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Help with Parasitic Drain - 05 Prius

Discussion in 'Gen 2 Prius Technical Discussion' started by e9coupe, Dec 3, 2024 at 12:40 PM.

  1. e9coupe

    e9coupe Junior Member

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    My wife's 05 Prius has a small power drain while parked, running the 12v battery down enough to not start the car after some time. It seemed random at first, with the car being fine for days, then sometimes after four hours it's down below 10v.

    The start with, I'm a notice when it comes to electrical issues. Give me a drive train to rebuild or body and paint work and I'm good to go. Electrical, not so much. I've read quite a few threads about parasitic drains on the second gen Prius' and I've tried what I can/know but have reached the not-sure-what-to-do-next point. I also managed to blow the fuse in my multimeter last night so that's another reason I stopped. (checking voltage and forgot to move the 10a lead) 8-/

    With the key away from the car, the only draw I find is with the Dome circuit, which I understand to be normal. All other fuses and relays in the two fuse boxes don't change anything. With the Dome fuse installed, doors shut, lights off and pretty much every connector disconnected on the dash (apart) I'm still seeing around 140ma draw.

    Edit: The 12v battery also checks out good (12.6v) and I'm using a battery charger while I work on it.

    Not sure what to check next. Opinions?

    Dan
     
    #1 e9coupe, Dec 3, 2024 at 12:40 PM
  2. Brian1954

    Brian1954 Senior Member

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    How old is the 12v battery?
     
    #2 Brian1954, Dec 3, 2024 at 5:49 PM
  3. e9coupe

    e9coupe Junior Member

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    2021 maybe? I'll check tonight. I have another good battery (2023) in my shop out of another Prius that I was going to swap just to check that off the list and that's when I shorted my multi-meter testing voltage! Nice eh.

    We have a battery load tester at work. I can use that too for the hell of it. It's the old school meter with a load button. We also have the electronic type tester too. What's the best way to tell if a battery is going bad? I've charged it and let it sit for two days before connecting it to the car and it worked fine. Didn't know what else to test.
     
    #3 e9coupe, Dec 3, 2024 at 6:48 PM
  4. bisco

    bisco cookie crumbler

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    get a free load test at many auto parts stores
     
    #4 bisco, Dec 3, 2024 at 8:27 PM
  5. rjparker

    rjparker Tu Humilde Sirviente

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    140ma is certainly too much.

    First, electrical troubleshooting of this kind can be tricky and can require experience and the right tools. Along with a good visualization and understanding of the circuits.

    Be sure no aftermarket obd2 scanners, alarms, gps or other rouge player is not causing issues. Sometimes wired straight to the battery or one of the main busses downstream of the 100 amp fusible link.

    Even when you find the fuse or fusible link sourcing the excessive parasitic draw the work has often just begun. So there are times hiring a specialist is far better than simply Mendel-ing it, eg leaving it on a battery charger 24x7.

    Of course we know some fuse or fusible link must have been missed.

    Fusible link symbol
    IMG_6844.jpeg

    Maybe you missed the fusible links in the hard to remove fuse box module?

    Prius Gen2 Eng Fuse Box & Fusible Link Mechanical.jpeg

    Did I hear you say "what fusible links"?

    Prius Gen2 Fusible Link.jpeg
    The nine in this ^ fuse box module.

    To measure ma current draws in that module is harder since the fusible links don't come out individually. It is possible that a millivolt test might help with the cover pried off. Otherwise it is necessary to find specific load wires and ideally use a sensitive dc clamp meter. Or find a wiring disconnect method for each.

    Its times like this that Ivan at Pine Hollow Diagnostics or similar should be called in.

    PS Don't leave the hatch open even if the dome light is out. Ecus are still scrambling to figure out what you are doing rather than going to sleep like nice boys and girls.
     

    Attached Files:

    #5 rjparker, Dec 3, 2024 at 9:51 PM
    Last edited: Dec 3, 2024 at 10:02 PM
  6. mr_guy_mann

    mr_guy_mann Senior Member

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    When I want to perform a key-off draw test, I open the tailgate, hood, and driver's door. Then use a screwdriver to trip the door and T/G latches to the closed position. If SKS, move the key more than 40 feet away.

    Wait about 20 minutes for all systems on the car to go to sleep.

    Then I use an amp clamp (the UniT UT210E is a decent budget tool). Turn the dial to the 2A range. Press the blue button to switch to DC amps. Hold the meter close to the negative battery cable before pressing the "zero" button. Now place the clamp around the cable for a reading.

    The 210E isn't the most accurate at very low current but it should be ok enough, just figure at least +/- 10mA to the reading.

    Typical for a known good Gen2 is 24mA.

    My preferred method to trace high draw is to measure mV across any & all accessible fuses. Anything higher than 0.1 mV is suspect.

    Posted via the PriusChat mobile app.
     
    #6 mr_guy_mann, Dec 3, 2024 at 10:34 PM
    Brian1954 likes this.
  7. e9coupe

    e9coupe Junior Member

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    Three
    Wow, awesome info guys. In the end I think I'm just a little dumb. I load tested the battery last night and it's definitely questionable, along with it being a 2019 date code. I got a new one this morning and will install tonight and update here.

    Regardless, I never consider troubleshooting a waste of time, even in this scenario. For one, the dash comes apart the easiest I've ever dealt with in a car and I always end up learning more about the cars this way. Okay, maybe electrical stuff doesn't get absorbed as much but you know what I mean. :cool:

    This info you guys posted will also get used over and over in the future. The beauty of vehicles staying on the road and info/data always being relevant. I run a classic car forum and get useful info from posts 20 years old!

    Thanks again,
    Dan
     
    #7 e9coupe, Dec 4, 2024 at 12:39 PM
    Brian1954 likes this.
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