Headlights flickering and air conditioning not working

So I have a problem with my 2010 F-150 and it’s really bothering/concerning me. Before I get to the problem I’m going to discuss of what happened before some repairs and the repairs I did.
One day out of the ordinary, I’m on my way to work and the battery light comes on out of the blue. Next day I take it to auto zone for a battery test and turns out it was the alternator. Okay cool. Buy a new one and replace it myself. Later on at night that same day I’m going to the movies and I get this strong burning smell coming from the vents. So I immediately pull over and check what’s going on but I can’t really see due to darkness, but there is some smoke and I couldn’t find where it was coming from. So I head home safely. The following morning I turn on my truck and found that the smoke was coming from one of the connections to the alternator. So I swap it out. But battery is now bad. Swap it out. All is good till I turn on my truck, a/c is barely blowing air so now I have a new problem and when I drive battery light turns on. As well as the headlights flicker but none of the lights from inside the cab do this. On another note when the engine isn’t on the headlights don’t flicker. Before I go further I would like to add that one of the pulley idlers (making noise) was bad and I swapped out the one i thought was bad, but I was wrong so then I swapped the one above it as well and there wasn’t a noise anymore. One thing I noticed was that the belt tensioner had some play and would jerk. When it would jerk that’s when the headlights flicker. Finally I replace the tensioner but still no luck it still jerks and headlights still flicker. I’ve tried following the wires making sure they’re not corroded or loose or dirty but still nothing. Anybody with more experience know what’s going on or might now what’s making the tensioner jerk (or if it’s normal)? Any help at this point would help due to almost $200 spent and not wanting to spend more on a shop.

If something wasn’t understandable let me know and i’ll try to clarify, I’m not the best when it comes to explaining in writing.

Did you replace the serpentine belt?


Not yet but I’ll look into it in the morning since shops are closed thanks though.

Is the alternator output the same as the original one that was removed? If there is a low load on the alternator then all of a sudden a big load that may cause the tensioner to move a little. Try to turn things on one at a time to see if you can replicate the problem. has anything aftermarket been added that may be having or causing a problem?

It seems as though this new alternator might be bad or the wrong one for your truck. If the batt light comes on with the new alternator, that is the warning you got when the old one was deemed bad.

Engine running should give you about 14 volts steady. If the voltage varies, the lights may flicker, and the belt may jump. The truck may be refusing to start the AC if it recognizes a problem.

Check your A.C. pulley for sloppiness. Sounds like it is intermittently locking up. Do the symptoms go away when A.C.Is not used? If it does your compressor is probably bad.

Yes, and remember the AC compressor will run sometimes if the AC OR defrost is on. You want all that stuff turned off - or disconnect the wire to the compressor clutch - to make sure the compressor is not part of the problem. Good luck and please let us know.

Serpentine belt is still good

The only after market item is the power steering cooler, as far as trying to replicate things no luck. Still does it on its own.

When the a/c is on or off nothing changes.

How do you know the serpentine belt is still good?


No cracks plus compared it to a new one and it was almost literally the same thing.

You can’t tell if a serpentine belt is worn by looking for cracks.

More people get fooled by this, and then can’t figure out why they’re having problems with components that are driven by the belt.

And you can get a free Gates serpentine belt wear gauge here.


I don’t know if you’ll get the one shown in the video, or this one.


So, I did swap the serpentine belt due to it hadn’t been changed since bought and maybe it was the problem but it wasn’t. Still, battery light on and headlights flicker.

Have you done the basic alternator/battery voltage test yet? Before first start of the day the battery should measure about 12.6 volts. Immediately after starting the engine, 13.5-15.5 volts. Measure at both the battery posts, and the connectors that attach to the posts.

The flickering headlights usually means that some of the diodes inside the alternator have gone bad. To prove it you can disable the alternator by removing the connection of small wires going to alternator. Then start the engine and see if the flickering of the lights is gone. If you replace the alternator you should have the charging system checked to make sure things are okay and the alternator isn’t over charging the battery or sending too much current to the battery. Your original post seems to indicate you have a over charging issue that is causing damage to the battery and the alternator. Especially if the battery "SENSE’ lead is grounded. That lead lets the alternator know if the battery is low on charge or not.