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More 12V Weirdness characterization

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by cwt, Jan 28, 2024.

  1. cwt

    cwt Junior Member

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    Location:
    Davis, CA
    Vehicle:
    2007 Prius
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    N/A
    My car has gone from intermittent to consistently going dead after turning off. Battery was replaced, new one was re-checked and found to be great. When I jump start it from a portable battery, it comes back on in accessory mode. When the car is on, voltage levels are at 14V at both the jump start point and the battery terminal. When the car is off, the jump start point goes to mVs and the battery terminal goes to 0.7V (so I think a diode may be in the mix). When the battery positive is disconnected (probably took me 1 to 2 minutes, the battery was down to 4.5 V. I went back out after about 15 minutes, disconnected the ground and the battery showed 3.3V. With the battery disconnected, there was about 450 ohms of resistivity between the battery cables. So, I am leaning towards a short somewhere after a diode from the battery and on the other side of that diode near the jump point? There were other weird things going on, such as being able to, after jump starting, push the power button turning the car from accessory to on without a key fob present. It also would at times go into accessory mode without a jump start: my neighbor knocked on my door once and told my wipers would wiping. A couple of times in the past, it even turned itself on without any action on my part and without a key fob present.
    I tried downloading electrical schematics, but the ones I found online were not of much help
    Oh, also, it used to recharge itself somehow after going dead if left by itself for a day or two. That has stopped happening.
    My thoughts are thinking about the start button and the electronics associated with that. It seems like the hybrid battery may be tied to the 12V system, so maybe that could be part of it Anyone else have any thoughts?
     
    #1 cwt, Jan 28, 2024
  2. ChapmanF

    ChapmanF Senior Member

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    Vehicle:
    2010 Prius
    Model:
    IV
    0.7 volts would indeed remind a person of a diode drop, but I don't think there is any diode involved. Currents to/from the 12 volt battery can exceed 100 amps, so that would be one honkin' diode if there were one, and I haven't noticed one in the wiring diagram or in real life. I'd go with the 0.7 volts being an artifact of some other difference between your measurements at the front and the rear of the car.

    Your story sounds a lot like the classic story of a lousy, flaky connection between the battery and the car on the ground side, or the battery and the wire harness on the + side. When the car is READY and its DC/DC converter is supplying 14 volts, everything is good, but when you leave READY mode and the 12 volt battery is the power source, as soon as the connection flakes out, everything goes black.

    The power source ECU, oddly, remembers its most recent status in nonvolatile memory. If it was in ACC mode when things went black, it will wake up in ACC when you restore battery power, no key fob required. If it went black in ON mode, it wakes up in ON mode. It sort of surprises me they would do it that way, but they did, and that's why you see the "other weird things" going on.

    Where were you trying to download the wiring diagrams? You should have several ways to get them as decscribed here:

    Toyota Service Information and Where To Find It | PriusChat

    The 450 Ω that you measured I would not interpret as a short, but just as the existence of the usual loads in the car that are powered even when the car is off. They are really too complicated to describe as a single resistance, but if the tool in your hand is an ohmmeter, everything looks like ohms ....
     
    #2 ChapmanF, Jan 28, 2024
  3. Tombukt2

    Tombukt2 Senior Member

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    Base
    What kind of mileage and condition is this car in for a generation 2 this is really strange I mean is there been some sort of electrical problems that have been going on other than these for some time has the car been parked for some time and now just getting back on the road or some such kind of thing? Have rodents gotten in the car and been eating away at wiring? California it's a toss up depending upon where you are...
     
    #3 Tombukt2, Jan 28, 2024
  4. GoodOldBob

    GoodOldBob Junior Member

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    Vehicle:
    2020 Prius
    Model:
    L Eco
    Try replacing the key fob battery with a fresh one and then see what happens.
     
    #4 GoodOldBob, Jan 30, 2024
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