Mystery Inverter Coolant Leak from Transaxle Bellhousing

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by user4321, Sep 21, 2025 at 11:49 PM.

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  1. user4321

    user4321 New Member

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    Hi everyone,

    I’m trying to get more info on a mystery inverter coolant leak from the bell housing of the transaxle on my 2007 Gen 2 (155k miles). For months I had slow loss of coolant, needing to top up the inverter reservoir every few weeks. Recently, after a long highway climb under load, I got the red triangle, smelled hot car smells and immediately knew I was overheating. The inverter coolant reservoir was empty, and there was a stream of coolant coming out of the transaxle bell housing (leak spot circled in red).

    The coolant is dripping steadily out of the weep hole at the bottom of the bellhousing cavity which houses the flexplate/damper wheel and should not be a place with any fluid. The Toyota dealer confirmed coolant on the flexplate/damper wheel and in the bell housing but didn’t go into detail about the cause, other than suggesting it was likely getting into the bell housing internally from the transaxle cooling loop.

    From what I’ve gathered, this sounds like an internal failure in the transaxle rather than something as simple as an external hose or seal. The dealership mechanic said he has never encountered this, and figured it's probably in need of a new transaxle.

    Has anyone here actually dealt with inverter coolant making its way into the bell housing like this, or have any thoughts? Part of me (the part that doesn't want to scrap it or pay the $4,600 quoted for installing a new (used) transaxle) wants to plug the weep hole and let the bell housing fill up with inverter coolant since it is a low pressure system and see if I can limp along with that.

    IMG_3946.jpg IMG_3918.jpg
     

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    #1 user4321, Sep 21, 2025 at 11:49 PM
  2. Tombukt2

    Tombukt2 Senior Member

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    Make sure that's not rundown from resvoir O ring leak . Just be sure . Theyres a loop in transmission . Look at picture of transmission breakdown . It'll become somewhat apparent.
     
    #2 Tombukt2, Sep 22, 2025 at 11:34 AM
  3. ChapmanF

    ChapmanF Senior Member

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    You might take a look in this video, around 34:07, to see the way the coolant passage around MG1 is supposed to be sealed off inside the bell housing.



    He doesn't make it sound very hopeful for fixing a leak ... sounds like that O ring might not be an orderable part?
     
    #3 ChapmanF, Sep 22, 2025 at 12:14 PM
  4. pasadena_commut

    pasadena_commut Senior Member

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    Not orderable doesn't mean not fixable. It means one has to pull out the failed one, measure it carefully, try to figure out what material it is, and order a replacement from one of the several companies that specialize in selling O-rings.

    For instance, half a year ago or so my fairly old HF floor jack started leaking. They don't sell replacement parts for that model and as far as I could tell nobody else does either. Nevertheless, HF does sell packs of O-rings and one was close enough to the right size to fix the problem.

    The bigger problem here is that getting to an internal O-ring in the transaxle is a much larger PITA, and the material used might be difficult to determine. So trial and error, which was an option for the floor jack, is not a great choice here.

    There is an O-ring under the inverter coolant tank which is less of a problem to replace, it can be ordered, and the OP states some work was done in that vicinity. So maybe change, or at least inspect that? I know, kind of like looking for a missing key under the streetlight even though it was lost elsewhere.

    Size of O-ring between Reservoir and inverter | PriusChat
     
    #4 pasadena_commut, Sep 22, 2025 at 1:32 PM
  5. Hayslayer

    Hayslayer Member

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    Personally, I would do a bit more investigation. Seems strange to me that you have pink residue from dried coolant all over the place, except at the weep hole.......

    VEVOR has a very nice steering endoscope that's about 90 bucks (I have one). The camera end can rotate 180 degress and look straight back from where it came. It would be ideal for sticking in that hole and looking around. The camera lens portion is 0.254" diameter so it can fit in some pretty small openings. The neck portion is like 3' long so it has plenty of reach if needed. Worse case, you may need to drill the weep hole slightly larger.
     
    #5 Hayslayer, Sep 22, 2025 at 2:09 PM
  6. rjparker

    rjparker Tu Humilde Sirviente

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    Regardless, plugging the weep hole would quickly take you from the desired limping to walking.
     
    #6 rjparker, Sep 22, 2025 at 2:16 PM
  7. pasadena_commut

    pasadena_commut Senior Member

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    I missed that. Has the OP actually observed fluid leaving the weep hole?

    If not perhaps stuff something absorbent, like a small piece of paper towel in there, then seal the hole with a bit of aluminum foil and tape. Drive around for a couple of minutes and then disassemble. Is the bottom of the transaxle wet without the inside of the weep hole being wet?
     
    #7 pasadena_commut, Sep 22, 2025 at 2:33 PM
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