i have an ‘07 2.5L and last month I had an intermittent burning smell coming from the alternator. I assumed it was on its way out and picked up a used alternator as replacement. The smell has returned. I’m doubtful that it’s the fault of this alternator too, and I’m just wondering what could be causing this burning smell from two different alternators. I’m not seeing any trouble lights and everything seems to be working ok, but I do notice that the starter is turning over a bit slowly on cold mornings. I’m wondering if this battery is beginning to break down inside and cause a minor short which is messing up the alternator?

Yank whatever drives the alt and then check all pulleys by rolling them by hand, you may have a pulley going bad there. The smell may not be alt, only close to it. Spin the alt too, if it has an overrunning clutch the pulley should lock one way and be free to spin the other.

A free battery loadtest at the parts store will quickly tell you whether the battery is any kind of issue there.

Wouldn't hurt to check the alt main output lead to battery, sometimes if they corrode then they begin to melt from the corrosion then adding resistance to make them get even hotter. The usual point is right at where the lead bolts to the output stud from alt.

Thanks amc49. I replaced the belt a few months ago and checked the idler pulley so that’s not it. What I did notice last night is he he negative battery cable is pretty corroded. I’m thinking that might be it. It would explain the slow cracking on cold morning (increased load). I’ll swap out the cable and get ge battery tested too. Thx for the feedback.

So, you don't think a pulley checked 'a few months ago' can be bad now? Bad thinking there bucko. And if the belt was one made to cover more than one length (typical 1/4" at a time) by being in between sizes then a few months ago could have a semi-loose fit OEM belt now slipping. I don't know if Nissan uses range indicators on their tensioner assemblies but if they do that might show you. The tensioner needs to be running close to the center of its range, or toward the tight side, toward the loose side they can slip. Even more if the belt has worn in to vary length down it to then make the tensioner arm action erratic. A belt can be loose with no noise at all. Pulleys sometimes make none either. You spin everything not just idler, the power steering and water pump even though usually not that and a/c for sure, alt and tensioner roller and the arm swing. Swing the tensioner arm through its' working arc and check for any sticking which will let the belt slip for a second to skid it and the smell.

The battery cable might be part of it, but the smell should be over by it, but smells being what they are are commonly misleading as to source. The cable gets hotter the more resistance it gets. Hot enough it melts the plastic insulation and you smell that.