How to Properly Test an Alternator (The Extra Steps the Parts Store Can’t Do)

Have you ever had someone come in to the shop and say that their alternator is bad and there reasoning is because a parts store tested it and it failed. I’m sure you’ve had this happen or even been the one having it tested. Well many times the tests are not accurate on these machines. So what do you do?

In this video the Professor will walk you through step by step how to properly test an alternator and be sure 100% that it’s failed, failing, or passing. You’ll see how to prove an alternator has failed even though it’s passing all the parts store testers.

We hope you enjoy this and as always if you have any questions or comments leave them below! Thanks for the support! Enjoy.

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[spelunkerd]

This is the classical old school testing I was taught. I find it surprising how often working pros today on Youtube don't bother. As soon as they see lack of charging they stop and order parts. New cars are much more difficult to do testing on. The alternator is often hard to reach, and the signals between PCM and alternator are much more complex. Even so, I think drop testing of the power and ground side are worth doing.

[pontiacsrule]

Great video as usual. Just remember a lot of us aren't in a shop. We are DIYers. That's why we watch these channels. Many of us don't own a lab scope. Many of us own a voltmeter. Try at all cost to teach us with basic tools if at all possible .Lots of time we condemn a alternator if the output voltage is low still at the terminal on alternator. We can't check to see it the PCM is controlling the input. I personally love the videos. Great story about the broken motor mount. Would of never thought about that. Also, thanks for your honesty about when cars come back to bite you. That's what makes your channel so good. Andy

[dnlmachine]

Top quality channel. A great source of information. Hopefully people can learn and share from this for years and years to come.
Stay gold.

[truckladders]

Great video, Im a 30 year heavy and auto mechanic still learned alot! Keep up this type of video, you can never know to much !

[williammargrave]

Thank you for taking the time. I am a master tech in TN and i like to watch this kind of stuff to stay on top of it so I dont get to over confident.

[AlHasenfratz]

Valuable procedures that really enhance accurate diagnosis ! Have seen quite a few no charge conditions corrected with a new alternator recur, due to bad grounds at the mounting bracket. Changing the part restored operation, for a while.... Read somewhere that half the warranty returned alternators actually are not defective. Sounds plausible? Voltage drop is

[SeanBZA]

New subscriber, and all great points people overlook. Especially about testing diodes, which people almost always, even experienced mechanics, overlook. Depending on the multimeter you can switch to AC volts, and some have a dual display, which will show both DC volts and the AC voltage on it, so that you can diagnose a failed diode pack without a scope. Others when in AC volts disregard the DC, so you get the AC voltage only. As well remember that there are 9 diodes in the alternator, 6 high current ones that feed the output, and 3 lower current ones that provide the excitation to the rotor, and those 3 also can have a diode fail. Most voltage regulators will have a terminal, or a place you can put a probe, to measure that field voltage as well, so your scope can test those diodes as well.

[scientist]

Good, refresh on the basics and tets methodologies.

[ThunderbirdRocket]

Better than a textbook ! You have a lot of knowledge ! Thanks for sharing this very clear lesson !!

[anthonysova]

Very good point about loading the circuit you’re checking for voltage drop it takes BOTH current and resistance to create a vd One question about A/C ripple why wasn’t the generator tested loaded and unloaded? Does the 0.5vac ripple still not exceed 0.5 loaded? CHEERS

[BenKlassen]

Thanks. This channel is a gem. You are a master teacher!

[mikerossman]

A simple Fluke meter can do what you need if you know how to use it.

[thomasherring]

Just to be clear, the same thing applies to testing the starter that applies to the alternator. When you are testing power and ground to starter, you need to have someone trying to crank the engine so that there is a load on the circuit. You can have a power or ground test absolutely fine with no load, and go completely open when loaded. This applies to any electrical circuit, always test for voltage drop with the circuit loaded.

[mamjacobs]

I must say from all the mechanics I watch on youtube, you explain t8he things at best. Keep up the good work

[vickenkaragozian]

That's 90 mV. not 900 mV..🙂. Thanks for sharing.

[henridekker]

Great Video. Always something new to learn.

[niallseddon]

Always worth checking voltage drop from battery post to battery terminal

[CoreyPfeiffer-hp]

"LET'S SEE A TOOL BOX TOUR" LOVE YOU GUYS. YOU'VE CHANGED HOW I DO THINGS.

[JamesSmith-xssr]

I fixed the charging problen in Oscar Goldmans 85 500 sel MBZ. I just replaced the "brush pack", she's back up to 14.23 volts!

[kevinkeown]

One other charge problem to check is the drive belt for the aultonator is it tight enough .good video though