What's the actual cause for no cabin heat (pump dead, stuck thermostat?)

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Rrrolfff, Apr 11, 2022.

  1. Rrrolfff

    Rrrolfff Junior Member

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    I'm replacing my coolant sensor and water pump - one of my symptoms is no cabin heat - is there a heater exchange valve? Thanks!!!
     
  2. PriusCamper

    PriusCamper Senior Member

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    I'm more experienced with Gen2 then Gen3 Prius and they changed so many things in stupid ways in Gen3 so it may not apply, but you have an electric heater as well as a heater from the engine coolant. The electric one that is the initial part of the system can have a blown fuse sometimes. Once engine warms up the coolant-based heater takes over.
     
  3. ChapmanF

    ChapmanF Senior Member

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    The electric supplemental element is about 700 watts ⇒ scarcely noticeable compared to what you (should) get when the engine is warm (on the order of 5300 watts). About the only way I can ever even tell whether the electric element works or not is that it does get the windshield to defog marginally faster on that kind of morning. Many people probably won't ever notice whether their electric supplement works or not.

    The HVAC is one of the systems that got simpler between Gen 2 and Gen 3. Gen 3 got rid of the thermos, coolant control valve, and whole separate electric pump that Gen 2 had. (Gen 3 just uses the engine water pump, which is now electric, so it can run even when the engine stops.)

    There's no one "actual cause for no cabin heat" ... you just have to figure out what's the cause in your car and your situation. How did you reach the decision to change the sensor and the water pump? And which sensor are you changing (the one on the cylinder head, or the one in the hose tee)?

    At the times when you're noticing no cabin heat, what is the coolant temperature? If you have a scan tool, you could read the temperature from both sensors (in case you're wondering whether one is bad; they're probably not both bad). Without a scan tool, you can fall back to older tricks like feeling hoses. Which ones feel hot?
     
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  4. Rrrolfff

    Rrrolfff Junior Member

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    I have a scan tool, from memory, there were two sensors (head gasket caused coolant to mostly empty - I know I need a new engine, but can't right now). The 7C0 and 7E0 - it works till the engine runs a bit, then goes to -41 degrees. I thought it would be the engine block one - once I put coolant in, as mentioned it works a bit, then fails.

    No heat could maybe be the water pumps failure due to dry run (it's why I'm swapping that too). The electric fans were running last I looked - but with the sensor jumping from working to not - it's hard to figure out. Thanks for your informed opinions!

    I'm also wondering if I need a 19mm flare socket, or if any deep 19mm will get it done. There's a YouTube of someone doing this in less than 30 min by just pulling the highvtage battery wires out of the way (and removing the wire bracket).

    Thanks again to all!!!
     
  5. ChapmanF

    ChapmanF Senior Member

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    The sensors have a negative temperature coefficient (lower resistance as temperature increases), so a reading of −41 degrees is about the same as an open circuit. Have you noticed anything about the wiring to the sensor?

    [​IMG]
     
  6. Rrrolfff

    Rrrolfff Junior Member

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    I'm quite sure the lack of a coolant bath /heat got my sensor - I bought a 19mm wrench, but it doesn't fit over the plastic end Harbor Freight didnt know anything about a flare socket - any tips on getting this silly coolant tool?
     
  7. JC91006

    JC91006 Senior Member

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    If you're in need of an engine, maybe you shouldn't be throwing money at this heat thing? If you live in Irvine CA, there isn't a real need for heat anymore until winter comes around.
     
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