IS this a sign of a faulty voltage regulator?

[reamer]

Headlights brighten up when revving, Dim a little at idle, But there is NO light flicker.
Change the regulator or it's fine and that's how the 40+ year electrical system works?

 

[DirtDonk]

Yes, no and maybe…
What is the actual rpm of your idle speed?
When you’re noting this there are no other loads on the system other than the engine running. Correct?
Is it the original style Alternator?

Basically when everything is perfect you don’t normally have that dimming at idle. But with age comes many faults. Including a weaker Alternator, or a regulator that’s not quite happy, or corroded connections. Or probably a few other things as well.

Give us a description of your charging system and your electrical system overall.
What condition is it in?

 

[DirtDonk]

And I guess something else to help diagnose the health overall would be to actually measure the voltage at the battery with the engine on and off.
With the engine running and no loads you should be seeing approximately 14 1/2 V at the battery. If it’s much more than that then, yes your regulator is probably at fault. If it’s much less than that then it could be a multitude of things.
Including an old tired battery. What’s the general age of your battery? And does it sit for extended periods of time and has it been drained before?

 

[reamer]

New Battery New (original style) Alt from napa. Old V.R. ..12.3V EOKO, 14.2 Engine running @ 1500 rpm,

 

[B RON CO]

Hi, before changing the regulator I would check all the connections and grounds for corrosion. Resistance could be adding up. Good luck

 

[bmc69]

Pretty normal; all of mine have shown at least some of that. Obviously lights shine brighter at 14.5V than at 12.7V...the difference a function of alternator RPM. On my '78 Bronco I have twin batteries and a Leece-Neville ambulance alternator that puts out over 100A at fast idle. That's probably the only truck I own that does not exhibit some sag in headlight brightness at dead idle.

 

[Broncobowsher]

14.2V sounds correct.
You may not have enough alternator. Or a winding/diode has failed reducing output.
How much additional electrical load has been added (EFI, fuel pumps, etc)

 

[Timmy390]

Once I upgraded alternators and got rid of the voltage regulator, mine stay same brightness all the time.

Tim

 

[Slowleak]

I would put some LED bulbs in it. You will have a lot less load on the electrical system.

 

[Rustytruck]

Are you using the tall original style mechanical regulator or the better more modern style electronic regulator. if your using the old style switch to the electronic version.

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[DirtDonk]

While it has generally been considered normal practice for probably 50 years to change the regulator any time you change the alternator, I don’t always follow that rule myself. I usually just replace whatever is bad.
Then wait for the next part to fail…

So yes, your regulator could just be getting old and need some help. But as the others have said, there are other things that can create this issue. So check those first.

When you replaced the alternator was the mounting point on the engine pretty clean and free of dirty oil, or rust, or paint? If not you probably should remove it again and clean those points.
Is the little tiny ground wire still attached to one of the mounting bolts of the voltage regulator on the fender? That connects directly between the alternator and regulator to give them the same ground potential.
Are the mounting points of the regulator to the body also clean and rust and paint free?

After all these years the original body ground may have been removed. Verify that you have a ground connection to the body still. Originally it was part of the main cable to the engine where it was bolted to the inner wheel well.
If it’s been replaced with new that connection has probably been removed so you need to verify that there is an additional wire from the ground log to the body. The usual spot to mount those is either under one of the starter relay mounting bolts, or one of the regulator mounting bolts.

While you are checking the ground wire, verify that the main ground cable has a good connection to the engine block.

And FYI, don’t unbolt the regulator while it’s still connected to power. Either pull the four wire plug off first, or disconnect the negative side of the battery. Then you can unbolt the regulator.

And last, but not least, what year is your bronco? If it’s a 66 through 70 model, disconnect the two rectangular firewall connectors were all the wires feedthrough to the fuse panel and check them for any corrosion that would need to be cleaned off. Especially on the large black with yellow stripe wires. Those are your charge wires.

Inside, just behind your ammeter, there is a large push connector on that same black with yellow wire. Pull it apart and make sure it’s not corroded. When it snaps back together make sure it’s good and tight.

And I guess last but not least for the second time, check that the black wire on the battery side of your starter relay/solenoid is clean. Corrosion there will do the same thing as it will do in any of these other places.

Once you’ve done all that and the system still dims at idle, you can have your alternator tested and then use that opportunity to replace the regulator.
But don’t forget to check idle speed. Anything less than 700 RPM is likely to drop below the threshold for that style alternator. especially if someone has installed a slightly larger pulley at any point in the past.

 

[reamer]

You guys are great, I got some things to look it, FYI, it's a new WH body, added @14 grounds (frame to body, body and frame to Battery, Installed headlight relay kit, New Alt with a dedicated wire, alt to regulator. Will check block cleanliness for proper ground....