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Here's the problem that raises it's head all the time. Nobody has an oscilloscope (I do) and with that tool, one can see the actual ripple on the output.

Like I've said before - I NEVER trust a parts store to test alternators (or starters, for that matter) since their machines are not even close in getting a true test. Taking an alternator out and throwing it on a bench looks cool - but it's rife with bad test values as it is not in the vehicle where it can act up as it sees fit.

OK --- that said ---- my standard of 0.015VAC on the DC side is at idle, no accessories on --- other than the fuel pump (be careful since these can add some noise to the test) and the ignition.

C/P from another site, discussing bad diodes:
"Any multimeter on A/C voltage measures RMS, root mean square.
It's a calculated value (sortta).
It's a DC equivalency(ish)
It's peak to peak voltage on an oscilloscope, divided 0.7 if I remember correctly.
Google away.
Anyway, I was always told 50MV (is) max on AC @ the alternator.
Never found that in a service manual of course.
I had one today the measure 98MV, about double what I normally see. Not sure how much is too much..... grey areas.... but it had a very premature 6 mos-new battery failure.... go figger"
The current problem is that diodes are now almost all Chinese and of questionable quality and therefor the bar gets lower and lower as the parts get worse and worse.

And (me here) --- the alternators on OUR vehicles are pretty high amperage so they sometimes use dual diodes and those can appear very noisy - so there's that.
♫ (mine is rated at 200 Amps; it tests out at 186 Amps @ 0.026 AC)​

They (dual diodes) seem to "allow" more noise as the individual diodes in sortta-matched "pairs" tend to not switch ON and OFF at the same threshold(s).

That's where a lot of excessive sine-wave action will appear on a scope - but don't be fooled by it. It's a strawman argument.

An excerpt follows form another site, chasing this same trouble and trying to use decent-enough values to believe:

(Original poster) On this Thursday, July 7, I went for my annual state inspection, and the auto mechanic told me my car almost didn’t start and my alternator was “acting up.”
At that point I had enough (having changed alternators 3 times), so I immediately headed straight for Advance to get another new alternator and my car died (AC was running) along the way. I got a jump and turned off my AC, then picked up the new alternator and went home.
I replaced the alternator Friday (myself), then the ripple tested 55mV (0.055VAC), putting out 13.9V under no load, and 13.6V loaded.
All of the previous alternator tests in the last month showed around 12V loaded or not loaded, with only one test (at night) showing around 13.6V and with 1.030 AC.
On a 12V system, I’m guessing I’ve been running too close to draining the battery under load, so given the higher voltage output now, and unhealthy ripple, I’m guessing the alternator was indeed the problem (whatever the ultimate failure inside the alternator was).
If anything indicating a different problem occurs, I will update this post.
Otherwise, that’s it.
Thanks for everyone’s input, and feel free to comment further if you like.



(Counterpost) Thanks for taking the time to come back and close up your thread…Glad to hear you have cured the problem.
(Another counterpost, different person) Well looks like you got your answer and replaced the alternator. I have argued with NAPA and Walmart on testing alternators and batteries and just plain don’t trust those tests anymore. I had two alternators under warranty that tested good but would would give me a fluctuating headlight at idle. I finally just threw the warranty away and bought a Delco and problem solved. I also had an 18 month old battery that wouldn’t take a charge but took over an hour for Wally’s to agree to replace it since it tested good. Might tell you if they are totally bad but not borderline stuff.
(Counterposter #1) Good job. The ripple voltage you are now seeing is normal.
The fact that the DC output voltage was lower and the AC ripple voltage was higher on the bad ones means some of the output diodes were bad.
That causes less DC output and higher AC ripple ... Just as you discovered


Now - here's an interesting post alluding to a strange event, talking about a digital meter not being compliant enough:

"OK, so it was the auto ranging that was screwing up the reading. Works fine if I set the range manually. This particular car is showing between 350 - 400mv at the alternator post with the brights, rear defrost and cabin fan on at 2000rpm. "
Notice the electrical load factor on that post overhead ( ↑ ) --- i.e.: the higher the load, the worse the alternator will look. It's because of the deepening valley (if you will allow that) as the voltage peaks get higher and there's more time between switching from one diode phase to the next.

