies ground to 85 on the relay, the fan comes on. Power to the gauge and terminal 30 on the relay comes off the same ignition circuit. The gauge is basically a voltmeter, so when the fan (big) kicked on, it lowered the voltage to the gauge, causing it to read lower. I moved the power to the relay to it's own feed from the battery instead of the ignition switch and the gauge no longer drops when the fan comes on.

    Thanks for all the suggestions,

    CT
     
    Laserguru likes this.

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