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Strange Electrical Problem


New Member
March 10, 2017
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City, State
Paulsboro, NJ
Year, Model & Trim Level
2003 Mercury Mountaineer
2003 Mountaineer 4.0L V6...When the dash light switch is turned to headlights or interior lights (but not to parking lights), the left turn signal indicator on the instrument cluster is on steady and the left headlight dims. I have removed, cleaned and replaced the driver's side fender ground and the ground wire off the negative terminal. I have replaced the battery's negative terminal and swapped the bulbs in both headlight housings AND the housings, left to right and right to left, just to see if it made a difference. The problem still exists...Any thoughts?

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First do you have all normal bulbs? No LED's anywhere (on outside lights)?

Here is the wiring diagram for the lights.


Thanks for the response, and no, all the bulbs are stock. This vehicle is actually a co-worker's and I'm trying to sort this out for him. Up to a year ago, I had been a longtime Ford owner, as was my dad, and I know a little something from working on my own vehicles. But this problem is puzzling. I asked my mechanic of many years ( and former Ford employee ) and he felt it was definitely a ground issue. I've metered all the connections and I can't anything overtly amiss. From reading this forum, I have replaced the multifunction switch and the other items mentioned previously. I just don't want to keep throwing parts at this problem and was hoping someone had seen this issue before. And thank you for the schematic.

Have you tried swapping brake light bulbs in the back as well? Do turn signals work normally left and right (light up on dash and signal lights)? 4 way hazard lights work?

Mine do not

With ignition on lamp off
Flashers work but cluster light flashes with

Left turn is rapid flash both work

Right turn normal both f/r work

As soon as the head lamp is in auto or on

Brake lights don’t work

Left flashes fast right didn’t work and left goes out st same time

Ditched the tail lamp LEDs, even the back up just in case, strange issues abated in 5 minute test, I hope forever, not worth the hassle.

Well duh, you didn't say you installed LED, that would cause it.

On the exterior you can only have LED lights for the 1. reverse lights 2. license plate lights 3. center brake light.

Here is the set I have installed in my center brake light: 6 x T10 Red 6 SMD5630 3W Super Bright 194 168 2825 W5W LED Car Lights Bulb | eBay

Very bright, looks like a modern factory LED center brake light.

I don't have a demo of them solid on, just with this 18hz strobe I tried out on them.


TechGuru, that's interesting info. I'd like to install center brake led lights from that link you posted.

Some distracted driver already rear ended me, so I'd like to avoid another in the future!

So it's plug and place, no weird electrical problems? Thanks.

what about headlights?
HID do the same thing?

Do the in series resistors solve the issue?

HID is illegal in the low beams and useless in the high beams because of the warm up time (can't flip the highs on when seeing a shadow ahead and light it up fast enough to see what it is).

LED conversions all suck badly ruining the focus of the fixture making it a big soft floodlight.They have not been perfected and other than the Philips H11 for Fog Lights there are no legit non-Chinese aftermarket manufactures of any.

The only time a LED conversion is sometimes better is when used in a projector housing, even then you're using a unreliable Chinese made that could go out at any time.

If you need more light than 9011/9012 in the factory fixtures can provide then get aftermarket 130-watt halogen flood/spot lights or a LED light bar.

Simply swapping the 9005/9006 for 9011/9012 gives a total combined lumen increase of 3,040