Voltage fluctuation - large swings

[Sean D 31]

Finally got my new engine running with new parts (and I mean everything), and at the same time rewired front end with new headlight harness and all other wiring too, including new 4 gauge battery cables. Has Pa performance 3g alternator with AGM battery. MSD electonic ignition. Now I am trying to figure out why my voltage across the battery terminal, when running, fluctuates so much. 

Please confirm that the voltage should not be swinging from 0 to 14.8 volts and everything in between when car is running. The reading on my volt meter across the battery terminals changes every second to some value between 0 and 14.8 v. Nothing constant. Also same result when testing at positive post on back of alt and negative battery terminal. Also same result throughout rpm range. Seems to get worse as things like electric fans start ramping up. When car is not running it is constant 13.1v.

What could cause this? Grounding? Bad alternator? 

 

 

[Midlife 814]

You have a bad connection somewhere or a faulty wire, including the DVM probe wires.  If you're measuring across the battery terminals, it can only be one of two things: a bad DVM (and its wires) or a very bad battery (pretty much unlikely---they don't fluctuate like your video).

[Sean D 31]

29 minutes ago, Midlife said:

You have a bad connection somewhere or a faulty wire, including the DVM probe wires.  If you're measuring across the battery terminals, it can only be one of two things: a bad DVM (and its wires) or a very bad battery (pretty much unlikely---they don't fluctuate like your video).

Thanks. I wouldn't think dvm probe as it is good constant voltage without engine running.

[Midlife 814]

Hmmm...how good are your battery terminal connectors: tight?  Are you measuring at the terminals themselves or at the connectors?  Find which set are constant and which are varying, and that will tell you what might be going on.  Batteries should always have steady voltage; the connectors may not.

[Sean D 31]

3 minutes ago, Midlife said:

Hmmm...how good are your battery terminal connectors: tight?  Are you measuring at the terminals themselves or at the connectors?  Find which set are constant and which are varying, and that will tell you what might be going on.  Batteries should always have steady voltage; the connectors may not.

I tested ar battery connectors and terminals with same results.

Interesting that the voltage gauge (converted ammeter) in the instument cluster  is pretty constant and doesn't bounce around.

[Midlife 814]

So you only get solid readings with the engine off?  I think that the vibration of the engine running is causing poor contacts of your DVM probes.  Oh...wait a second...your dial is halfway between 20V and 2000 mV.  Re-set your dial and try again!

[69RavenConv 286]

+1 on the bad meter/dial setting/probes. The laws of physics disallow the possibility of a lead acid storage battery fluctuating +/-14 volts in a matter of seconds. Try a different meter and/or meter leads.

[mwye0627 44]

I agree that something is likely wrong with his DVM or leads...

Batteries typically act as very large Capacitors which filter and smooth any fluctuations from the alternator.

An LED type test light would flicker if there were truly those kinds of variations in voltage.

 

[Sean D 31]

I tested my DVM on one of my other vehicles and it read constant voltage when the vehicle was running and not. What the heck could this be? Poor grounding? Bad voltage regulator inside the 3G alternator?

[Midlife 814]

Sean: try it again.  Based upon your video, your DVM was set halfway between two voltage range settings, so it was not making good contact inside.

[1969_Mach1 333]

I haven't followed this thread much, but it doesn't surprise me that providing your DVOM is okay, the voltmeter in the dash doesn't pick up on these rapid voltage changes.  It's simply not nearly as sensitive.

On kind of a side note.  Why does the wiring to the 3G alternator look so complex.  It only takes three wires, the charging wire, one 12V+ constant and one 12V+ switched source.  Then eliminate the original (or dummy original) regulator and wiring connector.  It seems midlife would know what wires to use that were going to old regulator. 

[Sean D 31]

19 hours ago, 1969_Mach1 said:

I haven't followed this thread much, but it doesn't surprise me that providing your DVOM is okay, the voltmeter in the dash doesn't pick up on these rapid voltage changes.  It's simply not nearly as sensitive.

On kind of a side note.  Why does the wiring to the 3G alternator look so complex.  It only takes three wires, the charging wire, one 12V+ constant and one 12V+ switched source.  Then eliminate the original (or dummy original) regulator and wiring connector.  It seems midlife would know what wires to use that were going to old regulator. 

It just looks that way. I used the PA Performance dummy regulator. only three wires are used.

[gordonr 4]

I would like to see some more voltage tests. Obviously your issue is  puzzling and any real legit swing like that will cause excessive heat in either a positive cable from a shorted line or starter or the supply to the alternator. I would assume Also do you hear a loading/ unloading whine coming from your alternator? Do you have a clamp on amp meter to verify. Your meter is in question for sure with me. I would borrow a Fluke as it has better shielding circuits and retest.

[Sean D 31]

All thank you for your help. I borrowed another meter and now move constant voltage at 14.6v.... thank goodness as I was about to loose my mind. Again thank you all.