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ACC B fuse blowing

Discussion in 'Gen 2 Prius Main Forum' started by theclarinetguy32, Aug 29, 2016.

  1. theclarinetguy32

    theclarinetguy32 Member

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    This may be familiar to some, but I really want to bring it up again to see if anyone might have some new ideas.
    2007 Pri, package 3. Only addition is a KarTrack GPS unit from the original dealer in San Diego.

    In the past few years, every time I have to brake suddenly, the ACC B fuse blows. Its the white 25A fuse 3rd from the right bottom if your looking up underneath the drivers area (I think it's right...)

    The braking isn't a slam on, ABS activate type, it's the , light turns to red faster, car pulls out with enough time to brake suddenly.
    Thankfully this isn't a lot, but when I do have to brake like this, poof, there goes the center display, clock, radio, cig lighter. Not a full impairment of driving, but certainly concerning.

    I've seen suggestions such as loose wiring behind the brake pedal to something else on the electrical grid that could be overloading. so far I haven't been able to find anything that gives a clue, including none of the above.

    After it happens, I've noticed it takes a decent number of hours (always varying) before I can successfully put another 25A fuse in to get things up and running. If I try too soon, either with the car off or on, it just blows within a second or so of insertion.

    so, can anyone think of something that would cause hard braking (or maybe the mechanisms involved) to pop this fuse?

    This seems to be a mystery that folks here, toyota techs and anyone else prius/toyota related haven't had a clue on.
    Any ideas are welcome!

    Thanks
     
    #1 theclarinetguy32, Aug 29, 2016
  2. edthefox5

    edthefox5 Senior Member

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    That fuse supplies power to the relay ACC RELAY that is powered on in ACC/IGN and provides B+ to Fuse 15 amp PWR OUTLET and 7.5 amp ACC fuse so probably whatever is plugged into the 12 volt cig lighter in the car is fubar.

    Its weird its not blowing the 2 smaller downstream fuses. Which makes me think there's something going on with the relay itself.
    I would replace the relay first as its cheap but not so quick. Its mounted to the left of that fuse panel your in under the dash.

    If not able to do that that try unplugging the 15 AMP POWER OUTLET fuse. May just be the cig outlet is shorting out. They go bad I had to replace mine. Look in the socket and see if its corroded. See if that stops the 25 from blowing.

    The 7.5 fuse is an input to the Gateway ECU. It tells the gateway its in acc not ign. Apparently the car does not malfunction when the 25 amp fuse blows so the 12 v to the gateway only matters if you want to go into just acc. Try unpluggng that next.
     
    #2 edthefox5, Aug 30, 2016
    Kevin_Denver likes this.
  3. Kevin_Denver

    Kevin_Denver Active Member

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    It sounds like a combination of a heavy load on that fuse combined with the voltage surge that will occur during sudden regenerative braking is blowing the fuse. The high voltage to 13.8 volt inverter adjusts to maintain 13.8 volts, but it's not instantaneous. So if the battery voltage suddenly increases from 200V to 300V as would occur during a sudden brake stop and engaging of maximum regen, there is going to be some voltage surge on the 12V battery from the inverter (such electrical events usually last less than 100ms, not long enough to coming close to damaging any electronics, but could cause a surge in current enough to blow a fuse already near max capacity). This voltage surge combined with a fuse already close to capacity could cause it to blow. This would explain why it blows during braking, but doesn't explain why the fuse would be at capacity which would cause the fuse to blow when adding a fuse with the car off.

    Does the fuse blow when the cigarette lighter is not used? The downstream fuses are a 7.5A ACC fuse, and a 15A cigarette lighter fuse. I would confirm that both of these fuses are the correct size, and for good measure, replace them both. Fuses have pretty high tolerance, so that 7.5A ACC fuse could be acting more like a 10A fuse, and that 15A cigarette lighter fuse could be acting more like a 20A fuse. If when you replace these fuses, one of them starts to blow instead, it points to a problem on that end on the fuse.

    If replacing these fuses does not cure the problem, the source of it is most likely the ACC relay. The second most likely source is wiring between the 25A fuse, the ACC relay, and the 15A and 7.5A fuses for the cigarette lighter and ACC.
     
    #3 Kevin_Denver, Aug 30, 2016
  4. theclarinetguy32

    theclarinetguy32 Member

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    Excellent, info, thank you both!


    I do recall having nothing in the 12V front port when trying to work with this issue, but I do recall that port being a tad bit finicky like you were saying Ed. Didn’t seem to make any difference if something was plugged in there or not.

    The fuses in place are all correct according the panel display or manual.

    I have to do some other work on the vehicle soon, so I’ll see what is needed for parts on both the 12V cig outlet and the relay. Seems easier to swap things out all at once and establish a new baseline.

    Would either of you have a recommended place to get these parts from? I figure the cig lighter assembly is prob not hard to find but I’m not sure the exact part number the relay would be.
     
    #4 theclarinetguy32, Aug 31, 2016
  5. edthefox5

    edthefox5 Senior Member

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    All I have is the schematic. You need the manual.

    Go to toyota.techinfo.com and you can get a one day subscription to the oem manual. Make sure your printer is ready to rock. There's much more inclusive info there including troubleshooting tree, dash disassembly and part numbers. The relay is call A12 ACC RELAY.
    I bought my cig socket online toyota oem.
    Btw, you said there's something the dealer installed. They probably chopped into the acc line for power so get a flashlight and lay under that area and take a look for errant wires and fuse taps poorly installed.
    But my guess is the cig socket contacts are corroded and shorting out. Dont know why the downstream smaller fuses aren't blowing which makes me suspicious about the dealer installed chopped into it right after the main acc 25 amp fuse..
     
    #5 edthefox5, Sep 2, 2016
  6. theclarinetguy32

    theclarinetguy32 Member

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    I actually had pulled down some of the manual info pertaining to the wiring, but in all honesty that didn't give me a whole lot to go on. I could see the layout on paper but that didn't allow me much to go on for hands on troubleshooting.


    The item that was installed was a GPS unit. The car was brought up from San Diego to the Toyota of Seattle, and from what the paperwork says, Southwest Dealer Services installed a KarrTrack device while it was down there. Sounds like it's the little brother to onstar although hardly known in the northern states.

    Two winters ago when I was having an issue with the car not recognizing key fobs, the only thing people could think of was this gps unit. The only info I was able to narrow down was for this model Prius, the device is probably installed way up in the passenger dash. Since it was the middle of winter at that time I wasn't terrible excited about tearing apart the entire dash. Maybe I can catch a glimpse of it from where the cabin air filter is...
     
    #6 theclarinetguy32, Sep 2, 2016
  7. edthefox5

    edthefox5 Senior Member

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    That possibly infers they taped off the cig lighter path. Look behind the cig lighter and follow that wire.
     
    #7 edthefox5, Sep 3, 2016
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