Going crazy with this one, 2005 GMC Sierra Denali, at idle or when engine braking the voltage meter jumps around and the dash lights and head lights dim and brighten. At idle you can watch the headlights dim and brighten as well. I have since replaced the battery with a brand new Interstate, replaced the alternator, cleaned every ground I can find, as well as add a ground from the engine to the body. Still doing it. Would like to fix it by this weekend, have to tow a truck from MA to NY and back.

Thanks guys

 

mine does that too. i think when the a/c cycles on or the fans come on. engine has slight stumble and i can see lights dim if they are on. i can also see the afr change a bit too

 

My truck used to do that, i put a new alternator in and it solved the problems. I never had problems charging or dead batteries either.

 

Did you use the original amp alternator? According to my VIN mine calls for a 145 amp alt which I used, but there is the option for a 165amp... For ***** I unplugged the alternator while it was running and it still continued to dim and flicker the lights.