Coolant temp light on all the time

Discussion in 'Gen 3 Prius Technical Discussion' started by HHammerly, Jul 11, 2021.

  1. HHammerly

    HHammerly New Member

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    I am a new member of this forum and purchased my first Prius to use as a commuter vehicle, it is a 2010 Prius with 170000 mi, my first modification was to install an ultragauge to monitor engine RPM and ICE coolant temp.
    after driving it 1000 mi or so I noticed that the engine had a vibration at 1500 rpm and after reading a bit decided to do an egr system clean, change plugs, thermostat (it was leaking a bit) , coolant and brake fluid.
    After this maintenance the engine vibration went away and I see no engine codes on the ultragauge but the coolant warning light (thermometer with wavy lines below) has been on all the time, I know that the engine coolant temp is in the normal range because I can monitor it, it quickly warms up from ambient temp to about 180 when the thermostat opens, I have driven the car for 1000 miles or so since this work and ICE coolant temp stays between 180 - 190 all the time will get up to 200 after a while in maintenance mode when the radiator fans come on and cools down to 195 as I would expect.
    The inverter coolant seems to be flowing in the system )I can see fluid circulating in the reservoir) but I cannot monitor its temperature, it does not feel substantially warmer that ambient temp.
    Is there a second sensor other that the coolant sensor bolted to the end of the head that will trigger the coolant light?
    Thanks!
     
  2. Tekken

    Tekken Member

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    Make sure you water pump is working correctly
     
  3. ASRDogman

    ASRDogman Senior Member

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    Check the connection on the sensor. Then check the sensor itself.
    I don't have to specs but a search online should bring them up so you know the resistance at certain temps.
    Or someone will put them up shortly...

     
  4. ChapmanF

    ChapmanF Senior Member

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    Because there are two temperature sensors and your Untragauge is showing you a normal reading from one of them, my suspicion lands on ... the other one. :)

    [​IMG]
    [​IMG]

    You might be able to get the Ultragauge to show you both readings, only I'm not sure how. If you can, I'm guessing you'll see the other reading is stuck high. Probably the normal reading that you're seeing now is from the one in the cylinder head (which is wired to the ECM and is in the Engine and ECT data list in Techstream), and the one with the hot reading is the one in the hose tee above the EGR cooler (which is wired to the combination meter and is in the Combination Meter data list).

    It would have been easy to disturb the wiring to either one while doing the EGR and other work you've reported. The sensors have a negative temperature coefficient (lower resistance shows as higher temperature), so my guess would be that you didn't just leave one unplugged, but may have pinched one's insulation one way or another, and made a short.
     
  5. HHammerly

    HHammerly New Member

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    Champ, Thanks! I found that the second sensor was connected however it has a short cable with a second connection that was hiding under the wire bundle that runs over the valve cover that was indeed disconnected, that solved the temp light issue, I suppose that the sensor on that line would trigger an initial warning light with the additional triangular warning when the head goes beyond a certain temperature and possibly be used by the hvac system to turn on the ICE engine when cabin heat is called for?
    Thanks again!
     
  6. ChapmanF

    ChapmanF Senior Member

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    Well, that sensor's in a peculiar place: measuring the coolant that returns from the exhaust heat exchanger way down under the car.

    [​IMG]

    Exactly what other use the car makes of that reading is unclear, but its wiring goes directly to the combination meter, and it will light the red wavy thermostat if it reads 248 ℉ or above. I guess it could be useful for telling you if the thermostatic mechanism down there has become stuck in the heating position. Of course you wouldn't know that just from the red wavy light, but would have to pull up the data lists and see which reading was elevated.
     
  7. mr_guy_mann

    mr_guy_mann Senior Member

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    Well, the heat exchanger is an emissions device, so there "has" to be some way to monitor its function, otherwise someone could just bypass it when there was a problem.

    Posted via the PriusChat mobile app.
     
  8. tony_2018

    tony_2018 Member

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    Yes!! The 2nd image is the sensor you need to check to make sure its connected. I had this issue when I swapped the engine out and notice the red temp light on. This was the issue.
     
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