I have a 2009 suburban LTZ that has a couple issues.
I have the usual problems but the headlight issue is about to drive me mad.

My headlights flicker a great deal when on low beam. On high beam I don't notice it flickering.

I've so far replaced the battery, the high amperage cables (all of them) with stock new chevy parts. Replaced and then had the original alternator rebuilt by a local source I trust. Rebuilt original is back in and running. Most of this was done because I also have the "something discharges the battery completely for no reason" problem as well.
I've watched live voltage readings from the system when it's flickering and its' running between 14.10 to 14.26 volts. And the change is slower than the harsh flicker in the headlights.
I have checked the ground points and they are clean and look good. It's both headlights with the issue and I'm pretty sure it's not one of the connectors on the back of the headlight assemblies.

Today i'm trying swapping the low beam headlight relay for one used by the read defroster. it's not used much and is the exact same relay as the headlight relay. I will have to night test this to see if it made any difference.

I do see the drop in voltage light change when the electric cooling fans switch on at idle. I do not however see that when running at normal speeds.

The light flicker is so bad at highway speeds it will make you think you are driving intoxicated. very heavy pulsing of the lights on low beam. Again it is not noticeable when on high beam. Both lights in each headlight operate when on high beam as they should.

I'm frustrated by this as I have been trying to figure it out for over 2 months now. I don't want to take it to the dealer and have them start with replacing everything I have already replaced.

Any ideas?

thanks

-bill

 

when they flicker are they dimming or are they just turning off totally?

 

with a fast cycle speed. very distracting to watch.

Saturday I pulled apart the lights and checked the main connectors and light connectors. no sign of heat or connection issues.

Replaced all the bulbs, but have not been able to take the car out late enough to do a full dark check of the system to see what it does now.

I can say the H11's in the low beams on both sides looked horrible. Black burnt spot on one side of the bulb, the filament not really looking like a good wound coil (looked half melted) and the glass was kinda broke up on the tip of one of them.

this may be no more than the bulbs had gotten really bad but not blown out.

Not sure.

 

I had a similar issue on my denali, I found the GM grounds not sufficient enough so I routed an extra ground to the front.

 

can you get a video of it?

 

This may sound crazy but you asked and didn't mention it in your description of the problem. My lights will flicker or dim about every 6-8 months while I'm slowing down to stop or sometimes while tooling down the road. I noticed that when this was happening the voltage gauge was always reading above 14vdc. An old mechanic friend of mine suggested cleaning the battery terminals(I said this would sound crazy). Sure enough it goes away. I have told several friends with Chevy trucks and all will agree that this works.

 

I want to update everyone after fighting with this that I believe it is whipped and the problem is not unknown if you have other kinds of GM autos.

I updated the ground to 12 gauge soldered connections run from just after the main connections to the light assembly direct back to the battery ground. That did not fix it.

My last try was to go get a fresh relay from Napa auto parts. That did fix it. I would say if you have the problem with your lights even if they are not set up like other autos with the DRL feature running the low beams, replace the relay. They are too light i think for the service they get under the low beam headlights. I didn't rip apart the original relay (haven't had time) but I'm willing to bet I find badly burnt contact. Oddly enough I think I need to replace all the relays of that type in the fuse panel but at $18 a piece that will be costly. I tried one of the relays that had been in service for another thing from that panel without fixing the problem. I will probably source them from Amazon or some other supplier where I can get them at a better price.

Hope this helps someone that is fighting this kind of problem.

 

Thanks for the update! Do you have a part number for the new relay or any specs on it?

 

On my 2005 Trailblazer the Alternator made the lights flicker because the Alternators Regulator was set to high. This was causing the PCM to shut down what is saw as over charge. The regulator should be limiting the voltage to a maximum of 14.5V, this was not happening. The PCM was then taking over by turning off the alternator momentarily to prevent this over voltage level to the battery. If the Alternator Regulator was set properly the PCM would only be a backup protection, as it was intended. I had the PCM doing the regulation ( very poorly) and the regulator in a backup position, backwards from what it should be.

I tried getting an alternator with a lower setting, no luck. So I add a 100A rectifier between the alternator thus lowering the regulated Voltage .8 volts. I used a bridge rectifier and wired with diodes facing both ways. You need a .5 ohm resistor or a diode invert across rectifier to excite the field coil to start charging. This will return the regulation back to the alternator and not the PCM.

Looking at posts, this is likely a large problem with GM vehicles that use the PCM controlled alternators.

 

I know this is an older thread, but if you're still around I'd love to know a little more about your fix. I'm having the same issue. Aside from my headlights flickering, I get surging from my little air compressor when it's plugged into the auxiliary power. That sounds like nothing else but alternator.