Good evening. I am having a strange voltage variation problem on my 2006 Lincoln MK LT with the 5.4 engine. I have looked at many posts on this forum but have not come to any conclusions yet. I noticed about a month ago that the headlights would go dim at times for a few miles and then brighten back up on and off. The voltmeter would move maybe a very small amount to follow the dimming headlights. I had a shop here in town rebuild the alternator mainly because the bearings were rough sounding. I put the rebuilt alternator on and same problem. I took a drive for about 40 miles with my Innova 3130c hooked up with live data enabled to ECU volts and noticed this; at idle for any length of time the scanner reads 13.9 volts to 14.2. At 1000 RPM it drops to 12.5 v. At 1500RPM it drops to 11.9 to 12.1 v. Can anyone make sense of this problem? Thank you.

 

our trucks are very picky on the voltage regulator in the alternator... the shop that rebuilt your alt probably used an aftermarket voltage regulator... I always tell people to replace a motorcraft alt with a motorcraft re-built alt.