Hello, this is my first post. I searched the forums and Google and couldn't find anyone with my exact problem, especially the Headlight part.

I have a tendency to write too much, so I'll synopsize my post here and put the long version under it. Thanks for your time and expertise!

BRIEFLY:
My Battery light comes on at stoplights. If I rev the engine, it fades down. But if I turn On the Headlights (without revving) the battery light just turns right OFF. [!]

1st: Why is this happening?
2nd: How can I fix it?
3rd: Why don't the Headlights make the battery drain WORSE instead of fixing it?

Thanks for your insights and knowledge!

* * *


THE LONG STORY:

I have a used 1992 Honda Civic DX. 5 Speed manual transmission.
Stock, with no performance enhancements.
iPhone power adapter always in the cigarette lighter (adapter led goes out when I turn off the ignition.)

Mechanic replaced the alternator about 9 months ago with a new alternator.
Was a weird situation because the old alternator (which was running fine for 2 years +) was a "random" model with a 3-groove pulley. All the new, proper belts and alternators use a 4-groove pulley.
I really didn't even "need" a new alternator (aside from it being random and not being the right model for my car), but the belt was starting to slip and squeal and my mechanic couldn't locate a 4-groove pulley to swap mine out with.

ANYWAYS... that's not the problem.

During the week after the alternator replacement, I noticed that the engine would occasionally "drop" into a lower idle while I was stopped at stop lights. This behavior never happened with the old alternator.

It could be a placebo effect, but it also seemed that the headlights were a bit brighter at night. My mechanic said the new alternator was a higher amperage and to possibly expect that.

The "dropping into extra low idle at stoplights" situation continued but the car never died. I do a lot of stop and go driving around town, not expressways.

4-6 months after the replacement, I pulled over to the side of the road on a drizzly afternoon and sat and idled for 20-30 minutes with the radio on, while I looked up something online on my phone. I don't believe it was plugged into the cigarette lighter at the time. This is unusual for me, I am pretty strict about turning off the car if waiting for a long period of time.

I then pulled out and busted a U-turn to drive off. Before I got to the end of the block, the battery light started to fade up. I pressed the gas and it faded down. I took a corner and it faded up again. I got really worried about getting stranded (it was 4:45PM Friday, so all mechanics were closed), so I just drove home. The light didn't fade up again.

I tried the car again Saturday and it was fine. No battery lights at all.

So for the last 3 months, the light has started to occasionally fade up again when I come to a stoplight. BTW, I usually put the car into NEUTRAL at a stoplight. Revving it back up causes the light to fade down again.

The 2nd or 3rd time it happened, I noticed the headlights were off, so I resisted the reflex to rev the engine and simply turned on the headlights. Doing so caused the battery light to IMMEDIATELY go out. No revving required!

* * *

I understand that perhaps my idle is too low or perhaps my alternator belt needs be tighter.

And I understand that this is under-charging the battery and that's what the light coming on is telling me, "Hey, FYI, I am no longer making enough juice to add a charge to your battery".

But I have no idea how the Headlights factor into this. Or why the replacement Alternator seems to have caused a lower resting idle.

Any solid tips, especially about the headlights are appreciated. Thank you!

 

The alternator's output is regulated, and based on the pull of your car's electronics. With the headlights on, the alternator is putting out more juice, which is letting it keep up with your car's requirement. The regulator is part of the alternator, and is not a serviceable part.

Option 1: Replace the alternator again. Look into the warranty of the alternator - Most parts stores have long, if not life-time warranties on them.

Option 2: Always drive with your lights on, until the alternator finally craps out on you. It could be tomorrow, it could be 5 years from now. Honestly, everyone should drive with their lights on all the time anyway. True statistic from IIHS: Cars with headlights are noticed at ten times the distance of a car without headlights, even in daylight driving conditions.

 

Thank you for the suggestions. But I'm really curious as to the underlying causes. If the Headlights are communicating with the Regulator in the Alternator, do you have any idea why the Battery (or something else) doesn't seem to be communicating with the Regulator?

 

I would have the ALT checked, I always recommend a NAPA or better factory alternator. The reman ones from Autozone O'reillys are junk. What did he install on yours?
Also I've come across where the alt belt is slipping on the pulley so make sure the belt is tight but definetly have it tested.