Noisy Alternator

I killed my battery yesterday by accidentally leaving the lights on. I had no problem jumping it and it has held a charge since, but not a full charge and when my truck is off it is sitting at 11.8V, (should be 12.6V) and when it is running it is at 12.01V. My alternator has now developed a loud whining noise. I narrowed it down to my alternator because when I unplug it the noise goes away. Has my alternator gone bad (i.e. a diode short) or is this an issue with my battery and the alternator is frying trying to keep the battery charged?

 

Yeah, you're right on the alternator not producing enough juice. Is the noise a bearing type noise? Could be a diode, but I'd think more the brushes are worn. You can service only the brushes, as they are a planned life wear item.

 

It's not really high pitch like a bad bearing. If you touch the alternator you can feel it vibrating and that vibration resonants through the cab and gets louder as you accelerate.

 

Idler pulley the belt uses is still good?

 

All of my pulleys should be good, they were still in good shape when I checked them about 2 months ago when I changed the timing belt.

 

It could be either the alternator or the battery

Do you have a battery charger to charge the battery??

Then see if the alternator works like it should

Shorted diodes are easy to check with negative cable removed .

Measure Resistance from the B terminal to the case

 

if your alternator is starting to make your truck sound like it has a supercharger it is about to fail...ask me how I know...

 

That's normal. Your alternator is working hard to recharge your battery now that is no longer has adequate voltage. That is the whine you hear. Anytime I am pushing my battery or I leave lights on for an extended period and then restart the truck, I always hear a whine as the alt is trying to recharge the battery. The noise goes away then the battery is properly charged.

 

So after going home there was a distinct burnt electronics smell. I tested the output of the alternator and it is only supplying 12.1V. That paired with the smell leads me to believe it's a failing alternator.

 

Good to know lol. Just narrowed my SC noise to it the other day but have had no issues yet. How long from when you noticed the noise until it failed?