angellscobalt07

Hi there, newbie here. I have a 07 Chevy Cobalt LS that I've been trying to fix up myself. One of the issues I've been having is the headlights will dim and fluctuate in intensity when they're on, noticeable at night. They would dim at the same pace of the dashboard which seemed to be in time with listening to the engine.

I replaced the battery, serpentine belt and alternator, in that order. Maybe about a week apart from each other. Anyway, I was dreading and putting off replacing the alternator because of how difficult it was to get to but after the last two attempts with replacing the 10 year old battery and super worn out belt, the alternator seemed it was going to be problem solver (after reading it was most likely the culprit on numerous forums/websites).

After tearing up my arms reaching in and cursing all morning, I was able to get the alternator replaced. Drives fine, arguably feels like it drives with a little more pep but maybe that's in my head.

Come nighttime, I could fully test to see if my issue was gone. At first glance, the lights were consistently brighter and the dash no longer dims at all. As I sat there and looked closely, I turned the heater on full blast and noticed the headlight intensity changed subtly. I drove the car around the block a few times to see if I'd notice it on the road and if maybe the new alternator needed a quick drive before doing its thing properly. I don't notice it much while driving since it was so embarrisingly obvious before. I pulled back into the driveway and stared at the wall being hit with headlights some more and that's when I noticed it was still dimming ever so slightly. The intervals at which it dimmed is much longer now, say 3-5 seconds. And the dim is very subtle, but I know it's there and my fear is that it'll get worse over time again like when I noticed it for the first time about a year or so ago.

Can anyone point me in the direction of my next step should be? Will it go away in a couple days with the new alternator? I made sure connections to the alternator and battery were solid and snug.

mountainmanjoe

no, alternators don't need any breaking in.

Have you considered the headlight bulbs are worn out? Have you check relays and fuses?
Does it happen with both sides? What about high beams?

Does the problem only happen with the blower on?

When you had the serpentine belt off, did you test all the other pulleys to make sure there were turning freely?

angellscobalt07

I figured that headlights are either working or blown, but it has been probably about a year and a half since they've been replaced. All the fuses looked good but I'm gonna check again tomrrow with daylight out.

What/where are relays?
It happens with both sides and even when high beams are on, although I did not step outside to verify which bulb is dimming or if its just the low beams.

It seems to accentuate the problem only initially when turning blower on. All pulleys were turning freely when the belt was off.

mountainmanjoe

headlights don't last worth a damn these days anymore. A year is about all you can expect and they dim over time.

Relays are together with fuses. They are the cube shaped things. They switch things on a off, but they do wear out and act funny.
https://en.wikipedia.org/wiki/Relay

The wiring harness connecting your headlamps will have grounding points (connection to the frame). These often get loose or corroded so you need to freshen them up.

angellscobalt07

Okay, I've seen the relays there. Would I check them like I would a fuse?

I'm gonna replace the headlights tomrrow, I need to replace a blown reverse light anyway.

I've got a feeling it might be the grounding points you mentioned, would I just follow the wiring harness till I find where it'll be connected to some bolt or piece of metal? Someone once told me "it looks like it's a grounding issue." But I had no idea how to check at the time and when I looked it up, all the information was confusing since I was looking up grounding points/issues, apparently there are quite a few throughout the car and I didn't know where I would even begin to look. Hopefully that'll be it. It's been driving me crazy and at times distracting when I'm driving on a dark highway.

mountainmanjoe

The easiest way to test relays is to swap them with one that you know works. The next best thing are these relay bypass:
You could also use a test light or meter.

ground locations vary by model and I don't know Cobalts well, sorry. But I think you have the right idea. Follow the harness. Yes each different sensor, light cluster, or electronic device will have a dedicated ground lug to ensure proper voltage. There could be hundreds.

angellscobalt07

Thanks a bunch for the advice, I appreaciate it! I'll let you know if it works out

tech2

gm issued a tsb regarding this ##02-06-03-008D.
its states at idle this can occur due to high electrical loads, added electrical accessories and or discharged battery. The alt output at idle can't compensate for this.

Speak with the dealer and see if they have an updated flash for the bcm. I have seen on other models they increase the pwm of the bcm to increase headlight output.

hanky

These are some checks that should shed some light on the problem.

The voltage at both headlamps should be checked for any appreciable voltage drop with the headlights on. What readings are obtained there will tell if there is a power supply problem.
Then voltage drop should be checked for the grounds to those lights.
If,,, the output voltage of the alternator and battery compared to the voltage reading at the headlights ,has a significant difference, power supply problem.
This requires someone that is familiar with voltage drop testing.
Depending on what results are found will determine what should be done.