AM2 fuse keeps blowing, unable to find cause

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Yaro, Mar 14, 2022.

  1. Yaro

    Yaro New Member

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    Been at my issue all day with no luck, even with the help of you fine people and previous posts. I know someone has to have posted about this before, but I can't seem to find it. There have been many things that sounded close but didn't change what is going on, ill get into it.

    My 2005 Prius with about 270,000 miles has died mid drive. Died in this case means everything turned off, a fule consumption warning popped up on the screen. After pulling off the road and tring to restart the car I was met with an instant shut off and some flashing on the dash. I am unable to even get Prius into accessory mode.

    After a day of trouble shooting, I have learned that it isn't the inverter pump and it isn't the power source controller(PSC) ECU. I check those things cause I found threads here and other places that said those could be the culprits. I tried two used PSCs. One was from the exact Prius I own, a 05 Prius, and the other was from an 08 Prius. And as for the inverter pump it was recommended to unplug it to see if the car would start with it unplugged to diagnose the pump, fuse still pops with no power running to the pump.

    The only previous issue I had with the Prius was the dash would go out sometimes only to start working again after a random period of time.

    I know I am not looking at something that I need to, whether it be a thread or something in the car. Any help would be awesome, Thank you for reading.
     
  2. ChapmanF

    ChapmanF Senior Member

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    One trick that helps, for pinning down an issue like that without going through a heap of spare fuses, is to plug in something like a "fuse buddy" where the fuse goes, and wire it through a 12 volt light bulb.

    [​IMG]

    That way, the short (wherever it is) doesn't keep blowing your fuses, it just lights up the bulb. Then you work your way through the wiring diagram (more info) to see everything that branches off downstream of that fuse, and disconnect one branch at a time until the light goes out. Then reconnect that branch and repeat with the specific end components on that branch. Before long, you've found the culprit.
     
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  3. PriusCamper

    PriusCamper Senior Member

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    Nice... Gonna have to research fuse buddy more. Got any tips for best one to buy that's also the best price?
     
  4. ChapmanF

    ChapmanF Senior Member

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    Well, you can see it comes in two flavors, one with its own built-in ammeter, one with ordinary test leads you can plug into your own DVM (or connect to a headlight, or something, if you're using it to trace a short). So the plain-leads flavor is less money, of course, and also more versatile.

    There's no rocket science to it, it's got a pair of blades the right shape to fit in a fuse socket, and two wires. :) It also has its own fuse socket on the side. so you can put in the fuse that came out of the socket, and the circuit is still protected while you're testing.

    The plastic case has squarish shoulders that prevent it from fitting in some locations in Toyota's fuseboxes. I took mine apart to see the shape of the actual metal bits inside, then cut down the corners of the plastic case to match, and put it back together with Plasti Dip for insulation there, so now it fits in more of the fusebox locations, anyway.
     
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  5. rjparker

    rjparker Tu Humilde Sirviente

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    The AM2 fuse is a difficult one since it is essentially a switched ignition power source that feeds many systems.

    1B3B62EB-9495-4AAC-B292-71ED3B00DA2A.jpeg
    If you have a hard fail, meaning a permanent short, a test light check looking for a ground or special tool like the fuse buddy gets you a test instrument. Then you have to start disconnecting wires usually at the connectors.

    Often techs might pick one circuit and work backwards.

    The real problem is the multiple circuits involved and the fact that Toyota schematics sometimes show black boxes such as the power source control ecu and assume you have been trained to know how and when the outputs are connected. Often one system is illustrated on a drawing, yet there are many other systems being fed by that fuse. For example the fuel injectors are powered by the fuse but you would not know that from this drawing.

    3E3C5B10-0BEA-4DFE-A213-934681981DC8.jpeg

    So you look for another drawing

    AECD6A1B-B422-4978-BD7F-617EBB105EDF.jpeg

    To add confusion, a fuse like this feeds different paths depending on your trim and options. Sometimes Toyota will show both circuits on the same drawing using * to differentiate.

    The online paid site has some advantages here since it will highlight the circuit better and link to explanations.

    https://techinfo.toyota.com/techInfoPortal/appmanager/t3/ti?_nfpb=true&_pageLabel=ti_home_search_docs_page
     
  6. rjparker

    rjparker Tu Humilde Sirviente

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    Here is a similar issue. The root cause and solution is pretty common even though the car is not the same.

     
  7. Yaro

    Yaro New Member

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    Thank you so much guys! I started doing troubleshooting day one. At the end of the second day I decided to replace the Power Source Controller ECU, the fuse still popped. So I replaced the dash, as it was going in and out sometimes, did my final test to find that the fuse was still popping and went home. At the beginning of day three I just put a fresh fuse in and the car started, without my dash working, but the car started. Why? I don't know. But the car has been working for 4 days now.

    I'll try to fix the dash going out issues this weekend where I'll try to solder on a capacitor.
     
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  8. Yaro

    Yaro New Member

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    Not sure if anyone is still following along here, but the issue went away and reappeared again. One thing different is right before the AM2 fuse popped again, the car kept screaming at me to repark the car on a level area or something about the parking brake or something. If anyone is still following along, anything would help. Not sure where to start taking things apart as far as the parking brake goes.
     
  9. rjparker

    rjparker Tu Humilde Sirviente

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    I doubt it has anything to do with the manual parking brake. Perhaps the Park position shift actuator was not working meaning the car could roll when off instead of being in Park. But that could just be a side effect of 12v switched power not properly operating hybrid controls or the Park mechanism itself.

    It would be interesting to know if the fuse continues to blow or will it take an intermittent short or device failure to overload it?

    The last post from March was pointing at the dash again. Was a problem found there?

    One troubleshooting fallacy is to assume something you already checked can't be the problem. The dash and inverter pump come to mind.

    There are auto electric specialists who have experience in these kinds of difficult shorts. Isolation of downstream parts is often the key strategy. Having an excellent wiring document for your vehicle year and trim is essential. Sometimes they have to de-pin connectors, insert probes through wire insulation, bypass power to certain devices through a different path, it gets tricky. Moving wiring and connectors may temporarily resolve the issue making a permanent solution difficult. Even dealers are not guaranteed solutions in cases with something intermittent. They and others may resort to wholesale part and harness changes.

    So if diy is still the plan, I would revisit the assumptions and verify if the short is continuous.
     
    #9 rjparker, Aug 20, 2022
    Last edited: Aug 20, 2022
  10. mr_guy_mann

    mr_guy_mann Senior Member

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    I might wonder if the warning message is a result of the AM2 circuit voltage going low as the high draw is overloading the fuse. I would scan all ecu's for codes.

    One way to isolate a fault is to add smaller fuse in part of the circuit. In this case, you can put a fuse holder in the ground side of the inverter coolant pump with a 7.5A fuse. If the pump has a short, it will pop the small fuse, not the AM2 fuse.

    Posted via the PriusChat mobile app.
     
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