Damien89

Hi everyone, its been a while since iv'e posted here. Today i went to start my dad's 89 vette. When it cranked it cranked a bit slow but its because it had been sitting for a month.

After it started i noticed a humming noise coming from the engine but it was really low this noise that i didn't take notice of it as i thought it was my ears playing tricks on me.

So i left it idling and went to do something in the other garage, after 5 minutes i go back and again there was a faint humming noise, i looked at the engine bay but i couldn't find were it was coming from. Then i went to turn it off and saw that the dash was really dim, so i checked the voltage and found it was 9.6v! So i turned the engine off. Then tried to restart it and the starter just clicked without turning. I hooked up the battery charger and left it for an hour.
After that i started the engine but the voltage was 11.1v and it was getting low 10.7, 10.5 etc after 2-5mins.

I checked the battery voltage with a multimeter and it was almost the same as the gauge on the dash.

How can i check the alternator without removing it from the car to see if its charging or not?
Could the humming be related? Maybe something in the alternator broke?

don hall

What are the charging amps of the batt charger? You may need to charge the batt overnight if the charge amps are less than 10.

It would appear from the lack of batt voltage, the alternator may have failed, and it may be the source of the noise you hear.

Removing the alternator is not difficult. It is recommended that you take the alternator to a parts store, and have it checked for output.
If necessary, buy a replacement alternator, or have yours rebuilt.

c4cruiser

First, you should fully charge the battery. A fully charged battery should show a voltage of around 12.6 to maybe 12.8 volts with no load. A discharged battery would be 12.0 volts or less. Auto repair shops will typically harge a battery using at least 20 amps for a good 45 minutes. If your charger is rated at 4-6 amps, it may take 5-6 hours or longer to get a battery to full charge assuming there is nothing wrong with the battery.

Once the battery is charged, start the car and check the voltage at the battery. With a good alternator, you should see a voltage of around 14.2 to as low as 13.5 volts if it's working properly. Check the voltage at the alternator terminal where the large red wire is connected. A good alternator will show about 14.3 to 14.5v with the engine running just after startup.

The humming noise from the alternator is not a good thing. They should be quiet. Not sure how easy it would be to get a replacement alternator in Malta but there are a number of online places that will do international sales. Also, there's nothing special inside the Corvette alternator, so you might search for a shop that does rebuilding. The parts would be common to GM cars of that era.

Damien89

Thanks for the reply, i dont know what amps are of the charger but i'm going to let the battery charge overnight and see what happens toomorow.

Damien89

Thanks for the reply, i will let the charger charge overnight, how do i check the alternator voltage? Do i probe the positive wire of the multimeter with the red wire on the alternator and the negative to an earth?
If i find the voltage to be low i will take it to a good shop i know that can test it and repair it for me.

c4cruiser

Yes. positive lead of the voltmeter to the red wire and negative lead to a good ground.

hippy

Here's how you test the alternator. Charge the battery, start the car, after you start it if voltage doesn't go over 12.6v you have an alternator that pooped out.

DanZ51

My guess is your diodes in the alternator are bad. That growl or hum is usually indicative of that. The bad things it it may have croaked your battery too (if it discharged low enough). Pulling the alternator is super easy for these cars. I'd take it (just the alternator) to a rebuild shop and have them bench test it. See how much they would charge to fix it vs. trying to source a rebuilt unit.