P1121 and inside the coolant valve

Discussion in 'Gen 2 Prius Technical Discussion' started by mr_guy_mann, Nov 26, 2020.

  1. mr_guy_mann

    mr_guy_mann Member

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    So I have read posts about problems with Prius coolant control valves as well as the HVAC air door servos. On the servos people have found faults with the potentiometer that provides a position feedback signal.
    When a Prius throws a P1121 the coolant valve is bad so often, that I just replace the valve first as a diagnostic step then recheck. I have used a scantool to command the old valve and the feedback voltage seems to achieve the correct value, but the code will usually reset.
    Last week I replaced a coolant valve on a 2005 and I was curious about it so I dissected it. There's probably another thread about this somewhere- didn't find it.
    First of all New Jersey is in the "lower salt belt" so this unit was rusty and nasty. All the screws broke off.
    Inside is a motor, some gears that turn the valve, and a potentiometer, which did have a bad spot. I might scope the signal on the next problem car I see. Hope someone finds this interesting. DSC_6735.JPG DSC_6736.JPG DSC_6737.JPG

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  2. spdracrm3

    spdracrm3 Junior Member

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    You can buy that pot , i took one apart and googled the number and found the part and mount on the web. Brought up a Japanese site with all kinds of crazy ricer stuff for the prius....

    SM-G960U ?
     
  3. ChapmanF

    ChapmanF Senior Member

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    I mentioned on another thread linking to this, but not here, that Lisle sells a "62140 seized fastener removal master kit" that works, like, miracles with stuck screws like that.

    It works with an air hammer, and it doesn't exert any twist force on the screws so doesn't break them off. It just steadily bops the screw on the head a couple thousand times a minute while you gently turn with a wrench on the tool. Screws come right out and you won't even flatten the corners of the Phillips head.

    I don't work for Lisle or get any cut, but I've worked some miracles I wouldn't have been able to without it.
     
  4. mr_guy_mann

    mr_guy_mann Member

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    There are some great tools out there, but I have trouble thinking about an air hammer being used on a plastic housing. Plus a couple of the screws were so deteriorated that the phillups slot was gone.

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  5. mr_guy_mann

    mr_guy_mann Member

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    Here's a scope capture. The feedback voltage should be an even, linear slope as the valve moves through its entire travel. You can see the circled area near the top where the signal briefly goes near vertical. That's the bad spot- there is a large voltage change while the valve position only changes a little bit. _20201128_144607.JPG

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  6. ChapmanF

    ChapmanF Senior Member

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    I've used the thing successfully on stuff you might not expect. That housing looked like most of the threaded inserts are located where you could back them up against something solid. And nice and easy on the air hammer trigger. It's not how hard the whacks, it's how many; if humans could swing a light hammer 2,000 times a minute, just that and a regular screwdriver would do it.
     
  7. Ruming

    Ruming New Member

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    Does it imply the failure of these valves may not actually be caused by the mechanical part (motor, gear, valve body), but rather by the potentiometer in it?
     
  8. mr_guy_mann

    mr_guy_mann Member

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    This particular valve definitely had a bad potentiometer. Would need to test this on more valves before I could say if it is a common failure mode for P1121. It almost certainly the case for valves that cycle back and forth (or hunt).

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  9. ChapmanF

    ChapmanF Senior Member

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    Gen 2 Prii used that kind of potentiometer position sensing in lots of places, and Gen 1 in even more. Gen 2 has it in that valve (which sounds like a washing machine when the bad position signal is making it hunt for position), and in the three HVAC door servos (which sound like mice in the dash when the bad position signal is making them hunt for position), and maybe others.

    Gen 1 also used the same design for the steering torque sensor (do a search for "steering gear jitters" or "wheel shakes like wet dog") and for the accelerator pedal sensor (do a search for "big hand syndrome", i.e., the pedal position gets a flaky read and it feels like a big hand has suddenly reached from the sky to slow down your car).

    The mechanicals I suppose can wear out, eventually, but the potentiometer position sensing does flake out, very commonly; it's the first thing to check, and usually easy to fix.

    If your valve motor or mechanism was mechanically worn out or jammed, you would probably find it stuck or not working at all. If it's making the washing-machine noise actively hunting back and forth for the right position, it's not mechanically dead.
     
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