Lights dim when I hit the brake. WHY???

[ronwest]

Lights dim (Gauges & Dome light) when I hit the brake. WHY???

I know it's been asked before but there are a few differences in my case. I am running a 2004 5.7 Hemi engine, with a custom harness from Hot Wire Auto.
I did "The Big 3" Upgrade with 0 gauge wire. I have a Duralast Battery, no stereo. I get 14V at idle and when I press the brake it drops 1-2 volts.

Any ideas on where to start with this?

[Rolling_Thunder]

have you updated any of the wiring in the car itself ?  dash, tail harnesses ?  still using the factory bulkhead connector ?    if so i would start cleaning all the brass terminals -

[myk]

Yup, there's a weak link in that system somewhere, no doubt because of that crappy original 60's wiring.  If you've gone as far as putting a modern HEMI in your car you might as well upgrade (tear out and replace) the entire electrical system too...

[elacruze]

As said, you have a poor connection somewhere, likely in the bulkhead connector. Simply stated, your brake lights put the poor connection past its ability to carry current so the voltage to everything else using that circuit has less to work with.

I found out about the 'vinegar and salt bath' cleaning method for connections on this list, and man I've taken that to the bank a few times already. Dip it, brush it, Q-Tip it, toothpick it-then rinse, grease, and go like new.

[b5blue]

Your welcome, it works (and tastes) better with "Balsamic Vinegar" that is 6% or better. 

[elacruze]

Your welcome, it works (and tastes) better with "Balsamic Vinegar" that is 6% or better. 

Props, that info was gold. Next up is the Molasses dip for rust on iron...I think I'll pour some into the rusting spot on my pickup floor and see what happens...

[b5blue]

Lemon, tomato or grapefruit on rust.    (Old motor oil will stop rust in its tracks, you just need to reapply it as it comes off) 

[myk]

As said, you have a poor connection somewhere, likely in the bulkhead connector.

The OP and the rest of us should just bypass that damned thing...

[2Gunz]

Lights dim (Gauges & Dome light) when I hit the brake. WHY???

I know it's been asked before but there are a few differences in my case. I am running a 2004 5.7 Hemi engine, with a custom harness from Hot Wire Auto.
I did "The Big 3" Upgrade with 0 gauge wire. I have a Duralast Battery, no stereo. I get 14V at idle and when I press the brake it drops 1-2 volts.

Any ideas on where to start with this?

Where are you metering?

And My vote is with the others...... bulkhead connector.

[TylerCharger69]

He idles at 14 volts  until he presses the brake pedal......then it drops to 1 or 2 volts.....it sounds to me that something is grounding out somewhere in the brake light circuit someplace......Does this happen with the turn signals as well???   That's quite a voltage drop there!!!

[Nacho-RT74]

brake lights sucks lots of power. Never have tested voltage decay but amperes decay with stock alt is HIGH and you can NOTICE IT on ammeter ( when ammeter is still on )... 4 21 watts bulbs is a lot!.


WOOOOPS... I didn't noticed it was for a newer car LOL, soorryyyyyyyyyyyy

[ronwest]

He idles at 14 volts  until he presses the brake pedal......then it drops to 1 or 2 volts.....it sounds to me that something is grounding out somewhere in the brake light circuit someplace......Does this happen with the turn signals as well???   That's quite a voltage drop there!!!

Yes it does happen with the turn signals but not as severely. I only have a few wires actually running through the bulkhead connector that are left. Most of the new wiring has bypassed that old thing. But it won't surprise me if that is the culprit... I will check it out!

Thanks guys!!!

[b5blue]

Check all bulb sockets and bulbs, corrosion would add resistance.

[TylerCharger69]

Okay...since you said not as severely with the turn signals as opposed to the brakes,  The trouble is in the brake light/turn signal circuit somewhere.  When the brakes are applied...of course both sides light up...when the turn signal is applied,  only one side lights up,...thus  power drop isnt as severe...im assuming by maybe half.   Yes  there is a weak, bad, or dirty connection somewhere,  and i DONT think it goes beyond the connector that feeds the rear lighting.   Its all about tracing the culprit  which can be a real pain in the arse.

[ronwest]

When you refer to the "connector that feeds the rear lighting" I assume you mean the bulkhead connector?

[b5blue]

There is a coupler behind the driver side kick panel. Near the door.

[ronwest]

cool, I'll check it out.

THANKS!

[b5blue]

Mine drove me nuts till I found it and repaired it!  Also that rear harness runs around the rear seat on the side, pull the rear seat to be certain it has not chaffed and is grounding.

[ronwest]

I know!! It drives me nuts too!

[Brock Samson]

 They're all like that without some serious updating mods,.. i bumped my idle up for a period a few years ago, to try to help, but what I generally do, is shift into N. at a really long light or some such, it'll bring up the revs a bit and the lights wont dim as much, and freeing up the breathing has a great side effect of improving the sound to a quicker airier breathing.  i have a quarter century of driving my beastie, and it seldom bothers me much, any more... 

[TylerCharger69]

FWIW.....I had a similar issue with the wiring harness going toward the rear.   Between the door and the rear panels there is a skinny chrome trim piece thats about 8 to 10 inches long that runs along the floor.   Theres a screw on the back side that holds it in place.  In replacing that piece, I accidentally pierced that wire  with the screw,  and took a while to track it down......For others who have that issue.....thats a good place to look as it seems like it could be a common mistake

[b5blue]

Installing the Denso 120amp alt. fixes the lazy idle amp problem nicely, I can sit in gear at a light with everything I can turn on running at max. just fine now. 

[ronwest]

I have a brand new Bosch 130, did the "big 3 upgrade" with 0 gauge wire and it still does it.
Also just did the bulkhead clean with salt and vinegar, plus I cleaned the kick panel connects. It seems to do it a bit less, but there is still a minor drop from 14 to 13 volts and a subtle dim on the gauges or dome light when I hit the brakes. Guess I need to trace some more wires?

[b5blue]

I chose the Denso 120 simply for output @ idle, it had the best ability over any others at 60+ amp. It uses a similar to factory wiring using factory regulator 2 wires and an output post that made it a simple add on. I don't know about the Bosh units idle amps? Are you feeding output to factory output wiring, or battery?     

[ronwest]

Mine's all custom because it's the 04 Hemi.
Although my first test was just parked and it had the result I mentioned earlier in this thread... I went on a test drive and it seemed to have fixed the issue when driving.
There is maybe 1/2 a volt drop (at times) when braking. When my fans kick on it really zaps it. (True 3000 CFM) just for a second it goes from 14 to 12.5 and jumps back to 14.
The next test will be with the lights on.