Do I have a bad alternator?

My truck has been making a whining noise that is proportional to engine RPM.

Yesterday evening, I accidentally left my headlights on for over an hour.

The truck started, but the whining noise was 5x worse.

This leads me to believe it’s electrical in nature.

Could I have a bad voltage regulator?

What would be the diagnostic steps?

Thanks.

 

Start the truck and put a multimeter across the battery terminals and check the charging voltage. Alternators tend to whine before they call it quits. You can also remove the alternator and take it to Autozone and have them run a test.

 

Unplug the alternator at the electrical connection for a few seconds while its running and if the noise goes away then you found your problem.

 

Thanks, I will try that. What is the nominal charging voltage?

 

That's a good idea. I will try that as well.

Is that b_tch difficult to unplug?

 

About 13.2-14.8 depending on temperature, when the temp is higher the target charging voltage will be lower.

 

Not really, the connector is right on top.

 

Thanks. Not to ask a stupid question, but will a bad alternator typically result in a higher or lower charging voltage?

 

Most of the time lower, the best running load test is to turn all loads on (blower on high, headlights on high, etc) with the engine idling. Battery voltage should stay above about 12.8 (ideally shouldn't drop below 13), if it drops below that you definitely have a charging system issue but the whining is more likely a bearing failing.
As stated above quick n easy test is unplug it and see if the noise goes away.

 

sounds like a bad bearing in the alt when under load
or could be a bad belt/ tension issue

take belt off and see if it spins free cold and then test again hot and check bearing

 

Would a partially depleted battery put the alternator under a heavier load after the vehicle starts?

 

Yes it would, still shouldn't make noise.

 

Yeah there is a little 4 pin electrical connecter (muted green if i remember correctly) right on top of the alternator. This controls the field magnets and when unplugged the alternator wont work. Often times you will get whining like you describe when one or more of the 3 phases of the diode board on the alternator are going bad.

This exact thing happened to me, i thought it was an alternator bearing. Pulled off the belt started the truck and no noise. Turned truck off spun everything on the front everything spun smooooooth. Put belt back on, started the truck with all electrical things on (lights, blower, radio etc) and whining noise was there. Unplugged the 4 pin connector on the top/side of the alternator, noise instantly stopped as the alternator wasnt energized. Plugged back in whining back.


Needless to say you can diagnose the bad diode board in about 30 secs. Pop the hood, locate the connector described. Turn on headlights, radio, blower fan at max etc, turn truck on.

If noise is there

Unplug the connector, ill bet noise stops. If it does bad diode board on alternator.


You can get diode boards for pretty cheap but its not the easiest of process to fix... OR get a good denso remanufactured unit from NAPA.

 

I’ve seen alternators fail a few different ways. Most of the time not charging. Sometimes intermittent not charging, whining noise due to bearing failure. Occasionally overcharging.

Yours sounds like bearing failure. The ideal tool would be a mechanic stethoscope, I don’t own one. Any time I’ve needed one I used a long screwdriver. Sometimes you can sorta feel the vibration the noise is making with your hand and not need to put your ear on it. Like what’s already been suggested, you could remove the belt, spin it to see how it feels.

If you need to replace it stay with a quality unit, not something from a chain parts store.

 

Better spank it

 

For OP @Plasmech

DIY alternator replacement:
https://www.tacomaworld.com/threads/step-by-step-diy-alternator-replacement.384165/page-3#post-29189196

DM me if you want help, I just did mine, confirmed with a voltmeter and I also had a whine before replacement. Not a hard job and you can save yourself some $$$

 

OK, I was able to do a quick test at lunch:

engine not running: 12.59 VDC at the battery terminals
engine running: 14.08 VDC at the battery terminals

 

I will try this test tonight, thanks!

Good intel.

 

If I have to replace it, I'll likely go NIB OEM.

I've not once heard a rebuilt alternator story that ends well.

ETA: strike that. Toyota no longer offers a NIB alternator for my truck. Factory rebuilt Denso it will have to be.

How long does a "starting to fail" alternator usually last?

 

my starting to fail Alt went on for 5 years but I didnt drive my 1995 F350 that much until my taco frame rotted out