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Trailerhitch dims aftermarket headunit...

Discussion in 'Gen 2 Prius Accessories & Modifications' started by R-P, Mar 29, 2017.

  1. R-P

    R-P Active Member

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    I've had this configuration for quite some time and never before noticed this. I think it did not happen before.

    I have an aftermarket headunit that works fine (apart from the radio reception being way worse than my Volvo with a window FM antenna). It is hooked up to dim its screen when I turn the lights on the Prius ON.

    Today I noticed the brightness was flickering. Eventually I noticed some kind of pattern: whenever I was braking, it would dim. Sometimes when having the indicator ON, it would dim with the frequency of the indicatorsound. But sometimes it would NOT dim with the indicator.

    One important thing to mention, is that I had a bike on my trailerhitch, which also means I had the extra licenseplate and an extra set of brake/indicator/rear-lights hooked up.

    I mounted the trailerhitch myself and even though I fabricated a relaybox for switching these lights with only 10mA, I never fitted this because I read on several occasions that the Prius CAN-bus doesn't care if you simply parallel an extra set of lights on the existing rear-light-clusters. So being a fan of the KISS* rule, this is what I did and what has been working flawlessly for years.
    Once I removed the bike, and obviously the extra light-cluster, all symptoms were gone and the headunit works fine again without any dimming.

    My theory is that the increased currentdraw of the extra (21Wx2) brakelights or (21W) indicator causes so much extra current that *somewhere* some computer thinks I have my lights switched ON and it should send a signal to the "illumination' wire in the radio-wiring harnass.
    Which doesn't make sense as I doubt the Prius is still so basic that there isn't a computer involved that actually KNOWS from the position of the lightswitch whether or not the lights are ON.

    Obviously it could also be the wiring in the 13pin trailerhitch plug that shorts one wire to another through inertia when braking, or it might just be gremlins.

    Any thoughts welcomed.


    * for those that don't know it: KISS means Keep It Simple, Stupid! Meaning that the simpler you keep something, the less likely it is to fail. [rant]Microsoft has never heard of this, which is why my computer is set NOT to adjust the time/date, yet it wakes up in the middle of the night, adjusts my date one day forward and goes to sleep again. When looking for WHY it did that, all info that I can find is that there are no waketimers and that Windows also has no clue what made it wake up and adjust the date.[/rant]

    Edit: this was moved from technical discussion to accessories and modifications. I do not agree. This has (if my assumptions are somewhat correct) very much to do with how the Prius operates and e.g. measures currents and the fact that an aftermarket modification caused this to surface is imho inconsequential.
     
    #1 R-P, Mar 29, 2017
    Last edited: Mar 29, 2017
  2. 69shovlhed

    69shovlhed Surly tree hugger

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    first response is always: did you check your 12v battery? next would be check all your ground connections.
     
  3. ChapmanF

    ChapmanF Senior Member

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    The 'illumination' wire to the headunit is the taillight circuit. When the headunit sees that on, it switches to night brightness.

    I've seen trailer wire harnesses or lighting setups mucked up in such a way that powering the brake lights will result in backfeeding the taillight circuit. Or, that can happen if the lights use dual-filament stop/tail bulbs, and the ground connection is bad.

    That's where I'd start.

    -Chap
     
  4. edthefox5

    edthefox5 Senior Member

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    It may help to get a external power supply that provides extra power to the trailer lights. If you buy a trailer light kit for a G2 from say e-trailer it comes with a n extra 12 volt supply just for the lights. You connect that power supply to the 12 volt battery. Works great that's what I have.