Both headlight low beam bulbs are dim. High beams are fine.

Tiny
COGENT1
  • MEMBER
  • 2010 FORD FUSION
  • 2.5L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 97,000 MILES
I have a 2010 Ford Fusion SE 4-cyl. Both headlight low beam bulbs began to dim about 2 months ago. High beams are fine. I had a free electrical system & battery check performed by Advance Auto Parts store. Everything passed their tests fine. Ford dealer nonchalantly said it's the wiring harness without performing any visual or hands-on tests. Is there one wiring harness that connects / controls both bulbs? Could a bad ground affect both bulbs? Could there be a fault in the Smart Junction Box that would allow both bulbs to dim without shutting them down completely? Any suggestions of what / how to diagnose are appreciated. Thank you.
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Saturday, May 11th, 2019 AT 6:02 PM

2 Replies

Tiny
KASEKENNY
  • EXPERT
Hi Cogent,

The reason they are saying it is wiring is because anytime a light dims is because there is additional load on the circuit then just the bulb. The way a circuit works is there is wiring which does not have much resistance but the bulb or load is what uses the voltage and then it completes the path back to ground. When you have wiring that is getting old and has higher resistance, it basically steals some of that voltage that the bulb would use to make it as bright as possible which causes it to be dim. Less voltage getting to the bulb means it is not as bright.

Looking at the wiring diagram attached, I don't see know that wiring is your issue. Going from the switch to the smart junction box there is one wire for the low beams. However, coming from the junction to each bulb it has separate wires and grounds so this is unlikely to be your issue. The fact that both bulbs are dim shows that you have an issue from the junction box back to the switch.

You can easily ohm out your wiring starting at each bulb by disconnecting it from the bulb to the junction box and test your resistance. Then do the same from the junction box to the switch. You should have about 0.5 ohms of resistance or less on wires.

If you don't have high resistance in any of your wiring then I would agree that your issue is in the junction box.

https://www.2carpros.com/articles/how-to-check-wiring
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Tuesday, May 14th, 2019 AT 10:56 AM
Tiny
KEN L
  • ADMIN
Hello,

It sounds like you could have a bad ground but to be sure here is a guide to help check the voltages and ground connection with the headlight wiring diagrams so help you see how the system works.

https://www.2carpros.com/articles/how-to-check-wiring

If the smart junction box is bad here are instructions to replace that as well. Check out the diagrams (Below). Please let us know what you find. We are interested to see what it is.
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Tuesday, May 14th, 2019 AT 11:01 AM

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