Piston Slap: Foiled by a Dome Light?

Sajeev Mehta
by Sajeev Mehta
piston slap foiled by a dome light

Peter writes:

I drive a 2010 Toyota Prius. From top to bottom, I’m thoroughly impressed by the technology in this car. And yet this engineering marvel is so easily disabled by its inferior owner leaving a dome light on overnight and draining the 12V battery.

Is there any technical reason cars allow the 12V battery to be drained down beyond the point where the car will start? Who needs that extra 6 hours of dome lighting?

Sajeev answers:

And here I thought every modern vehicle implemented a battery saver relay/switch/doohickey, eliminating human error! The Best and Brightest better find us a reason not to isolate such “human-error only” items, as hybrids aren’t exempt in my book.

Since when did a map/dome/cargo light play such an influential role in an electrical system’s design?

Let the people have an always-on USB port/cigarette lighter for an emergency phone charge (they stop charging at 100%) or for a hit of that sweet, sweet nicotine monster. Everything else open to human errors should shut off after 10-30 minutes of inactivity from a human or proximity key.

I’d recommend one of these:

Easy: change the offending (reading/dome/cargo) interior lights to LED bulbs. Not all are created equal in terms of light quality, but that’s a bigger concern for exterior lighting. LED’s lower power consumption could be the difference between a start or a no-start on that tiny 12V battery.

Hard: Install a battery saver relay intended for another application, a battery saver system, or a trigger module: all require extensive knowledge of the Prius’ chassis wiring to make sure the 12V system won’t go berserk over such a change.

[Image: By Mariordo Mario Roberto Duran Ortiz (Own work) [ CC BY-SA 3.0], via Wikimedia Commons]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry … but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

Sajeev Mehta
Sajeev Mehta

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  • Manu06 Manu06 on Dec 06, 2016

    Instead of an on/off switch, why not a mechanical timer you twist ? Maybe15 minutes max ?

    • DenverMike DenverMike on Dec 06, 2016

      We want a switch that's easily slapped on, hit it with a knuckle if hands are full, kick the door closed, and that's it. No need to reinvent the wheel here.

  • LTDwedge LTDwedge on Dec 08, 2016

    Call me aware ! Any ORANGE colored high voltage wiring - read "LETHAL" needs to be left alone. Just having safety gloves and a wiring diagram may not prevent DEATH. Just don't want to read about someones unplanned demise. Insufficient knowledge will kill.

  • ToolGuy "Mr. President, no government agency, no think tank, and no polling firm knows more about the automobile customer than us. We talk to customers every day. As retail automotive dealerships, we are agnostic as to what we sell. Our business is to provide customers with vehicles that meet the needs of their budgets and lifestyles.”• How many lies can you fit into one paragraph?
  • Spamvw Three on the tree, even Generation X would have a hard time stealing one of those.
  • ToolGuy This trend of cyan wheels needs to end NOW.
  • Kwik_Shift Interesting nugget(s) of EV follies. https://x.com/WallStreetApes/status/1729212326237327708?s=20
  • SaulTigh I've said it before and I'll say it again...if you really cared about the environment you'd be encouraging everyone to drive a standard hybrid. Mature and reliable technology that uses less resources yet can still be conveniently driven cross country and use existing infrastructure.These young people have no concept of how far we've come. Cars were dirty, stinking things when I was a kid. They've never been cleaner. You hardly ever see a car smoking out the tail pipe or smell it running rich these days, even the most clapped out 20 year old POS. Hybrids are even cleaner.