Alternator Alternating current question

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spycam

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Sorry if this has been answered before, I'm picking up some Alternating Current Voltage on the B post of my old (slant six 1966) alternator. How much should that voltage be before I know the rectifiers are toast ?

Thanks
 
less than 1 volt a/c, anything more and you got a diode out
 
Thanks guys, I thought that was one hell of a ripple ! :}

I guess it would drive the regulator crazy ?

Would that cause the alternator to get pretty hot ?

Would that by any chance account for the cabin lights flickering at low RPM (idle) ?

SC
 
Depending on what's wrong, shorted stator, shorted diode, YES could run hot
 
I would not worry about any other electrical problems until you get that fixed. It may be driving the VR nuts
 
Yeah, thanks, that's what I meant. I thought the giant "ripple" could cause all that.

Interesting thing, although there is a huge AC voltage (33 AC volts !? measured separately with 2 different VOMs ) there is no AC leakage (0 mA measured with the VOM in line and in general there are no leakages anywhere in the system) which could mean that the diodes are not leaking. So it might be something else.

You mentioned a shorted stator ??
 
33 volts!!!!?????? Get that thing outtta there!!!

Let's review..........you put the meter on AC volts and connect from alternator output stud to ground and that is what you get? You will be lucky you did not damage something in the system.

'N ya, I'm not surprised the alternator runs HOT!!
 
spycam said:
measured separately with 2 different VOMs ) there is no AC leakage (0 mA measured with the VOM in line and in general there are no leakages anywhere in the system) which could mean that the diodes are not leaking.
Click to expand...
Tell us how you did the leakage test. "In line", with engine stopped, no AC there because the alternator is not putting out. There might be DC current however, and that might even blow fuse in meter. Best to use clamp probe meter for current measurements.
 
Yes, engine stopped, of course no AC, and the ammeter inserted "in line". There can still be DC leakage through the diodes from the battery. But there wasn't. There are no shorts anywhere in the car, I went around and checked everything.

Now it turns out the whole ripple thing was a complete fluke. The result of using "el cheapo" VOMs (two different ones), but it turns out they were not "True RMS". The NON-True RMS meters make the assumption that the AC is a nice sine wave of less that 450 Hz and calculate "RMS" on that assumption which of course is totally wrong since the ripple coming from the diode bridge is nothing like a sine wave. The resulting measurement gave an outrageous value of 33 (!!??) volts of AC, which was the clue that something was wrong there because 33 volts would have burned a few things in the car, but that of course didn't happen. The fact that two different meters of different makes gave the same value is what befuddled me. Not all of us can afford a Fluke I guess.

The real ripple turned out to be 0.57 volts. Slightly higher than the recommended 0.5 max but not outrageous (funny thing I afterwards read somewhere that for the 66 Barracuda there could be as much as 2 volts with no ill effects).

The whole thing started because I noticed that the cabin lights were flickering at low RPMs (idling). Which made me think there might be a higher ripple in the alternator, which led to the fluke measurement.....

Moral of the story : use better equipment if you can, and don't believe everything you see without a lot of thinking first.
 
What are these meters you used? "True RMS" or not they should not have been so far off. I've had several meters before Fluke (I'm 68, used LOTS of analog instruments before digital)
 
Thanks for your comment, I'm older than you :} and I grew up with analog meters myself ! In fact one of the meters I used was an old analog Radio Shack from the 90s.

But look at this guy on YouTube using a "Horrible Freight" orange meter doing the exact same thing I was doing and he is pulling 29-30 volts on a ripple measurement (almost the same amount as mine) and he is casually saying "you don't want to see much more than that.." instead of being alarmed:



So if you use that type of common meter you are going to get those kinds of values. Believe me the answer is in the internal algorithm that assumes a sine wave of < 450 Hz, instead of the rectified half-wave pattern that comes out of the diode bridge at probably much higher frequency. I'm just pissed-off that I didn't think before bothering you all with my question. (In another life I was a mathematical physicist and I should have used my now depleted brain).

In fact another guy on Youtube makes a comment about that problem exactly saying that a "spurious voltage measurement" comes out of some cheap meters (about 1 min into the video):



Thank you very much for rushing to my help though.
 
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I know everyone can't justify owning one, but I check alternator ripple with my Uscope and an amp clamp. It makes the job fast and easy.
Alt Uscope test small (800x600).jpg
 
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