Instument Cluster and headlights dim when slowing down....
 

Sometimes while driving at night when I'm decelerating (either braking or just coasting), my headlights and instrument cluster dims. Then I turn the dim dial on the headlight switch and both come to full brightness. My mechanic thinks there is a very slight possibility the headlight/dimmer switch needs to be replaced but that is a VERY slight possibility. In the two years and 20,000 miles I've owned my 04 3.9 liter V-6, I've had to replace the alternator three times. All three times the alternator was under warranty from O'Rielly's (it's warrantied for life) so I was thinking maybe this is the cause... that the headlight switch is shorting out.

Ford wants about $45 for a new switch, which I'm OK with spending if this solves the issue.

Thoughts?

alternator was the first thing i thought. maybe get the battery checked out also

+1 sounds like the alternator

Battery is less than a year old and alternator is a life-time warranty from O'Reilly's installed this past March or April. I'll check both with my volt meter but I don't think these are the cause.

being that you had to replace the alternator three times in two years, there must be something with that alternator. good thing they offer a lifetime warranty. are you running an aftermarket sound system at all?

I agree, everything points towards your alternator or battery.

I just replaced the stereo but that's it on the sound system. I did add 55w fog lights though but the issue with the alternators started way before that. I find it funny that when the lights dim, they come back up as soon as I turn the dimmer knob. I'm just trying to find the cause of this. I can't believe that THREE alternators were defective. I really don't have the funds to buy a brand new alternator.

What brand of alternator did you buy?

Is it an OEM motorcraft alternator or an aftermarket replacement one?

OP's first post states the alternators are from O'rielly Auto.

The best option would be to measure the voltage across the battery terminals with the car running at idle. This will tell if the alternator is doing it's job.

My headlight /dash dimmer has some intermittent issues as well which clear up by wiggling the knob. Need to get it out one day and run it through the ultrasonic cleaner. betting after 14 years there is some dirt in there.

14 years? It's a 2004... by my calculations that's only eight years.... and if I'm going to go as far as taking the darn thing out, wouldn't it just make sense to replace it with a new one if it's only $45?