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All, anybody know why I am seeing 29VAC at 13.9VDC on an alternator?

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Backstory: My original 211K alternator was outputting 35-45VAC and normal DC voltage. With some input from Mopar1973Man, all I could gather was that a diode in the rectifier in the alternator was really acting up, but the regulator was keeping the DC voltage in check allowing a AC waveform through. Speed up today: I bought a new premium NAPA alternator and replaced it. The voltmeter is reading a beautiful 13.9VDC, and 25.8VAC. ETA: The standards I have read online for an alternator is .09VAC MAXIMUM, and that if you are reading over that, your diodes are screwed up. I am so confused by this. Anybody have any ideas?

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Try a different meter, if possible, an analog one. Digital meters have interesting ways of determining voltages, some work some don't. You changed out the alternator so I don't see why it would still be like that so I am guessing the meter is to blame. The voltage coming out of the alternator is not exactly DC but is rectified DC, meaning instead of the usual AC sine wave (which alternators are 3 phase so 3 sine waves 120* apart), instead the diodes only let the positive portion of the wave through, so you end up with a bunch of mountain tops. 3 phase makes it a lot better since the mountain tops run into each other without dropping all the way to 0V, so the sine wave is straighter. The problem is that digital meters interpret these mountain tops as a sort of weird sine wave which is obviously related to alternating current. I can put my meter on Hz (frequency) in my truck and I get around 500-600Hz, how can that be when it's DC?... Now when I put my digital meter on AC, I get something in the millivolts range, which falls within your maximum limit. So in this instance it works, but I have used the same meter on a bridge rectifier with household power (60Hz) and since it inverts the negative peaks it ended up being 120Hz on my meter, even though it was only seeing positive waves. It also showed 30VAC from the rectified 120V outlet.. Something to think about :smart:

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Try a different meter, if possible, an analog one. Digital meters have interesting ways of determining voltages, some work some don't. You changed out the alternator so I don't see why it would still be like that so I am guessing the meter is to blame. The voltage coming out of the alternator is not exactly DC but is rectified DC, meaning instead of the usual AC sine wave (which alternators are 3 phase so 3 sine waves 120* apart), instead the diodes only let the positive portion of the wave through, so you end up with a bunch of mountain tops. 3 phase makes it a lot better since the mountain tops run into each other without dropping all the way to 0V, so the sine wave is straighter. The problem is that digital meters interpret these mountain tops as a sort of weird sine wave which is obviously related to alternating current. I can put my meter on Hz (frequency) in my truck and I get around 500-600Hz, how can that be when it's DC?...

Now when I put my digital meter on AC, I get something in the millivolts range, which falls within your maximum limit. So in this instance it works, but I have used the same meter on a bridge rectifier with household power (60Hz) and since it inverts the negative peaks it ended up being 120Hz on my meter, even though it was only seeing positive waves. It also showed 30VAC from the rectified 120V outlet..

Something to think about :smart:

Tried an analog tester as well. I really need to find somebody with a fluke tester... Unfortunately the Flukes we use at work are on the other side of the state and probably will be for about a month or so. I am going to replace the regulator with an old school 1st gen regulator and see what happens, if that doesn't work I am contemplating buying an oscilloscope, and actually getting a look at the waveforms coming out of the damn thing! I am at my wits end here with this.

--- Update to the previous post...

OK.

I figured it out.

As I sat in the NAPA parking lot it occurred to me to try testing the VAC while the ignition is on and the engine is off. (as I was using test leads through the power outlet to test while driving) It showed 26.5V in ac, and 12.5V in dc.

INTERESTING!

Same results when testing from the batteries and when the alternator feed is disconnected and the enigne is running.

So instead of putting an external voltage regulator in, I went and found an 01 and 97 Cummins pickup at a dealer to test the voltage. Both trucks, show EXACTLY the same "AC" voltage on my meter.

I am chalking it all up to instrumentation at this point. There is currently no evidence to show that my trucks charging or electrical system has any issues. It appears that I have been chasing my tail with a miscalibrated cheapie multimeter showing 18V dc, and then instrumentation on a mid range $100 multimeter showing ac that doesn't exist, or even if it does exist its normal!

Off to put the Edge CTS back on... hopefully it works this time. Hopefully, somebody else learned something from this or will if the search for "ac voltage" or something later. :shrug: At least I have a spare alternator now! :lmao:

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I might have to try mine with a different multimeter and see what I get. My digital one is a fluke. Good fluke not a fluke fluke lmao. It shows millivolts but maybe a crappy one will show what your's shows.

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I might have to try mine with a different multimeter and see what I get. My digital one is a fluke. Good fluke not a fluke fluke lmao. It shows millivolts but maybe a crappy one will show what your's shows.

Yeah, flukes are nice. Someday I will own one. :spend: At any rate. I just tried putting the edge juice w cts on again. It didn't work and did the same thing it did last time before I sent it back and Edge Products "Fixed" it. I checked every ground, I cleaned every connection with contact cleaner. I checked the resistance of every fuse. I checked to see if I was getting 12V at the right places. I even tried only plugging in the minimum of wiring (no turbo timer or temp probes). Nothing would get the CTS to connect to the juice. It was seeing the OBDII output, but not the Juice. I wish I had a 12 valve ISX. :evilgrin: lol. At this point, I am convinced it isn't my truck. I have a NEW ECM. New alternator. And every bit of wiring on the truck has been meg'ed and checked out for any stray voltage.

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Does your meter have a frequency setting? I would be interested in seeing what it picks up.

It doesn't do frequency unfortunately.

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A great guy from ARFCOM offered to loan me his Fluke 87 meter, great guy. I am seeing a spike in VAC every 15-30 seconds or so of several hundred millivolts when the engine is running. Base VAC is 13 millivolts. I am going to do some further investigation when I get back from vacation... and once it stops raining / snowing. :hyper:I am going to troubleshoot the alternator and voltage regulator, by disconnecting the feed of the alternator and looking for the spike. If this eliminates the issue, I am going to look for a new PCM or a solid state external voltage regulator. If the regulator and alternator test have no influence on the voltage spike when running just on batteries, then i probably will start looking at individual relays and systems again. I wonder if the heater grid has a big relay on it or what... I will need to spend some more time with the wiring diagrams and make sure the grounds are good as well. A high power relay could make some noise, and I am hoping that with this excellent high sample rate meter, I can figure it out! If anybody has any suggestions while I am away, please post away!

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Check for shorted fuses......:lmao2:Ok only thing I can think of is how is the ECM gorund? What about running a dedicated ground wire to the battery from one of the ecm mounting screws....err the pcm actually.I think your meter is just measuring DC ripple.

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