Checking AC Voltage On DC Systems

[Keith Browning]

I came across a couple topics on other forums and basically we now have hundreds if not thousands of people connecting their digital multi-meters to their truck's battery terminals and measuring AC voltage.

 

Just in case I have missed something over the last 24 years I don't want to state the obvious just in case the obvious isn't apparent to me. I have never heard of doing this. Is this some kind of a valid test and what would such a test be used to indicate?

 

Yes, I do understand that automotive "alternators" create AC voltage which is then rectified... but really? Checking AC voltage on a DC system? I would have expected to have seen this in all of my years in training courses and shop manuals. I even have Googled the subject! Nothing.

[mchan68]

Yes, I do understand that automotive "alternators" create AC voltage which is then rectified... but really? Checking AC voltage on a DC system? I would have expected to have seen this in all of my years in training courses and shop manuals. I even have Googled the subject! Nothing.

Now, now Keith. If you're referring to any vehicle newer than 1996 (OBD-II compliant), you mean GENERATORS right?

 

So...... who was it that you actually saw testing for voltage using AC volts today that sparked this post?

 

I know I tested an automotive component using AC volts today actually, the block heater cord for a 2000 7.3L truck.

[mchan68]

So...... who was it that you actually saw testing for voltage using AC volts today that sparked this post?

 

Never mind.........

 

I just remembered you mentioning that you were in school this week. You should've asked them if they even completed their basic electrical diagnosis classroom course.

[Jim Warman]

Yes, it is a valid test (anyone got an AVR with a "Diode Test" function?). We are looking for excessive alternator ripple - hoping for that "clean" power that modern modules have a thirst for FWIW, alternator ripple has only become a concern with the growing use of electronic modules.

 

The ideal test would be to use an oscilliscope - In the past, depending on the scope, you might be measuring VOLTAGE ripple or CURRENT ripple - and this might be a cause for some confusion.

 

If you go to "Service Tips" in the online manuals, there is what has to be one of the best write ups on current ripple testing that I've seen.

In the end, if you suspect alternator ripple as being a cause for concern, you are going to disconnect the alternator and retest, anyway.

 

Using a voltmeter to test a block heater.... not sure I would venture there... If we are hoping to measure volt drop across the heater, the DMM will read the same whether the circuit is open or "good". I like to use an old incandescent trouble light and a female plug rewired so that the trouble light bulb is going to be in series with the block heater. If the bulb glows at about half brilliance, we can see that (a) we have continuity through the wire and element and ( we have a better than even chance that there aren't any "unintended resistances" in the circuit. Important consideration.... the customer 'may' express a concern that the block heater "blows the breaker" even though he really means "it trips the GFCI" -

[Keith Browning]

Yes, it is a valid test (anyone got an AVR with a "Diode Test" function?). We are looking for excessive alternator ripple

So I can assume that you have heard of this? Would you consider looking for AC voltage in a DC sysem more of an "indication" that the alternator is failing in some way? What readings would you expect to see and what is acceptable and what is not?

 

The file you referenced is "Understanding Generator Ripple Waveforms" for those of you wish to read it. The file is too large to attach to a post so if anyone who does not have access to the PTS website I can email it to you... send me a PM.

[Keith Browning]

Originally Posted By: mchan68

So...... who was it that you actually saw testing for voltage using AC volts today that sparked this post?

Never mind.........

 

I just remembered you mentioning that you were in school this week. You should've asked them if they even completed their basic electrical diagnosis classroom course.

Actually Mike I saw this over on the FTE forums. I am not sure if the tech posting about this is a member here. I wish I had seen and thought about it a few days sooner as I would have asked the instructor.

 

I rarely have a need to check for this kind of a failure or symptom and when I do run into it is always because the check engine light is on and the DTC brings me to the silly scope and the ripple test... never an AC voltage test at the battery terminals. If doing so is valid enough to at least try it is quicker than hauling out the VMM and setting up the scope on IDS... at the very least valid enough to prompt me to proceed further with the scope.

[Bruce Amacker]

The AC voltage test across the battery terminals has been around since the 60's when alternators were released. It's a simple test and works very well with one exception- the "industry standard" of .5ACV max was written in the 60's and does not apply today. On most vehicles I measure .02VAC at the battery and .05VAC at the alternator. If it exceeded .1VAC I'd be concerned.

 

Not too long ago I was doing an electrical class in WA and used a new IH ambulance for our hands-on testing. It measured .4VAC idling and I told the guys to send it back to IH. They contacted me a few weeks later to tell me IH did replace the alternator.....

 

[2006]

I always find the 6.0's have about .2 volts ac at the alternator output stud and ground with all the accessories turned on. Anything more gets a full charging system test.

 

For a better quick test, I also use the older Snap On Vantage tester and use a .01uf capacitor in one of the test leads. Connect it to the alternator or battery or even into the powerpoint. You will see the voltage ripple. After you see a few good waveforms, the bad alternators ones are easy to pick out.