Rapidly fluctuating voltage

[MaximRecoil]

The voltage in my car ('69, but with the later style dual field terminal alternator and electronic voltage regulator) used to be very steady; you could hold a meter on it and it would only move up or down by .01 volts every so often.

Recently I noticed the illumination of my radio rapidly fluctuating in brightness slightly when the engine was running. I measured the voltage with a multimeter and it is rapidly jumping back and forth from about 14.1 volts to about 14.6 volts. Is the alternator or the regulator the more likely culprit? They were both junkyard parts in unknown condition when I installed them in 2011.

[Mopar Nut]

Is the alternator or the regulator the more likely culprit? They were both junkyard parts in unknown condition when I installed them in 2011.

I would replace the regulator/alternator with new parts.

[MaximRecoil]

I would replace the regulator/alternator with new parts.

That's the "shotgun approach", which works, but I'd rather not replace anything that is still functioning properly. I'm guessing that it is one or the other, rather than both, that is not working right.

[Mopar Nut]

Most parts places will check the alternator for free.

[MaximRecoil]

Most parts places will check the alternator for free.

I don't have a place nearby that is competent when it comes to testing Mopar alternators. They have the machine, but they don't have anyone who knows how to use it right. The alternator I have in there now was declared completely dead (zero output) by them (after all 4 employees, each with a different idea of how it should be connected to the machine, gave it a try) ~2 years ago before I installed it.

[Mopar Nut]

I don't have a place nearby that is competent when it comes to testing Mopar alternators. They have the machine, but they don't have anyone who knows how to use it right. The alternator I have in there now was declared completely dead (zero output) by them (after all 4 employees, each with a different idea of how it should be connected to the machine, gave it a try) ~2 years ago before I installed it.

I hear you on this, I had two different places check mine. 

[PlainfieldCharger]

I would start with the voltage regulator first. As you said they are both from the yard. .5 volt fluctuations tend to be there. You could make sure the ground wires are clean and tight too. Intermittent problems are always the hardest to find. If they could just fail..then you know what to replace.

[Pete in NH]

If you're using a digital type meter the last digit of the reading will always be + or - one digit. So, if you're using a meter with a three place readout, the reading will always be + or - .1 volt. Also, digital meters have a sampling rate and may be slow to respond to the full range of voltage swing. If you have an analog type meter i would try it as well and you may see the voltage is moving around more than .5 volt. There of course can be many reasons for the voltage fluctuations and intermittent connections. One thing you might check since the alternator is used with unknown miles on it is the remaining brush length.When they get real short the spring tension on them decreases and they can start to bounce around.

[PlainfieldCharger]

If you're using a digital type meter the last digit of the reading will always be + or - one digit. So, if you're using a meter with a three place readout, the reading will always be + or - .1 volt. Also, digital meters have a sampling rate and may be slow to respond to the full range of voltage swing. If you have an analog type meter i would try it as well and you may see the voltage is moving around more than .5 volt. There of course can be many reasons for the voltage fluctuations and intermittent connections. One thing you might check since the alternator is used with unknown miles on it is the remaining brush length.When they get real short the spring tension on them decreases and they can start to bounce around.

The carbon dust build up can be a problem too on those brushes as they get small and spring tension decreases. Very true about Digital meters on the speed of response 

[MaximRecoil]

I would start with the voltage regulator first. As you said they are both from the yard. .5 volt fluctuations tend to be there. You could make sure the ground wires are clean and tight too. Intermittent problems are always the hardest to find. If they could just fail..then you know what to replace.

I'm leaning toward the regulator too. I'll have to see if there are any more in the junkyard that I can try. If it steadies out the voltage I'll put it back and buy a new one.

I'm particular about wiring, so all of my grounds have always been as good as they possibly can be (every one sanded to bare metal, greased, and bolted down tight). I also have overkill on grounds: 1/0 gauge from negative battery terminal to front of engine block, 4 gauge from front of engine block to inner fender, 4 gauge from inner fender to radiator support, and 4 gauge from firewall to back of engine block. Plus, in addition to sanding to bare metal around the mounting holes of the regulator and around the holes in the firewall it screws into, I also have a short 10 gauge wire bolted to one of the regulator's mounting holes on one end and bolted to the firewall on the other.  

If you're using a digital type meter the last digit of the reading will always be + or - one digit. So, if you're using a meter with a three place readout, the reading will always be + or - .1 volt. Also, digital meters have a sampling rate and may be slow to respond to the full range of voltage swing. If you have an analog type meter i would try it as well and you may see the voltage is moving around more than .5 volt. There of course can be many reasons for the voltage fluctuations and intermittent connections. One thing you might check since the alternator is used with unknown miles on it is the remaining brush length.When they get real short the spring tension on them decreases and they can start to bounce around.

I don't have an analog meter; I have a cheap $4 DMM from Harbor Freight, and I have a $90 DMM that I got for free. I use the cheap one for most things because I like it better and it's the one I keep in the car. I check my voltage at the battery often just for the hell of it, and it has always read pretty steady with that cheap meter (I've never checked it with the other meter). The readout for the 20 volt DC range setting is to two decimal places, e.g. 14.38 volts. So when I say it has always been pretty steady I mean that it would read say 14.40 volts and stay there for 20 or 30 seconds, and then it might go to 14.41 volts and stay there for 20 or 30 seconds, and then go back to 14.40, and so on. So recently when I measured it after noticing the illumination on my radio flickering slightly, it rapidly jumped back and forth between about 14.10 and 14.60 (sometimes reaching the 14.70 range), and by rapidly I mean every half second or so, barely even enough time to take note of the low and high numbers.

I know those readings aren't perfectly accurate and that the range of fluctuation is probably more than is being displayed, but regardless of what the actual range is, I am definitely getting significant voltage fluctuations now whereas I wasn't previously.

[PlainfieldCharger]

You could try lightly tapping the alternator with a small hammer to see if the fluctuating goes away. It is like hitting a starter to get it to turn over when try start getting tired...Could be a quick test....

[Mopar Nut]

Any luck?

[MaximRecoil]

Any luck?

I went up into the junkyard and got another regulator. That one was truly junk, no charging at all with it hooked up (12.25 volts at the battery when running). So I went back up into the junkyard and got yet another one. That one was good, and the voltage was quite steady with it, only showing fluctuation on the meter of a few hundredths of a volt (e.g. from 14.29 to 14.33). It is an old aftermarket one that was on an '85 Ramcharger. I'll buy a new Mopar one when I get around to it, but for now the issue is solved.