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I noticed last night when I locked the car the front left side fog light was not on, the right side was as it should be. My 2010 C300 has the two round fog lights with H7 bulbs.
I hardly ever use the fog lights and always keep the light switch in the Auto position, due to the issue with the parking lights randomly turning on and off at night when the car is parked. The only time the fog lights come on is unlocking/locking the car at night, i guess thats the feature.
I was hoping it was just the bulb but the bulb looks perfect. I have two other bulbs that I tried and still nothing. The message on the dash now says Check Left Fog, last night I never saw it come on.
I tested the voltage and seems to be around 8 volts. I tried testing the ground for continuity with a ground point in the engine bay and nothing. I tried looking for the wiring diagrams but cant find a diagram on the fogs. From searching the manual it appears the fogs run directly from the front sam.
Any ideas?
I'll post what I could find. A diagram showing the fogs running to the front sam. Then what it says about the fog light activation. My build year is 3.12. 09
I drew in the red line showing where the diagram pages meet.
I tried testing the ground wire with my multimeter. Im hardly an expert so wasnt sure what I was doing. Hooked one probe to the ground on the connector then I touched the other probe to 3 separate ground points in the engine bay and nothing. To double check I connected the multimeter to the ground points and got a beep. So now im thinking its the ground wire. The connector itself looks fine and I cleaned it a few times just to be sure.
Finally tracked down where the left fog light ground goes, to ground point W9. It shows the headlight connected to the same ground and the headlights work fine. Will try again tomorrow. Will have to trace the wires and see where W9 even is.
- Tested for continuity from the fog light connector ground to where the battery is grounded to the car and got nothing. Just to check I tested from the battery ground over to a ground point in the drivers side engine bay and it was beeping away.
- Turned the fog lights on and tested the green/black wire to the drivers side ground point in the engine bay and was getting over 10 volts.
I was hoping it was the ground and not an issue with the sam or the power wire. I did come across some more electrical diagrams so I at least know which connector the fog lights are at the sam.
I spliced in a new ground wire and the fog light is working again. I have no idea how the ground wire was damaged, the two wires go up into a larger bundle of wires so cant really see and inspect the wire. Ended up just cutting the ground wire and adding a new wire and ring at the factory ground location.
Used my Bernzomatic soldering iron for the splice. It comes in handy outside under the car.
Tested the repair and it worked.
Cleaned the brake hard lines and sprayed with some Blaster surface shield while in there.
How did it happen? 15 years of ocean air and northeast winters. Down here it’s T-storms that dump 5 inches of rain in an hour in summer and just rain every day in winter. All that stuff eats older electrical systems.
Glad to see some ingenuity applied to repairs. And the experience gained by owning a Grand Cherokee helps, I’m sure. I think I rebuilt everything except the engine on my wife’s at least twice while we had it.
I did some work in that area last so maybe knocked the wire around. If I did its odd it took a week for the light to fail.
Since ive owned the car I noticed a plastic piece up in the bumper rattling around loose and part of it was zip tied on. It was missing the rivet that holds it on and I dont have a rivet gun so I just epoxied 2 nuts on it so i can use a screw instead. I did this because I didnt feel like removing the wheel well liner to reach up in there and hold the nut by hand.
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