I have a 77' 280, with the car running and the headlights off there is a whiny noise coming from the alternator, I can hear a slight grinding noise in the background. What's confusing me is that when I turn the headlights on the alternator goes quiet, the whiny grind noise goes away. Now, logic is telling me that it should be the exact opposite, like when I turn the headlights on the noise should be there because the headlights being on would put a load on the engine, and when I turn the lights off the noise should then go away.

That's not the case... Either way, its a bad noise.

Alternator is relatively new. Anybody have any ideas?

RECAP:

Lights on- no noise

Lights off- weird noise

New alternators can go bad quickly. Where did you get it?

Have it tested by an automotive electric shop. If it's bad have them rebuild it.

I've had better luck having my starters and alternators rebuilt by such shops than having to rely upon an alternator or starter with lifetime warranty from most of the chain stores.

I have a bias because I had a car die on the bay bridge going into San Francisco with a new "lifetime warranty" starter.

Dave Ruiz

With the headlights on, the load on the alternator goes up, changing the stress on the bearings - whatever is causing the mechanical noise.

If it was a rebuilt alternator from a chain parts place, relatively new means nothing ....

Replace it, and if not done already, with at least a zx internally regulated unit - a simple conversion, before it strands you.

Yeah, sounds like a bad alternator bearing. FAIW, I use Autozone's lifetime warranty parts if they're easy to replace (like the alternator). They're not the best parts in the world, but AZ is really good about replacements, and pricing isn't bad.

Welcome to the forum!

ok, new alternator it is... I will look into doing the zx conversion. I still don't get why it sounds like there's a load on the engine with the lights off, and sounds like there is no load with the lights on... oh well...

Thanks for the welcome & the advise!

You don't really need the ZX alternator conversion. The problem you're having could happen to either type of alternator. The upgrade is commonly done on the earlier S30, i.e. 70-73 240Z because of the lower output of those OEM units. There's really no need to change to a ZX type alternator on a 77. They'e both 60 amp units.