Brake light switch or 12-volt replacement?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by chaelra_prius, Feb 5, 2020.

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  1. chaelra_prius

    chaelra_prius New Member

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    Hello all,

    I'm suspecting my 2008 Prius has one of two problems: either the 12-volt battery or the brake light switch needs to be replaced.

    Symptoms: car will start fine after sitting overnight, but after being driven and sitting for 30-60 mins, the brake light will stop turning on, and thus the hybrid system won't start. It's as if I'm not pressing the brake at all. This can be frustrating and leave me stuck for quite a while as I keep attempting to start it.

    At first I thought there was a problem with the brake light switch only, after doing some searching here and online, but I noticed last night, when starting it with no problem, I had barely pressed the brake and could see the red light. So I'm leaning towards a battery problem, but really not sure.

    I followed the 12-volt battery check, and got the following numbers:
    12.1 ACC mode, 11.8-9 under load, 13.9 ready mode

    The only other symptom that could be related to battery is dropping to about 38 MPG in near-freezing weather, versus 50 MPG in summer. I usually drive short trips and take the bus/rail sometimes. The battery was once discharged after the car was not driven for 2 months and needed to be jump-started. I'm the second owner of the car, so some unknown history there. However, I don't see any signs of dash lights being dim, no dash warning lights staying on while in ready mode, nothing that screams 'battery replacement needed!'

    Thoughts?
    Thanks for reading.
     
    #1 chaelra_prius, Feb 5, 2020
  2. bisco

    bisco cookie crumbler

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    Battery may be on the edge, how old is it?
    If the switch works very time you test it, it isn’t that
     
    #2 bisco, Feb 5, 2020
  3. TMR-JWAP

    TMR-JWAP Senior Member

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    @chaelra_prius recently stated:

    Symptoms: car will start fine after sitting overnight, but after being driven and sitting for 30-60 mins, the brake light will stop turning on, and thus the hybrid system won't start. It's as if I'm not pressing the brake at all.
    Thoughts?
    Thanks for reading.


    It sounds to me like this is the problem you need to resolve....
     
    #3 TMR-JWAP, Feb 5, 2020
    Raytheeagle and dolj like this.
  4. sam spade 2

    sam spade 2 Senior Member

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    The "ready mode" number doesn't count but those other two are too low.
    Your battery appears to be weak.
    If it is more than 5 years old you should replace it......even if that is not the cause of the brake light problem.

    When the brake light is not responding, a quick jump should prove whether or not the battery is the culprit.
     
    #4 sam spade 2, Feb 5, 2020
  5. dolj

    dolj Senior Member

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    As well as agreeing total with TMR-JWAP above, my three additional thoughts are:
    This will be a problem both for getting good MPGs and also for keeping a good charge on the 12 V battery. You would be a good candidate for using a battery charger on the car when it is not being driven.
    This is not good for the 12 V battery. It could have significantly shortened its life.
    The only warning the car will give you to warn of a low 12 V battery is to flash the MCL (master caution light – / ! \ ) momentarily when the car is started.
     
    #5 dolj, Feb 7, 2020
  6. sam spade 2

    sam spade 2 Senior Member

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    OR maybe a multitude of seemingly unrelated lights and codes all at the same time.
    And sometimes refusing to start......sometimes but not others.
     
    #6 sam spade 2, Feb 9, 2020
  7. ChapmanF

    ChapmanF Senior Member

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    My first and probably most useful reaction here is:

    How did you go from those symptoms, to deciding you had a list of two and exactly those two possible causes to check?

    By setting it up as a binary choice, you're leading yourself into "well, if it isn't A, then it must be B" sorts of armchair thinking, where if it were my car I think I would just be out there with a wiring diagram and a multimeter to see what I was going to find out the real issue was.
     
    #7 ChapmanF, Feb 9, 2020
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