Testing diode's in the alternator.

[DCinVA]

I was testing my Battery/Charging system last night and I read the following in the bently manual.

Quote ---
Alternator diode packs can also be quickly checked.
Failed diodes are indicated by AC voltage in DC electrical system.

Caution
AC voltage in the car electrical system can lead to running problems and damage to sensitive electronic components, namely the DME control unit.

To check for excess AC voltage, switch multimeter to AC volts and check across battery terminals with engine running. There should be approximately 300 mv of AC voltage present. If significantly more AC voltage is present, alternator should be replaced.

End Quote ---

I have never heard of this test before!

Does anyone have any experience testing the diodes using this simple procedure?
I have searched the net, and only found a couple vague references to it.

By the way, I have 28 volts AC with the engine running, 0 volts AC with engine off.

Thanks in advance!

And happy holidays

[brianlay]

28 volts AC definitely indicates a problem. Are you sure about the meter scale you're using? Any chance you could compare measurements on another car.....like your Boxster?
Many manuals will describe using an oscilloscope to diagnose diode failure....the AC ripple measurement is just a less precise way to do it.
Any other symptoms in the car....like a loud whine in the audio system?

[DCinVA]

Hmmm, I just checked my truck, and I have 26 volts AC with the engine off.
Must be an issue with my Digital Multi Meter. I only have two scales for AC,
200 volts and 600 volts. I don't have a scope.

[brianlay]

What problem are you trying to solve? What are the symptoms?

[HawgRyder]

An alt is a 3 phase generator.
It has slip rings to get the voltage to the rotor...and the diode packs are in pairs (in series).
Each of the diode pairs connects to one phase of the Stator...so that the pair is in line ..so-to-speak...with the coil of the phase.
This forms a full wave rectifier for each of the 3 phases.
If any one of the diodes goes open...you lose a phase (and the power goes down) but not necessarily to zero.
If a diode shorts...different story...it can push through the AC factor of the coil voltage...and give you a very large AC voltage out from the unit.
If all the diodes are working...you will still get a little bit of ripple because there is not very much capacitance to dampen the output.
This is why a lot of older cars benefitted from having a big capacitor installed at the output of the alt...to get rid of the AC factor and let the radio (back then) be much more quiet.
You might remember the old 50's cars that the radio sounded like a jet engine varying with RPM.
Hope this helps.
Bob

[DCinVA]

Sorry, I guess I should share the issue that generated the question.

My problem is a bad alternator. There is no issue with diagnosing the problem, it needs to be rebuilt/replaced.

My question is about the AC Voltage test mentioned in the Bently Manual. In my 40+ - years of playing with cars, both professionally (a long time ago), and for fun, I have never heard of testing for AC current at the battery.

I was hoping that someone might have experience with this test, and would chime in with their experience. Based on my research, this test seems like a very uncommon procedure, but given the extensive base of knowledge on this forum, I was hopeful someone might chime in with their experience.

Thanks in advance

[DCinVA]

Sorry, I guess I should share the issue that generated the question.

My problem is a bad alternator. There is no issue with diagnosing the problem, it needs to be rebuilt/replaced.

My question is about the AC Voltage test mentioned in the Bently Manual. In my 40+ - years of playing with cars, both professionally (a long time ago), and for fun, I have never heard of testing for AC current at the battery.

I was hoping that someone might have experience with this test, and would chime in with their experience. Based on my research, this test seems like a very uncommon procedure, but given the extensive base of knowledge on this forum, I was hopeful someone might chime in with their experience.

Thanks in advance

Perhaps I should rename the thread to something like "Testing for AC voltage at the battery?"

[T77911S]

Originally Posted by DCinVA

Hmmm, I just checked my truck, and I have 26 volts AC with the engine off.
Must be an issue with my Digital Multi Meter. I only have two scales for AC,
200 volts and 600 volts. I don't have a scope.

it probably reads "26 or 026 mV" which mili volts, it could also read ".026 or .26V" IE 0 volts.

[T77911S]

Originally Posted by DCinVA

My question is about the AC Voltage test mentioned in the Bently Manual. In my 40+ - years of playing with cars, both professionally (a long time ago), and for fun, I have never heard of testing for AC current at the battery.

"

you are checking voltage, not current. they are 2 different things.

it is a common check. we do this check on power supply's all the time

[wwest]

Originally Posted by DCinVA

I was testing my Battery/Charging system last night and I read the following in the bently manual.

Quote ---
Alternator diode packs can also be quickly checked.
Failed diodes are indicated by AC voltage in DC electrical system.

Caution
AC voltage in the car electrical system can lead to running problems and damage to sensitive electronic components, namely the DME control unit.

To check for excess AC voltage, switch multimeter to AC volts and check across battery terminals with engine running. There should be approximately 300 mv of AC voltage present. If significantly more AC voltage is present, alternator should be replaced.

End Quote ---

Old multimeters and early DVMs, had a DC voltage "blocking" capacitor in the AC voltage reading position. Many more modern DVM do not have this capability so you get a "strange" reading in the AC measurement mode if this is DC content along with the AC.

A 1MFD capacitor in series with one of the test leads should do the trick.

I have never heard of this test before!

Does anyone have any experience testing the diodes using this simple procedure?
I have searched the net, and only found a couple vague references to it.

By the way, I have 28 volts AC with the engine running, 0 volts AC with engine off.

Thanks in advance!

And happy holidays

....

[timmy2]

Originally Posted by DCinVA

Sorry, I guess I should share the issue that generated the question.

My problem is a bad alternator. There is no issue with diagnosing the problem, it needs to be rebuilt/replaced.

