2012 Hyundai Sonata Headlight Switch

Tiny
ADOG
  • MEMBER
  • 2012 HYUNDAI SONATA
  • 2.4L
  • 4 CYL
  • FWD
  • AUTOMATIC
My two back brake lights stop working as soon as my headlights are turned on, but the brake light above the trunk in the back windshield stays lite up constantly. However when the headlights are turned off, everything works properly. The cruise control also does not work when the headlights are on, but works correct when headlights are off.
Tuesday, October 6th, 2015 AT 7:37 PM

1 Reply

Tiny
CARADIODOC
  • MECHANIC
  • 33,833 POSTS
These symptoms are typical of a bad ground wire for the rear lights. You'll find with the head lights off, the brake lights really aren't working properly because they're not as bright as they should be. Current is indeed going through the brake light bulbs, but to get to ground it has to also go through the tail light filaments. That reduces their brightness but that's often hard to notice. Turning on the head lights / running lights removes that ground path for the brake lights so they stop working. The loss of the ground puts voltage from the tail lights onto the brake light circuit, and that voltage is what the cruise control system sees and thinks you're pressing the brake pedal. That's its cue to kick out and stop working.
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Tuesday, October 6th, 2015 AT 8:02 PM

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