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12v battery died parasitic drainage AM1 fuse?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by JohnStef, Jul 10, 2017.

  1. JohnStef

    JohnStef Member

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    Yesterday morning I was on my way to church only to find a very dead 12v battery and no response from the car as I pressed the start button. I found that the system was drawing aprx 5 amp with no load (closed car doors) After reading thru the forum a bit I got help from my wife and looked at the parasitic drain by pulling relays and fuses one at a time. Under the dash I found that removing AM1 the draw went from 5 amp to 1.5 amp. I figured this was it, and it seemed with a label like AM1 it was something to do with the radio. It is actually the Hybrid system. I pulled this out and in a couple of times and with the fuse out tried to start the car. It did not work at all (of course, since it disabled the hybrid system) but after replacing the fuse and a freshly charged 12v battery, the car started and runs with no problem.
    The AM1 fuse is the only thing that would cause the amp draw to go down.
    My cars is running but I am spooked by this.
    Any suggestions on how to get to the bottom of this?
     
    #1 JohnStef, Jul 10, 2017
  2. JC91006

    JC91006 Senior Member

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    Do you have a SKS smart key? If you turn that off, that may minimize the draw
     
    #2 JC91006, Jul 10, 2017
  3. Sam Spade

    Sam Spade Senior Member

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    A little clarification is necessary, I think.
    As soon as you open the door, some (most?) of the computer systems "come alive" in preparation for "starting".
    If nothing actually happens, they will go to sleep again after some time.....a minute or two ??
    If you keep doing "things", they will stay "alive"......and this is not necessarily parasitic drain.
    To actually measure the parasitic drain, all doors, buttons and switches must be in the "off position" for that de-activation period.
    I think it is only a couple of minutes.......but a half an hour should be safe.
     
    #3 Sam Spade, Jul 10, 2017
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  4. JohnStef

    JohnStef Member

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    JC91006 I do use the SKS key, always

    Sam while pulling the fuse/relay under the hood and in the cabin on Sunday, the amp draw was about 5 amps. Now that things are working again, the "normal" draw with the rear hatch closed and the car is "awake" as you describe above is 1.4 amps.
    I am pretty sure there was something out of the ordinary where excessive current drainage took my 12v battery to zero. The fuse I pulled on Sunday brought the current draw back to normal. From an on-line Toyota Elec diagram is says: the AM1 fuse fees the following:

    Interior Light / Push Button Start System / and Hybrid Vehicle Immobilizer System / Shift Control System / TOYOTA Hybrid System.

    file:///C:/Users/d29041/Downloads/prius%20wiring%20(2).pdf

    One of these characters is the bad guy
     
    #4 JohnStef, Jul 11, 2017
  5. Patrick Wong

    Patrick Wong DIY Enthusiast

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    1. I am aware of an AM2 fuse, but not an AM1 fuse.
    2. The most likely reason for abnormal quiescent current draw is a relay stuck in the closed position.
    3. It would help if you are able to observe a circuit which remains powered up when it should not be, while the car is IG-OFF. For example, if you saw the multifunction display was on, or the cabin ventilation fan remained on. That would provide a good clue about which relay to look at.
    4. Lacking such clues, you would need to use the electrical wiring diagram to look at all systems which receive power via the fuse in question, and look for relays downstream from that fuse. Tap each such relay with a screwdriver to see if that causes the relay to release. If that doesn't help, then you would need to remove each relay to see if that makes a difference.
    5. When the Prius is IG-OFF and locked, the quiescent current draw should be around 0.02A after the various systems settle down and assuming the Smart fob is a reasonable distance away.

    Good luck.
     
    #5 Patrick Wong, Jul 12, 2017
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  6. JohnStef

    JohnStef Member

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    Patrick it looks like I have a 2004 model according to the pdf I found online - see below
    Below is a visual summary of the AM1 fuse location and connected devices.
    So far so good with my driving experience - no issues. And as you suggest observing the draw above, it has not happened again so I cannot. I agree with your suggestion that a relay stuck and drained power, but it is OK for now.
     

    Attached Files:

    #6 JohnStef, Jul 13, 2017
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  7. bisco

    bisco cookie crumbler

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    perhaps you just had a 'senior moment'. i'm finding them more and more frequent.:p
     
    #7 bisco, Jul 13, 2017
  8. Patrick Wong

    Patrick Wong DIY Enthusiast

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    This is very interesting. The newer 2G Prius do not have an AM1 fuse. It was removed from the 2006 electrical wiring diagram, for example.

    It appears the only job of AM1 is to provide a permanent 12V source to the power source control ECU. If you look at the four snapshots of the AM1 fuse, in all cases they show the same circuit where the AM1 fuse feeds power to the AM1 terminal of the power source control ECU. That is probably why it was eliminated in subsequent 2G models.

    When you were pulling fuses, did you by chance try the AM2 fuse and DOME fuse to see if their removal would result in a current reduction? I suggest you try to find other fuses which have an impact, as AM1 isn't doing much.

    If you really were to find that AM1 was the only fuse which drove a quiescent current reduction, that would lead me to believe the power source control ECU was having a problem. However if true this would be an exceedingly rare event.
     
    #8 Patrick Wong, Jul 14, 2017
    Last edited: Jul 14, 2017
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  9. JohnStef

    JohnStef Member

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    The AM1 fuse was the only one that had the effect of reducing current draw. I agree that it was likely "something" in the ECU was stuck, so to speak. So far and 2000 miles later no more issues.
     
    #9 JohnStef, Jul 24, 2017
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