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Thread: Thermostat resistance threshold

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  1. #1
    Join Date
    Feb 2017
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    Default Thermostat resistance threshold

    I drove the Zetec for the first time in about a year today (long story), and I found the radiator fan would not trip. Fuse good. I jumped the fan to ground, and it spins just fine. Non-Focus setup, with radiator from PMA, U.K. I tested the thermostat on the stove-top, and it tripped w/in about five degrees Fahrenheit of target. I took the water hotter, and resistance hovered between .8 and .9 all the way to boiling.

    Would that level of resistance stop juice to the fan? Clamp connections were solid and tested to '0.0'. I don't see any other players in the circuit that could cause a problem. Fluid system was bled, and I could visually see fluid in the radiator covering the level of the thermostat.

    How is the connection bridged inside the thermostat? From the outside, the system appears pretty fail-safe.

  2. #2
    Join Date
    May 2010
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    Laurel MD
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    I think you tested the temp sensor, a thermostat is a mechanical device. The temp sensor is probably firing a relay, so the relay might be bad. Did you try shorting out the two wires to the temp sensor, that should start the fan.

    Graham
    Last edited by fastg; 02-17-2020 at 08:27 AM. Reason: Spellings

  3. #3
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    Feb 2017
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    Yes, I was able to jump the sensor to get the fan to run without a problem. I don't see any relay on the circuit, though it may be somewhere I can't see on the fan housing. I'll take a look.

    Edit: Looks I'm direct wired from the fuse with a hot wire to the sensor, then straight to fan with a ground coming out. Temp sensor has to be the culprit, but its a first for me to see one fail.
    Last edited by David K; 02-17-2020 at 01:43 PM.

  4. #4
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    There will be a relay somewhere, the fans can draw 15 amps, but if the fans come on your tray is good. Are you sure the sensor is getting hot enough, you now know what temp the sensor trigger's at, so run the engine and see if you reach that temp. My fans do not even come on for track days.

    Graham

  5. #5
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    Quote Originally Posted by fastg View Post
    There will be a relay somewhere, the fans can draw 15 amps, but if the fans come on your tray is good. Are you sure the sensor is getting hot enough, you now know what temp the sensor trigger's at, so run the engine and see if you reach that temp. My fans do not even come on for track days.

    Graham
    It's definitely hot enough. I took the coolant to almost boiling and it filled my expansion tank. Separately wired gauge was showing much higher temp than usual. I could see the sensor was fully submerged in the radiator. Time to find a sensor.

    As long as I'm moving on a track, no need for it. But for lazy Sundays in traffic...

    Many thanks Graham.

    David

  6. #6
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    Laurel MD
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    Check pegasusautorscing.com great people, fast shipping. They list Setrab high quality sensors at a couple of temp ranges.

    Graham

  7. #7
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    Feb 2017
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    I'll do that, and I'll wire for a relay. Honestly, there is none. When that sensor switches on, it's directly carrying whatever amps the fan is asking for. Perhaps that led to the problem in the first place.

  8. #8
    Join Date
    Feb 2017
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    Lost my temperature gauge yesterday, so I figured it was an air bubble and not my sensor. I needed to clean the system anyway, so I drained all. Refilled with cleaner and water. And of course, now all works fine.

    Over the year of inactivity, I must have developed a nasty air bubble somewhere in the system. I tried all manner of burping it last week when the fan would not work, and I was sure it was full of water. Turns out not so. Good news is I don't need to get a new sensor from the U.K.

    Much appreciated, and apologies for crying wolf, and all.

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