Intermittent electrical start problem 2007 Prius

Discussion in 'Gen 2 Prius Main Forum' started by RafaelRobyns, Jul 27, 2018.

  1. RafaelRobyns

    RafaelRobyns New Member

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    Location:
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    Vehicle:
    2007 Prius
    Model:
    II
    Over the last two weeks my Prius has started and run without issue except for three times when I had absolutely no electrical. That is, I push the start button and nothing comes on, no dashboard lights or sounds. A jump start even without the source engine on starts it right up. There is a clicking sound from the fuse box when the jump start connection is made, then there are dashboard lights and the car starts, drives fine. It is as if there is a relay or something stuck in the wrong position that is corrected by the jump. Aux battery test showed it is fine. Please let me know what to check, if there is a relay to check or replace or something more serious. Thx.
     
  2. Patrick Wong

    Patrick Wong DIY Enthusiast

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    1. Check the tightness of the battery cables at the battery and the negative cable where it bolts to the body.

    2. Regardless of the battery test, the 12V battery may need to be replaced.
     
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  3. TMR-JWAP

    TMR-JWAP Senior Member

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    Vehicle:
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    Touring
    I'm curious about the "aux battery test showed it is fine". Can you explain how that was performed and what the results were that determined it to be fine?

    here's the long version of PW's answer

    The 12v battery in your trunk has a positive and a negative terminal.

    The negative terminal post is connected directly to the car body by a cable that is about 6 inches long. It clamps to the negative terminal post and bolts to the car body. Both of these must be tight. If you grasp either end of the cable and can make it wiggle then the fastener is insufficient. I've seen a negative clamp terminal incorrectly installed and it would spin on the terminal post. This was done by an installer when the battery was replaced a few weeks earlier. Very intermittent problem since it would sometimes work and sometimes not. Easy to troubleshoot and fix though if one understands the 12v system.

    The positive terminal post has a "fuse box assembly" (for lack of a better term) clamped to the post. It has two fuses installed in it. It also has two cables (one large gauge and one small). The large gauge cable goes directly to the "jump" terminal in the engine compartment fuse box. The small cable goes to the inverter, iirc. The large gauge cable supplies power to the fusebox and the fusebox distributes power to the remainder of the car. There are no other components in that line. Again, the clamp at the positive post must be tight. The two cables must be fully plugged into the "fuse box" clamped to the positive post. I've never seen the cable at the engine compartment fuse box be loose. That's a very unlikely problem.

    If the car jumps fine, then you are not getting proper 12v power from your battery to the engine compartment fuse box jump terminal. This can only be due to a bad connection at one of the cables or the battery is not holding a charge.
     
  4. RafaelRobyns

    RafaelRobyns New Member

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    Location:
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    Vehicle:
    2007 Prius
    Model:
    II
    Funny coincidence: this problem developed on the day that installed a new bulb in the driver side headlight. Because that involved opening the fuse box, removing a couple of relays, and pushing wires around (all to gain access), I was focusing on potential problems I might have caused under the hood. But following your advice, I checked the connections to the aux battery and found the negative connection to be just slightly loose. After tightening the car seems to be starting fine. Thanks for your help, and may this thread help some other misguided soul.
     
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