My question is about the AC Voltage test mentioned in the Bently Manual. In my 40+ - years of playing with cars, both professionally (a long time ago), and for fun, I have never heard of testing for AC current at the battery.

I was hoping that someone might have experience with this test, and would chime in with their experience. Based on my research, this test seems like a very uncommon procedure, but given the extensive base of knowledge on this forum, I was hopeful someone might chime in with their experience.

Thanks in advance

Perhaps I should rename the thread to something like "Testing for AC voltage at the battery?"

HawgRyder explained it well earlier in this thread.
You are testing for the magnitude of the AC voltage component that is always part of the DC voltage output. The best way to measure this is by checking for an AC current. (AC Ripple current) There is always an AC component in the DC output from our alternating current generators (Alternators) The Diodes imperfectly clip the ac output of the alternator and convert it to DC.
We test Industrial battery systems for the ripple current output from the chargers as a way of measuring the performance of the chargers. As the ripple current baseline begins to increase, (degradation of the charger) the result is noise in our systems as well as degredation of the batteries themselves.
Replacemet of electrolytic capacitors usually fixes the problem in the chargers.
Diode replacement on these large chargers is the next fix if the caps are working as rated.
(Note when installing new electronics it is recommended to install an electrolytic capacitor on the dc supply to "clean up" the DC. I just installed one for my Bitz kit and Wideband controller power)

For our little alternators, diode replacement will/should repair the problem.

Basically, too much AC current means your battery and electrical components are being cooked because they don't like AC.
Hope that helps a little.

[DCinVA]

Thanks for all the replys.

This makes perfect sense, and is a very simple method of testing the diodes.
I am just surprised that I had not heard of it before.

This forum has an amazing base of knowledge. A great group of people, who are willing to help others learn.

Thanks again

Dave

[wwest]

There appears to be some confusion about "AC" current. AC current in a strictly literal sense would be current flow that reverse directions on a cyclic basis, as does our AC house current.

What we have in our car alternator is "alternating" DC current, alternator output current flow that varies in volume as a function of alternator rotor rotational position.

In the case of a fully discharged battery with the engine at a fairly high, "adequate", RPM you would undoubtedly measure ~10 volts "AC" (15 volt peak DC X 0.707) uing the AC range on a DVM.

Whereas with a fully charged battery only about 0.600 volts "AC" would likely be present. Pulsating DC atop the battery's 13.5 VDC.

In other words the "AC" you measure across the battery terminals will most likely be a indication of the battery state of charge, only indirectly, possibly, an indication of charging system health.

I wonder if this "urban legend" actually involved testing for the "health" of the old generator type charging system. A few commutator segments shorted in one of those and you would truly have AC current flow, battery current flow "back" through the shorted generator segments as they rotate under the brushes.

[big911fan]

I'm in way over my head on this but I've been following the discussion.

Wiring a house plug is the extent of my electrical knowledge.

My question to those experts that have reponded is this:

If you measure the AC voltage across the battery at idle, then measure it at a "reasonable" rpm as stated, what will that indicate? Does it still measure AC battery health or will the difference, if there is one, tell you something about the alternator?

Thanks for the knowledge imparted here by the experts.

[wwest]

Originally Posted by big911fan

I'm in way over my head on this but I've been following the discussion.

Wiring a house plug is the extent of my electrical knowledge.

My question to those experts that have reponded is this:

If you measure the (DC "ripple" voltage) AC voltage across the battery at idle, then measure it at a "reasonable" rpm as stated, what will that indicate? Does it still measure AC battery health or will the difference, if there is one, tell you something about the alternator?

Thanks for the knowledge imparted here by the experts.

What you would normally see on a fully charged battery pretty much regardless of engine RPM will be 2 waveforms "impressed" upon the battery terminal voltage.

The first of these will be the result of the rotation rate of the alternator and the 3 phase voltage output of same. The second will be the result of PWM, Pulse Width Modulation, of the alternator rotor current in order to regulate the battery charging rate.

Basically, the regulator will supply the rotor winding with the maximum current until the resulting battery terminal voltage rises to the "set" voltage. At that point the rotor voltage/current supply will open circuit until the battery terminal voltage drops to some "set" minimum.

When my car goes in for WDOT emissions testing that plug a monitor into the cigarette lighter in order to discern, display, engine RPM. There is a KNOWN divisor between the phase output "pulsing" of the alternator voltage and the engine RPM. Were an alternator phase to be non-functioning the emissions' RPM indication would be 2/3s of actual.

Bottom line: Should a phase fail, open diode (most rare event IMMHO), I doubt if it could be detected via the DVM method, just results in a slower charging rate. A phase failure due to shorted diode (more common IMMHO) would be a CONSTANT 12 volt system load (not only no charge rate but a QUICK battery discharge path) and therefore not be detectable via the DVM method.

Methinks a O'Scope would be the only viable method.

[wwest]

Originally Posted by big911fan

I'm in way over my head on this but I've been following the discussion.

Wiring a house plug is the extent of my electrical knowledge.

My question to those experts that have reponded is this:

If you measure the AC voltage across the battery at idle, then measure it at a "reasonable" rpm as stated, what will that indicate? Does it still measure AC battery health or will the difference, if there is one, tell you something about the alternator?

Thanks for the knowledge imparted here by the experts.

The method I use, rely upon, assuming a beginning battery terminal voltage of at least 12.5 volts (***) is to measure the battery terminal voltage at 2000 RPM with very little electrical load and then again with a high load, headlights, blower(s), etc. At 2000 RPM and with the charging system in good health there should be virtually no voltage decline as you add electrical loads.

*** Ideal = 13.8V w/OAT above 65F, 14.7V w/OAT below 35F.