Prius 2011 overheat, no code even though pump not functional

Discussion in 'Gen 3 Prius Technical Discussion' started by Sal Delivery Driver, Jan 7, 2024.

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  1. Sal Delivery Driver

    Sal Delivery Driver New Member

    Joined:
    Jan 7, 2024
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    Vehicle:
    2011 Prius
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    First thanks to all at Prius chat, I was able to repair my overheating problem thanks to you guys and some providence...

    My car had VERY intermittent overheating (like once every other 2 months for less than a minute), and then the cabin heating would occasionally fail but only a few times, then 12/30/23 it could not cool off, and the cabin heat was totally non functioning.

    Thankfully bought an ODB2 VPOW scanner for $20 or so on Amazon, and I could finally monitor the real-time temperature long before the red dashboard overheat light came on...

    Supposedly the RPMs of the water pump are supposed to be reported back to the car's main computer, and if there are not enough RPMs it's supposed to give a check engine code P261B or p261C or something like that. But NOTHING.

    I removed fuses and relays and once disconned the harness and I got the expected P261 codes. Once, however, even with the pumps electrical wiring harness disconnected, after I cleared the check engine code, the code didn't come back -- everything looked good from the standpoint of no warning lights.

    Anyway, I changed the water pump. The old pump clearly had a rotor falure. It was somewhat stuck and not moving.

    All this to say, at least in my case, the car didn't report a problem with the pump like I would have expected.

    Just mentioning it explicitly in case someone is getting the same problem but not getting expected codes.

    Thanks everyone who is a part of Prius Chat, especially since, up until recently, I rarely try to do work on my own car. I've never even changed oil by myself!

    I felt obligated to at least make this small contribution of information in case it helps anyone.
     
    #1 Sal Delivery Driver, Jan 7, 2024
    CR94, Brian1954 and ASRDogman like this.
  2. ChapmanF

    ChapmanF Senior Member

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    Vehicle:
    2010 Prius
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    The car doesn't have a main computer, just a couple dozen different computers networked together. The water pump is controlled (and its RPMs watched) by the ECM, the computer responsible for the gasoline engine.

    Did you have a chance to use a scan tool that showed live data, and see what the target and actual pump RPM were?

    P261B is the code you get when the actual RPM is less than 900 and the ECM wants it faster. That depends on the actual RPM signal getting back to the ECM.

    P261C and P261D are problems with signal getting through: C when the voltage is lower than expected, D when higher than expected (which translate into a probable short or open, respectively, given the signals are high when open and pulled down to drive them). But there's some ambiguity in the wording of the DTC detection conditions for these two codes. They say "water pump output voltage is less than (or higher than, respectively) ..." So what's the "water pump output voltage"?

    On the ECM, the terminal labeled WPO, for water pump output, is the signal that comes out of the ECM to the pump, telling it how fast to run. But the detection condition as written doesn't use that mnemonic WPO, it just says water pump output. Could somebody else reasonably think "water pump output" refers to the signal that is output by the water pump, reporting its RPM back? That one comes to the ECM terminal WPI, 'cause to the ECM it's an input.

    Who knows?

    From the way the Monitor Description is written, you'd think they meant WPO: it says "if the actual drive duty cycle does not correspond to the target drive duty cycle", and WPO is where the drive duty cycle is seen. But what if somebody (either in Japanese or in the translation) said "duty cycle" where they meant "RPM"? The actual RPM would be seen on WPI.

    It turned out in this post that the pullup for WPO is in the pump, and the ECM drives it by pulling down. So it's possible if you have the plug flat-out disconnected, the WPO voltage isn't seen as excessively high or excessively low, it's just floating. Kind of a funny way for the test to work.

    I remember while I was playing with the pump in this post, I did end up getting a P261C stored, I thought because I hadn't added enough DC offset in a spoofed signal I was generating ... but that would suggest it's more to do with WPI. Hmm. It was a while ago now.
     
    #2 ChapmanF, Jan 8, 2024
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