Thread: Lights dimming rhythmically

As the title describes, I'm having an issue with the lights dimming rhythmically. It is not limited to a particular set of lights, the problem affects all lighting (headlights, interior, dash & DIC, etc). Here's a list of things that I have performed/checked:

Battery is less than 3 months old, tested and performing excellent. CCA & voltage all well above minimum spec.
Terminal connections are free of corrosion and tight.

Alternator appears to be OEM (116k miles), voltage was 14.1 unloaded and 13.8 under load. Connections are also clean and tight

Now here's what I have added/upgraded on the car electrically:

Big 3 upgrade: 4 gauge power wire from alternator stud directly to positive battery terminal (150A fuse)
4 gauge ground wire from rear alternator bolt to frame ground by battery
4 gauge ground wire from battery negative to frame ground

Frame ground by battery was cleaned to bare metal
Ground underneath driver side headlight was cleaned to bare metal

I do have all upgraded interior speakers driven by an amplifier. The amplifier is a Pioneer GM-D8604 4 channel amp. 600W RMS rated power, hooked up with 4 ga power and ground. The gains are set very low to not overdrive the new speakers. All connections on amp are clean and tight. Ground is on one of the strut bolts, bare metal and tight.

Here's my speculation ...

I do not see, or know of a ground between the engine and firewall. However, I am aware of a ground that runs from somewhere on the engine block to subframe. I did not upgrade this ground, as I safely assumed that the ground wire from the alternator to frame ground would handle this task better (path of least resistance and whatnot) and I read that it is already a considerable size (4 gauge IIRC)

One of my neighbors is an electrician, and he said that it could be a connection shorting out somewhere in the vehicle. My other neighbor, who was a mechanic for a number of years, looked over my work and said he believes that the alternator is the most likely culprit. I cannot quote exactly what he said (I think it was rectifier) but it was one of the internal components he believes is failing and is causing a "pulsing" throughout the charging system. This pulsing did not show up on my DVOM, and I would imagine that it would have to be at least half a volt for the lights to dim like this (about 3 times a second).

I did notice that when I use one or both of the power windows, the dimming will get worse as the motors are operating, then when I release the switch all lights will not dim for a few seconds, then it resumes again.

I hope you guys have some good ideas for me! Thanks again for your time!

- Ryan

I would suspect the alternator is on its way out. My truck i had was same way and had an amp test on it and at 2000rpm it was only pushing 20 amps. I had the whole dimming light problem

Do you mean with your stereo banging? If so a capacitor would do wonders.

I have a video on my phone, but the dimming is hard to see. I'll upload it to photobucket when I get home.

It does it regardless of the stereo being off, or at max volume.

It's very rhythmic when it happens, but I noticed last night that it comes and goes. When I first started the car, it was really bad. After about twenty seconds, it stopped for about half a minute and then resumed again..

Could the alternator be producing low amperage, but still have good voltage? As I mentioned above, I measured unloaded idle voltage at 14.1, under load at idle it only dropped to 13.8

Do you have the amp set for a 4 or 2 ohm load? The standard amps for this amp is 60a, that is a definite draw from alternator. Then factor in what else is being drawn from the alternator i.e: radio, lights, fuel pump and others. You might want to invest in a second battery or a bigger alternator. If the load is at 2 ohm expect more amps drawn than a 4ohm load.

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The amp is set for a 4 ohm load on each channel for my interior speakers. I used crutchfield's amp draw formula to determine that the amp's maximum draw is just under 20A (19.3 to be precise) at maximum volume.

Taken directly from Crutchfield:

To calculate the current draw of an amplifier, multiply the number of channels by the RMS watts per channel (a 2 channel amp rated at 300 watts RMS per channel would be 600 watts). Double it to account for amplifier inefficiency (600 watts X 2 = 1200 watts), then divide by the average output Voltage of an alternator, 13.8 volts (1200 divided by 13.8 = 87 amps). Since the average music signal requires about 1/3rd of the average power in a test tone, divide by 3 (87 amps divided by 3 = 29 amps). The result is the amplifier's approximate average current draw while playing music at top volume.

Kk 4 ohm is way different than 2 ohm, this is why I asked. 4ohm will generate about half of needed power as to 2ohm drawing max power. I was better hoping for the 2ohm load than the 4ohm. I'll dig a bit more. Sorry can't help right away.

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All the same get a capacitor or two, they will help if not take the problem away

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