Voltage drops / dimming with lights.

I drop about 2-3 volts when hitting brake lights, which also dims all interior lights and headlights very slightly. Turn signals and headlights do the same. I have to set a very aggressive idle in order to prevent draining the battery at stops (which causes a hard jump when shifting from park into drive / reverse usually, which is probably a question for another thread, though if anyone has ideas on that, I'd appreciate it).

I have a 2-wire Duralast alternator, relatively new. New battery. 4-5 month old voltage regulator. Is this normal operation? How do I go about improving this system for my car?

'66 Charger, 361.

Check all grounds. Body to engine and so forth.

Was it always like this on your car ? Don't know the exact voltage drop as I have not measured it, but at low RPM's the stock alternators / electrical system in these cars did not keep with loads well, certainly not like modern cars. Though it's not a problem at higher RPM's.. Sounds like you may have bypassed your alternator gauge as many do. If you have an ALT meter what does that show when you apply the brakes ? It should be centered and instantly deflect momentarily to discharge, but just as fast return to center as the load goes away. Similar with turn signals you will see a small discharge when they light up, but should return to center when they go out. At idle with headlights on, discharge is very pronounced with anything electrical but when at higher RPM's not an issue. I would add that my car is all stock with no modifications to electrical system.

As long as it charges at higher RPM I wouldn't raise the base idle speed.
You've seen the bad consequences from too high an idle but you also have to stop it and youre putting more heat into the transmission.
Your battery shouldn't go dead in 2-3 minutes at idle even with the lights and brakelights on.
With the engine off it should be fine with the lights on for 5-10 minutes.
A couple things you might want to consider:
1. Change out the headlights to something like an H4 conversion.
I replaced my old halogen bulbs and unplugged the high beams didn't need them anymore.
2. LED taillight bulbs (but this gets pricey)

If you drop 2-3 volts something is way Way off. A fully charged battery is 12.6 so are you charging 15.6?
A good battery wil only drop .3-.5 volts with All the lights on and engine off.

What is your charging voltage at 1500 RPM?
What is battery voltage after is sits overnight?
What is battery voltage at a slow idle in gear with lights on?

What size alternator are you running?
I had the same problem with the original 35 amp alternator. Replaced it with a 55 amp and it is MUCH better, but in hindsight, I probably should have gone bigger.

I noticed you said Duralast aka AutoZone "Don'tLast" That could be your problem. The only thing I'd buy from AZ is "dumb parts", fluids, bulbs, wiper blades etc. NEVER mechanical or electrical parts.

Some of that to a point is normal on the older 60's Mopars. In my 63 I had the stock 35 amp alt on it all the years I had it and I added an electric fan I could turn on if needed. When I came to a stop at night with my headlites and fan on my volts would drop from 14.5 to about 13. But as soon as I hit the gas and got over 1600 rpm it went right bck to 14.5 and all the years I have had it I never had a charging problem or dead battery. But last year I decided to put a one wire 100 amp alt on it that looks like the factory Mopar alt. Is a new style though and will put 65 amps or more out at idle if needed. Now if I turn on my fan and headlites at idle the volts starts to drop and then shoots right back to 14.5 at an idle with all loads on because this alt will put out much more at idle. The older alt would not reach the full 35 amps until 1500 to 2000 rpm so it is normal for the volts to drop some at idle with many loads turned on. But the battery should not go down or dead as if it does then there is a problem. And you can always update to a newer alt that will charge more at idle also. Ron

Aron Gleason said:

Check all grounds. Body to engine and so forth.

Click to expand...

Absolutely agree.
If you are still dealing with the original wiring it's time to check everything.
http://www.mymopar.com/downloads/1966/66ChargerA.JPG
http://www.mymopar.com/downloads/1966/66ChargerB.JPG
Concentrate on the main power distribution connections, bulkhead terminals, ammeter connections, voltage regulator and alternator connections. Clean and tight is a requirement. Original Mopar bulkhead terminals are always suspect. Always check the fit on push on connectors like the ones on the alternator. A few times wiggling them to get them off and on will spread them and the fit is compromised. Another little trick, after you have gone through the wiring is possibly replacing the regulator with a Mopar P3690731 constant output voltage regulator.