Why are my headlights dimming when braking? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 11-20-2012, 10:57 PM   #1
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Question Why are my headlights dimming when braking?

Hi everybody. Since this is a completely different issue, I decided it best to make a new thread.

This problem has been plaguing me for a couple years, but Im just now motivated to fix it.

The issue: My headlights dim (at night obviously) when the heater is on and I hit the brakes. I can see it on the road as the light output goes down. Also, my blinkers tend to slow down at stoplights with my foot on the brake. This can be at night and during the day.

What Ive tried:
-cleaned the battery terminal connections
-cleaned alternator connections
-cleaned power wire from battery to the relay strip on the firewall
-cleaned the ground on the passenger fender behind the battery
-cleaned the two grounds on the drivers fender behind the PCM
-cleaned and repaired a crappy connection on the alternator bracket under the coil packs
-cleaned the ground in the trunk on the drivers side wheel well
-inspected rear blinker bulbs, same in both sides (read a heavy duty one on one side would screw up the system)
-inspected all rear light sockets for corrosion, none found, applied dielectric grease to keep them that way
-inspected front bulbs, same bulbs on both sides
-inspected light sockets, all clean and applied dielectric grease to keep them that way
-load tested battery, 650 cca on the label, at 575cca
-load tested alternator, passed
-cut out the ground bus on the drivers floorboard and reconnected it all. It was very corroded and the windows and fuel pump wouldn't work, that'* how I knew to inspect that bastard.

What is next? It'* clear that there is a power drain when the brakes are applied, heater running, and all lights on. What confuses me is the flasher lights in the dash slow up sometimes, but not all times.

Help!!! What am I missing??
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Old 11-20-2012, 10:59 PM   #2
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I've also ohm'ed out the ground cable from the battery to the block, the power cable from the alternator to the battery, and the cable from the battery to the relays on the firewall, and all are great (around .5 ohms).
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Old 11-20-2012, 11:59 PM   #3
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I have the same issue as well, and always attributed it to the alternator not being able to keep up with the load. Usually happens only at idle, like waiting at a stop light. The lights dimming is likely to be directly associated with a voltage drop, which you should be able to see with the volt gauge. I would expect everything to come back up as the rpms pick back, like taking off from a light.

What does the voltage gauge have to say about all of this?

Honestly they didn't use the best of wiring in our cars. Nice little spaghetti factory doesn't always work in our favor :(
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Old 11-21-2012, 08:45 AM   #4
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I didn't see anything mentioned of how old the battery is and if the alternator is still OEM?
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Old 11-21-2012, 08:59 AM   #5
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i think they all do it, danthurs has a write up on adding a sense wire because oem didnt hook that up. the sense wire gives the alt a voltage reading at the load, i would hook it up indside the car in the fuse box over the drivers feet. that would help. also that battery is a little small, a bigger interstate or something would help even out the load
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Old 11-22-2012, 12:21 PM   #6
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Yeah I've hooked up the sense wire in my car (Boost helped way back in the day) and it was one of the best simple mods I ever did. Voltage holds 14+ almost all the time, and is far more consistent. I know the issues mentioned here, they are gone with that.
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Old 12-01-2012, 01:01 PM   #7
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yes, this only happens at idle, like at stoplights or idling down the gravel road as I go home.

The battery and alternator were both load tested and came out good, so the age of the components should not be relevant in this case, correct?

I read up on this sensing wire, and it seems there are more problems doing this then benefits. Am I wrong here?
"With the added voltage you can shorten the life of everything from light bulbs to your battery."

I am not exactly sure how the sensing wire works, so please feel free to educate me if I am way off base here.

As always, thanks everybody for all of the input! I really appreciate it!
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Old 12-01-2012, 05:27 PM   #8
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the sense wire makes it able to react to the loads better, not just have more voltage but have voltage stay right when you have everything on
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Old 12-03-2012, 02:23 AM   #9
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If this is the case, then why didn't the factory add this? Im just trying to figure out why the car doesn't already have it if the factory didn't think this was an issue? And, what are the negatives about doing this.
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Old 12-03-2012, 07:32 AM   #10
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most other gms do have it thats why its an easy thing to do. not sure why they didnt, ease of wiring probably. the factory is all about cost cutting, i think having the mind set gm is going to do the right thing is a mistake
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