a/c stops/starts intermittently

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by alan0714, Jun 17, 2026 at 10:48 AM.

  1. alan0714

    alan0714 New Member

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    I started my 2008 (Gen 2) Prius 4 door a few days ago and the a/c would blow hot air (both sides of car) but no cold air. This is Missouri in June, 85+ degrees so I was concerned. Finished my chores and drove home. Next day I start the car and the a/c works just fine. Now I am confused, since I performed no maintenance... in fact, I did nothing at all to fix the problem, but now it's cool while driving around doing my chores, so I'm happy. Next day I start the car and again the a/c only blows hot air. So I check the a/c 10A fuse, and it's fine. The hot air is coming out pretty hard, so there is no problem with the blower motor or the blower filter. So I'm thinking about checking the "air flow gate" controller next and maybe finding out how to check the freon level after that. But today I start the car and the a/c is immediately blowing cold air again. I have no idea what is going on or what to check next... anyone else ever have this situation? If so, I'd really appreciate knowing how you fixed it. Thanks!
     
    #1 alan0714, Jun 17, 2026 at 10:48 AM
  2. JC91006

    JC91006 Senior Member

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    You should check if your inverter pump is working. Often when that pump has failed, and your inverter gets hot, the AC starts to shut down. So in the morning when everything is nice and cool, the AC tends to work until the inverter gets hot again. So you should check if the pump is working. Easiest way to check is to open the cap to the inverter coolant reservoir and see if there's any movementment in the liquid when the car is in READY mode. If no movement, the pump has failed
     
    #2 JC91006, Jun 17, 2026 at 11:17 AM
  3. alan0714

    alan0714 New Member

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    Thanks, JC, I will check that!
    OK, minor problem: I cannot locate any place in my owner's manual that identifies the inverter pump or it's coolant reservoir. Nor do I see anything under the hood that appears to be such a thing... do you mean the "engine coolant reservoir" or the "power control unit coolant reservoir"? If so, I can easily check them, but if that's not it, can you describe where to find the "inverter pump" or the "Inverter pump coolant reservoir"? Thanks again!
     
    #3 alan0714, Jun 17, 2026 at 1:07 PM
  4. JC91006

    JC91006 Senior Member

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    upload_2026-6-17_11-18-24.jpeg
     
    #4 JC91006, Jun 17, 2026 at 2:18 PM
  5. rjparker

    rjparker Tu Humilde Sirviente

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    It is the small reservoir by the inverter. The pump is a small electric pump basically under the inverter's right side. Only use oem which often is the same price as inferior aftermarkets that fail quickly.

    Low refrigerant is common and could be on the edge of working or not working. The ac txv valve will compensate for some degree of low charge especially when its cool out.

    Checking the basic ac is easy by looking at the sight glass on the passenger side right next to the H refrigerant access cap. AC running max you should see liquid refrigerant moving without any frothy bubbles.


    This pic shows white frothy bubbles on the top two thirds of the sight glass with liquid on the bottom. It is low. This one still worked but needed r134a pure refrigerant (no oil no sealer no dye $10 walmart can)

    IMG_1863.jpeg
     
    #5 rjparker, Jun 17, 2026 at 2:49 PM
  6. alan0714

    alan0714 New Member

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    OK, JC, thanks for the picture with the circles and arrows (tag Arlo Guthrie)... on my car this is marked rather prominently as a "Hybrid Synergy Drive"... anyway, I got the car "ready" and took the cap off the reservoir; the liquid was moving around quite a bit in there so it seems the pump is doing it's job... on to the next possibility; low refrigerant...

    RJ, thanks for your suggestions and your picture. I cleaned off the sight glass beside the "H" cap and put the a/c on max... the little window is completely full of what appears to be a steady stream of 'white' fluid that has so many tiny bubbles that I would describe it as dense shaving cream moving rapidly through the tube... I reckon that means I need to top off the coolant...

    I assume that it will use the r134a "pure" coolant you mention... but the rest of your finishing comment confused me -- do you mean that I should NOT use the stuff from Walmart? Or are you saying that the $10 can from Walmart WOULD work (IF it's pure refrigerant: no dye, no sealer, no oil)?

    I have never added refrigerant to an a/c system. I can probably figure out how to do it, and I've seen the little kits for sale at Walmart and other places. The question is going to be "how much" refrigerant do I put in? One can? Do I add it with the engine turned off or on "ready"? Also, do I stop when the stream of fluid stops being white (i.e. there are no more tiny frothy bubbles) or at some other identifiable point? Is it possible to "over-fill" and cause severe damage?

    Thanks for getting me this far, guys; I enjoy learning new things (at 85 it's amazing how much I don't know) and I appreciate good teachers!
     
    #6 alan0714, Jun 17, 2026 at 4:11 PM
  7. rjparker

    rjparker Tu Humilde Sirviente

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    It's possible the inverter/transaxle coolant pump is intermittent. Don't drive if the dash lights for the hybrid system come on.

    bad
    5674.jpeg

    good
    20260617_163400.jpeg

    If you have never put in refrigerant you need help. It is dangerous and can cause freeze burns to your face and hands or worse. Always wear gloves and face protection. Age 85 is a bad time to end up in the er.

    Too much refrigerant is bad for your compressor which is a very special three phase high voltage motor and pump.

    Air injected into the system is bad, the adapter hose should be purged.

    One can should be enough, the system only holds 16 oz and already has some.

    Your system has a leak. A recharge might hold for the summer but a proper repair at a minimum includes leak detection, repair, vacuum and recharge. This will be needed sooner or later. Maybe now is good.

    There is no coolant in the ac system. Using that term might cause someone to question your ability to safely handle refrigerant. Freon is a word some use but is a brand name.

    Inject through the low side (L) with the hybrid system in Ready. Max ac with windows down to load the system. Engine may or may not be on but the radiator fans will be on and are dangerous. Fill until sight glass just clears and stop.
     
    #7 rjparker, Jun 18, 2026 at 10:08 AM
  8. ChapmanF

    ChapmanF Senior Member

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    Because the Prius uses a subcooling design, the sight glass clears of bubbles a bit earlier than the proper charge.

    They specify to continue adding—using a scale—100 g past where the bubbles disappear, though hitting anywhere 50 g to 150 g past is "good enough".

    [​IMG]

    However, many of us, including me, who's just some guy, and Tom Lech, who may have been our most knowledgeable A/C tech while he was posting here, have all found the sight-glass method to be so variable as to not recommend relying on it at all, but rather just to evacuate the system and charge the right known amount of refrigerant by weight.

    Some of Tom's threads about that.
     
    #8 ChapmanF, Jun 18, 2026 at 11:45 AM
  9. rjparker

    rjparker Tu Humilde Sirviente

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    Which is why I suggest clear and stop for guys who will add refrigerant anyway.
     
    #9 rjparker, Jun 18, 2026 at 12:34 PM
  10. Mendel Leisk

    Mendel Leisk MMX GEN III

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    It's in there, albeit easier to chase down in the pdf format. Page 360
    :

    upload_2026-6-18_9-48-37.png

    and page 367, ref'd above:

    upload_2026-6-18_9-51-11.png
     
    #10 Mendel Leisk, Jun 18, 2026 at 12:51 PM
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