I got a '89 350 5.7L T/A and has 120,000 miles - I live in Boston and temps are
down in 8 - 20 degrees overnite. Car sits out all nite too.
Soooo it's real real cold nite & most of days - for past 2 weeks.
I take good care of it & original owner.

My dash VOLT guage started (especially at nite with lights on and heater running) to start dipping below 13 reading to the 9-10 mark area.

But when I shut off the lites and the rev engine and RPM go up
the needle goes back up to around normal area of 13.
This goes on continously now especcially during nite driving.

Two nites ago, I started hearing a low, whining, whirring constant noise (not too loud). Could this be the windings inside the Alternator ?

Oh yehhh... My fan belt is kinda old and I have sprayed some belt dressing & I know/think it has been slipping especially earl morning start ups it make on & off again screeching noise then goes away once engine gets warmed up.

PLUS... I checked the voltage of battery bot with engine off and while running with a good voltmeter.
Battery seems ok : Engine OFF = 12.5 - 13
Engine ON = 14.5 or so

Questions:
1. If belt is screeching a little at startup could this prevent the ALTernator from charging the battery properly and /or not giving the car and dash VOLT guage the right amounts of voltage.

2. Should I first just try cheapest things like:
a. replace the battery
b. replace the fan(timing) belt (I know they're expensive - like over $100 - 125, right?
c. replace the ALternator ?

3. Is there a way or where around the Alternator can I sheck the
voltage outputs wires with car running with my voltmeter ?

3a.. Is this dangerous & possible to get a bad shock by checking the running alternator ??

Thanks for all the help - I hope...
BLUETA89 - trying to make iot thru another BADasssss freezn' winter !

 

The problem is alternator related. Your running and non-running battery voltages are exactly where they should be.

One of two things is happening. Either the alternator is on its way out and is losing some low speed charging ability (regulator/rectifier assembly is starting to go bad), or the belt is slipping at low RPM.

I'd do the belt first. If the problem doesn't go away you've lost nothing. If the problem persists take a look at the alternator.

BTW, you can check the voltage of a running alternator safely. See what's coming out of the power lug on the back of the alternator (post that runs back to the battery, labeled BATT). This is the voltage that the alternator is putting back into the system.