That is why MOST high capacity alternators use dual diodes for each phase - not only to carry the load, but to try to pass/no pass the current together with less individual switching heat and ultimate thermal damage.

As always --- shoot for the best you can get and just remember that batteries do NOT like ANY AC Voltage! Period. Full stop!

I demand 0.015VAC on my alternators.

I've even seen 1999 GM Tahoes screw with their simpler - less demanding electronics, having alternators put 0.350VAC on the DC side.

Back to an electrical lecture I attended once upon a time --- in a kingdom long defunct ....

"All rectified DC alternators produce ripple voltage. Measuring ripple voltage has historically been used as a way to determine the condition of the alternator. Too much ripple voltage indicates an alternator problem.
We measure ripple voltage with a voltmeter that has a DC filter, a circuit that blocks the “non-varying voltage” and only passes (and reads) the “varying voltage.”
Many test benches and on-the car testers do this with a good-bad scale or bad diodes light. Good quality handheld digital meters usually have a DC filter and can read ripple voltage as a specific number value.
What are some of the things that affect the amount of ripple voltage?
Since the alternators were all operated on the same test bench under identically repeated conditions then the answers are: alternator design such as the number of rotor poles, rectifier configuration (additional “wye” tap rectification), rpm, rated alternator amperage output and amperage load.
Often overlooked factors are those that are external to the alternator.
This would be anything that filters or “smoothes” ripple.
The number one item is the battery, including its plate capacity, condition and distance from the alternator.
Excessive voltage drop (resistance) in the circuit between the alternator and battery will also increase ripple voltage when measured at the back of the alternator.
How about battery-less test benches with “simulated batteries?” The only straightforward answer is you will need to use “simulated” specifications!
On many battery-less test benches, ripple voltage for good alternators may be as high as three or four volts!
Without the benefit of a battery, other forms of filters (typically capacitors) must be used to reduce not only the alternator ripple voltage, but also the 60 Hz line ripple in the “simulated” battery.
So when you measure ripple voltage on a test bench with “simulated batteries” are you measuring alternator ripple, power supply ripple or both?
It’s different even for each alternator!
I believe that’s why I have never seen (oops, I used that word, never) any OE alternator manufacturer give ripple specifications to the outside world.
My point is we can no longer use 0.015 ripple volts as a rule of thumb. (me here: <sniff>)
Of the alternators we tested, there were only two of the good alternators that were under .50 ripple volts!
Five of the metered "good" alternators have a significantly higher ripple voltage.
There are too many factors that affect ripple voltage (both alternator design and external factors) for us to continue to use ripple voltage as an “accept” or “reject” measurement. "

Me here again ----> I typically disagree with that last statement because we have to have some numbers with which we PASS or FAIL any parts on our vehicles and the alternator is considered the most important part of a vehicle.

I learned that last statement about it being the MOST part on a car important from a different test through the California Air Resources Board of Vehicle Emissions" in 1968, 1978, 1988, 1998, but I retired before the 2008 test.

Don't ask me why it's an alternator-thingy --- since if you don't have a steering wheel, that kinda sux too.

Same with engines - gotta have one at least............ and wheels.......... and a horn ...........


Something else to consider ----> our vehicles monitor the battery temperature - it must be one of those algi-rhythms because I've never found a heat sensor under the battery..... but as the battery heats up - the alternator will not be asked to generate high voltage on fear of actually boiling the electrolyte out of it.​
This will cut back the field strength and then the diodes will have a hard time switching as their floor voltage is lower - but it's not in the 800 Hz low RPM (read: idle) range and as yet not artificially inducing it to charge harder.​
 

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Now I'm a-gonna eat my Oreos and drink a glass of ice cold milk and go to bed.
 
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