Get the items used in this video here

Phone Charger with Voltage Reading - https://amzn.to/3KTEV5M
WD40 - https://amzn.to/3uMPNws

Other tools that are useful for working on Alternators:

Voltage Meter - https://amzn.to/3vr1Kan
Portable tool kit - https://amzn.to/3xxrPad

This is the tool kit I use to fix my cars and boats - https://amzn.to/3h58UtI
This is the Scanner I use to Diagnose ALL Autos - https://amzn.to/3k56zjQ

A running engine alternator output should NOT be below 12.6 V, 12.4 is too low, 11.7 is way too low. Modern  cars with current sensors can charge at voltages around 12.8-13.0 volts, but honestly it should be 13.8-14 volts. Also unhooking a battery terminal on a modern automobile with many modules, and multiple networks, while it's running is a Terrible idea, most every OEM service manual warns against the damage this can cause, I have been a technician for 33 years, and a master drivability tech for 20, seriously, doing this on a Nissan with an Intelligent Power Module, or CDJR with a TIPM can cost you thousands.

99% of alternator (and starter motor) failures are simply the spring loaded carbon brushes either wearing down, or the spring that pushes on them sticks or hangs up, allowing the carbon brush to lose contact with the rotating slip ring, thus the failure to charge. A simple light tapping on the alternator or starter motor housing with a hammer will often times free that stuck spring, or give the little nub of carbon brush that's left, just that tiny bit of movement enough to make contact again, and last long enough to get you home, with a few miles to spare while you go get your new or rebuilt alternator or starter. The tapping trick has worked for me personally on 3 alternators and 2 starters over the years. No need to be towed home if you remember this!

My alternator was acting up the night before my sons graduation. Sprayed a bunch of wd40 in it. The light in the dash went out and and voltage went from 12.3 to 13.9. Saved the day as the next morning we had to leave at 6:30 am and had no time to do a alternator change
THANKS!

Your Lose battery terminals can cause the issues described, Also never pull the Positive off first. Negative goes on last and comes off first. It's much safer to do it this way.

Thank you so much for posting  this video, I just wanted to say that the WD-40 trick saved me from getting stranded this week on a business trip about 300 miles away from home. When I was about to arrive at my destination, the voltage indicator on my dash started to intermittently flicker between 12 and 15 volts (instead of the usual steady 14-15) with the battery indicator light coming on, with my headlights dimming at the same time.  

Right then, I figured my alternator was about ready to crap out.  I managed to get my car to a service shop to get looked at when I arrived at my destination.  And sure enough, they found that the alternator was faulty.  Unfortunately, that shop and none of the other shops in the area that I called were unable to order a replacement alternator right away because of how old my car was (a ‘92 Olds Achieva, albeit with only 45,000 miles on it).  It would’ve taken a week or two for them to order in an alternator for my car.  Resultingly, the shop didn’t charge me for the diagnostic, thankfully, but it meant that was pretty much up the creek as far as getting it repaired before I could head back home.

But then, while I was in my hotel room browsing videos here on YT, I came across this video, and tried applying WD-40 in all of the vent holes on the alternator as instructed in this video, and it was enough to partially revive the alternator enough to be able to drive back home without any issues!  It only puts out enough volts (14.5-15 V) when I’m driving at highway speeds (65-80 MPH) with the engine RPMs around 2k-3k.  But since my return trip is via the highways anyhow, it all worked out, and I’m glad to announce that I’ve made it home safely!!! A new alternator is on order at my local repair shop, ready to be installed next week.

Once again, thanks for posting this, your advice allowed me to get home safely.  👍👍

It's so funny, you can go all your life working around something and still learn a neat trick. You never stop learning in this line of work. Thanks.

I figured it was cleaning off the rotor and the stator.  Fortunately my alternator is easy to get to and with a 10mm gear wrench I can change the thing out in about 20 minutes.  Can't get a socket on one of the bolts and a standard wrench would do it but it would turn that 20 minute job into a two hour job as even with the gear wrench there's only room to turn that one bolt back n forth about two clicks with each swing of the wrench.  Replaced it about 3 years ago with a generic no-name brand and that lasted 1 year.  Paid more and got one by Bosch and it's been good ever since.

I was an auto mechanic early in my career as a mechanical technician and have been doing nearly all my own maintenance for almost 50 years and have never heard of this hack, well done! Excellent advice, just shows again that one is never too old to learn, thank you

Thank you for your super cheap tricks we all need to teach the world about super trip  Tricks.

Many thanks for sharing this for us not so rich people who didn't have money to take care of the alternator right away.

Came back here to say, thank you so much man. My car wouldn’t start in a Walmart parking lot. My car had been having issues but wasn’t sure if it was battery or alternator. Car wouldn’t turn over in a Walmart parking lot and almost had to call for help. I remembered watching your video a while back so I went back into Walmart, bought a can of WD40 and did exactly what you said. Got back in, and it started right up and I got back home no problem. Seriously, you got me home dude. Thank you!

Not a good idea to disconnect the battery while the vehicle is running on a modern car. If the alternator was working properly (already established it wasn't by the voltage test), it would cause a voltage spike that could damage electronics (like the ECU). I do like the WD40 trick to unstick or clean the brushes! Keep sharing your ideas!

WD40 and little tap on alternator worked, best and honest advise on YouTube !! Note: do not spay WD40 on the belt.

Wow, WD-40 is like magic

Thanks for the tips, I'll have to try it!

Nice trick when you're stuck miles away from home.  My 2002 Tacoma showed the same voltage as in your vid.  I removed my alternator, took off its cover plate and found that my brush springs had had my brushes fully extended, but they weren't making contact with the rotor's shaft.  I took a small round file, elongated the holes of the brush holder, and pushed the brushes closer to the shaft before tightening the screws.  This fix lasted thousands of miles before I changed the brushes.  Still working to this day.  It's  better spending $25 than $250+.

Awesome  thanks alot , FREE INFO YOU JUST SAVED MANY DRIVERS MONEY THAT WOULD OF WENT TO SOME TOWING COMPANY THATS MONEY TOWARDS A ALTERNATOR

The most practical mechanic out there. Nice

I came to say that old school alternator test was cool back in the 70s but it will fuck up some shit way more expensive than your alternator today. I had a ground wire short on an 05 Silverado. 750$ computer got smoked. Even with seven grounds on the ECU the start circuit got smoked. You gave me a good tip I had not known but you should edit that pull the ground right out. Just use your gage or idiot light for side of road test.thanks for the tip.

Bought my 94 Accord for $500. Fuel leak when running. Needed a vapor return valve for the fuel tank. Chiseled a hole through the trunk to find the source, had it fixed in 20 minutes, not counting the travel time to/from the wrecking yard. $500 cars are the ONLY WAY TO GO.