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I don't believe you have an issue that warrants any concern if i'm being honest. I couldn't see the dimming, but I did hear when it took place. What I think is happening there is this: As you're coasting down, the engine cuts fuel and is freewheeling. Once you drop below to a certain speed/RPM, it will switch back to feeding fuel and to the IAC for idle speed control. The PCM is likely cutting the alternator output momentarily at the same time to make the overall transition smoother and to prevent stalling, but this will also result in a drop from charging voltage to battery voltage and then back. Non PCM controlled alternators monitor the voltage and self regulate based on that. PCM controlled alternators have the PCM monitor the voltage and the draw, and can adjust the alternator output as required. It also means they can reduce or cut the output completely in certain states to prevent other issues. If the alternator output is completely cut momentarily, you will immediately drop from charging voltage to battery voltage and then back once the alternator ramps back up. Halogen lighting output and blower speed will change along with this voltage change.

More or less, yes. What matters is where the voltage drop is occurring. If the voltage drop is only at the fuse box itself due to an undersized feed, but the battery is maintaining ~14 volts the entire time, then moving the lights to run directly off the battery will have a positive effect, since they will no longer be subject to the drops affecting the fuse box. However, if the voltage drop is due to the PCM briefly turning off the alternator, or the alternator not yet having caught up to the demand, that drop will be at the battery itself, so it effects everything in the vehicle. Moving the lights directly to the battery wouldn't have an effect there, since they would still be affected by the same drop. In the first video it occurred in conjunction with the AC compressor turning on, which I call normal operation for these cars. My 2000 Regal did that and my parents 97 Lumina always did the same thing. The 2nd video was harder to tell, but it seemingly always happening around the same speed when coasting down makes me think that's when the car is coming out of it's deceleration fuel cut off, and the alternator output is briefly being cut to make the transition back to the IAC smoother. Older vehicles didn't experience this in the same manner, as the alternators weren't controlled by the PCM. Newer vehicles with electronic throttle bodies don't seem to experience this either as the PCM has full control over the throttle itself. I really don't think there is an issue that needs solving. If you want to keep the headlights from showing the effects, you could look at switching to LEDs. The LED chips typically run at less than 12 volts, so they have a voltage regulator to keep things in check. This would prevent them from dimming whenever the vehicle voltage fluctuates slightly. Also, ignore what I was mentioning earlier about the ABS. The ABS kicking in will cause the lights to dim briefly due to the load, but after seeing the driving video, i don't think that's what's happening here.

I don't think that would make a difference here. It's not a headlight specific issue, it's the overall system voltage dropping momentarily and noticeably affecting incandescent/halogen light output and the audible tone from the blower. You can sort of duplicate it by leaving the lights on when you shut the car off. There is a slight but noticeable dim that takes place as the overall voltage drops from ~14 volts to ~12 volts.

Maybe you'll want to look into something like this: https://www.summitracing.com/search/part-type/headlight-conversion-harnesses/product-line/united-pacific-headlight-relay-harness-kits

Okay I will look into it! And are you referring to the post that is on the top of the under hood fuse box? The pics in the forum thats what it looks like. Not the positive battery terminal itself right?

Check how hot the auxillary battery terminal gets...mine would get hot and cause voltage drops until I replaced it...previous owners thought it was just going through alternators. I had one of the zinc ones (and one with the bottom threads stripped so it was had a couple of washers for spacers and what appeared to be a Delco radio post nut as well) and replaced it with brass...common issue on the B bodies too, and it's the same process and part number for replacement. Might not apply on a 2000, though... Replacing the Auxiliary Battery Post | Page 3 | Chevy Impala SS Forum

Please do. That slight voltage sag (dimming/fan speed drop) that took place above when the AC compressor kicked in is 100% normal operation for these cars in my view, and its just a byproduct of how the charging system is programmed to function. It can happen whenever a large electrical load comes on line at low RPM, and It is definitely visible in the headlights when they are on. My 2000 Regal did this all the time, and the factory alternator and the 6 year old battery were still fully functional when I got rid of that car in early 2015.

I'm going to be honest, I don't see an issue there. It cranked just fine, and the dip sounded like the AC compressor kicking on. The slight electrical dip is likely just a product of a slight lag between a large electrical load kicking in and the duty cycle of the alternator being increased adequately to compensate.

The Monte. This was a long time ago and it's putting out less now for sure.

3.5l LZ4 with an F23 in a FWD race car. Port work, WOT-Tech VVT Delete with Big Race Cam Kit. 75mm Throttle Body, Megasquirt MS3X. All credit to Dan @ R&R Motorsports for putting this together and sharing the results.

HERE'S MY CAR AFTER ALL THE VISUAL UPGRADES

1993 3.4 DOHC in an 88 Fiero with a Getrag 282 5-speed. The graph says "Flywheel horsepower" but we set the correction factor to "1.0" which means no correction, so this is at the wheels. The 10ft-lb torque dip corresponds to a bad AFR there; I still have to tune it. Engine details: 1993 3.4 DOHC, balanced and blueprinted... Ok, more details: 91 Beretta iron flywheel (16 lbs), Sealed Power hypereutectic pistons with coated skirts, balanced rotating assembly, .030" overbore, honed with a torque plate, exhaust valve guides tightened up a bit, opened the bottom ring gaps to prevent flutter, gasket matched the lower intake manifold, custom upper intake manifold (shorter runners, direct shot from the TB to the runners), stock throttle body, stock ECM, stock injectors, stock cam timing, 3" cold air intake with K&N cone filter, 2.5" exhaust with Magnaflow cat and Spintech muffler. Pic so you can see the intake setup:

That's the one. It looks like you took care of it with your wiring upgrade though.

And it worked as designed yesterday...