Lights dimming, voltage sinking

[itschuck]

Hey everyone new guy here. First time owning a GMC/Chevy, normally I buy Fords..

I have a 2011 GMC Yukon XL 1500 6.2L Denali

I've noticed the voltage goes down a lot when I'm at a light or if the music/bass is going. Lights dim, dash lights dim, etc. So far I have replaced the battery, replaced the alternator with a higher output (260a), I've replaced the positive wire from the alternator to the battery with a 1/0 copper wire properly crimped, I've verified the main ground does go through the hoop sensor thing, i've tried unplugging that sensor as well and no improvement.

My next step is to add a second battery, but I am wondering if there's anything else I can try or something I am missing? Truck runs great otherwise without any issues at all.

Thanks for the help

 

[B-train]

Does it change if your foot is off the brake? Just wondering if it's trying to activate the ABS pump for some reason. Have you tried pulling the fuses(s) for the radio or amp?

 

[itschuck]

Lights still dim when foot is off the pedal as well as while in park. If I have the ac full blast or if i have music playing I can literally watch the volt meter go down from 14+ to maybe 12 or less. I'm going to put a second battery in tomorrow and see what happens. Its not even a large amp, 1200 watts or so I think. Pulled this same system out of my old truck and it didn't dim lights at all in that (ford f350).

 

[iamdub]

Hey everyone new guy here. First time owning a GMC/Chevy, normally I buy Fords..

I have a 2011 GMC Yukon XL 1500 6.2L Denali

I've noticed the voltage goes down a lot when I'm at a light or if the music/bass is going. Lights dim, dash lights dim, etc. So far I have replaced the battery, replaced the alternator with a higher output (260a), I've replaced the positive wire from the alternator to the battery with a 1/0 copper wire properly crimped, I've verified the main ground does go through the hoop sensor thing, i've tried unplugging that sensor as well and no improvement.

My next step is to add a second battery, but I am wondering if there's anything else I can try or something I am missing? Truck runs great otherwise without any issues at all.

Thanks for the help

Did you upgrade the ground to the same size? A circuit it only as strong as its weakest link. You could use a 1,000A alternator and 4/0 for the charge cable and you'd still only be able to support what that factory 4 gauge ground will carry. I have 1/0 for my charging system- it'll fit through the RVC sensor with room to spare. Also, I have a ~1,000 watt Alpine that, according to dyno videos, should do 1,200-1,300 watts. A new battery (previous one was old and abused) greatly reduced my dimming lights and a 220A alternator showed further improvement. Now, the dimming is barely noticeable when I have the stereo cranked. A/C stays on max at all times here in Louisiana.

Having said all that, I think you have something wrong. I'd first upgrade the ground and verify all the terminations are clean so you can actually make full use of that new battery and 260A alternator.

 

[swathdiver]

Hey everyone new guy here. First time owning a GMC/Chevy, normally I buy Fords..

I have a 2011 GMC Yukon XL 1500 6.2L Denali

I've noticed the voltage goes down a lot when I'm at a light or if the music/bass is going. Lights dim, dash lights dim, etc. So far I have replaced the battery, replaced the alternator with a higher output (260a), I've replaced the positive wire from the alternator to the battery with a 1/0 copper wire properly crimped, I've verified the main ground does go through the hoop sensor thing, i've tried unplugging that sensor as well and no improvement.

My next step is to add a second battery, but I am wondering if there's anything else I can try or something I am missing? Truck runs great otherwise without any issues at all.

Thanks for the help

You mention the radio, is it stock or customized?

With a stock truck none of these modifications are necessary. Alternator output does decrease when the battery has been charged to the satisfaction of the computer system. Look up the terms RVC for Regulated Voltage Control and EPM for Electronic Power Management with reference to GMC trucks.

https://www.gmupfitter.com/wp-content/uploads/2021/05/2011_LD_ElectricalPickupsChassisCabs_100813.pdf

 

[B-train]

Lights still dim when foot is off the pedal as well as while in park. If I have the ac full blast or if i have music playing I can literally watch the volt meter go down from 14+ to maybe 12 or less. I'm going to put a second battery in tomorrow and see what happens. Its not even a large amp, 1200 watts or so I think. Pulled this same system out of my old truck and it didn't dim lights at all in that (ford f350).

Keep in mind that a F350 probably has a higher output alternator due to the fact its built for work. Also, depending on if it was a diesel, you probably had more battery power available.

A second battery is easy to install. I put one in my 2008 yukon and it's nice to have. Just do what @iamdub stated about cable size. Think of cables like hoses......flow is based off size. Electricity is no different.

 

[Joseph Garcia]

In agreement with the above cable size recommendations for grounds. Also, as @iamdub suggested, I also believe that you have some underlying issue, and not the alternator capacity, that is causing the dimming.

 

[mikez71]

corrosion in the factory battery cables? Have heard that mentioned in a few threads here...

 

[j91z28d1]

do you have an amp clamp to clip on the alt lead?

what kinda alternator did you install? I have a feeling it's not pulleyed or not putting out what you need at idle. you probably need at least 120amps at idle.. it might be time for the alternator upgrade iamdub did. newer style 6 pole.

 

[Marky Dissod]

May I suggest something you'll likely ignore?
1. Have your ecm tuned (preferably, your ecm AND your tcm, in person / on site)
1a. Among the many other things that should normally be addressed by a tune to suit your particulars, have your four idle RpMs - Park/Neutral, & In Gear, with AC On, and with AC Off - raised by at least 100 RpM.

The extra RpMs at idle would likely have been enough to make even an OE alternator put out just enough extra juice.

Instead of merely ignoring it, you also misconstrued it.

 

[j91z28d1]

May I suggest something you'll likely ignore?
1. Have your ecm tuned (preferably, your ecm AND your tcm, in person / on site)
1a. Among the many other things that should normally be addressed by a tune to suit your particulars, have your four idle RpMs - Park/Neutral, & In Gear, with AC On, and with AC Off - raised by at least 100 RpM.

The extra RpMs at idle would likely have been enough to make even an OE alternator put out just enough extra juice.

there's a whole alternator section in the tune as well. lots of stuff in there that should over ride the idle settings when voltage falls because of load, some kinda boost mode.

 

[itschuck]

May I suggest something you'll likely ignore?
1. Have your ecm tuned (preferably, your ecm AND your tcm, in person / on site)
1a. Among the many other things that should normally be addressed by a tune to suit your particulars, have your four idle RpMs - Park/Neutral, & In Gear, with AC On, and with AC Off - raised by at least 100 RpM.

The extra RpMs at idle would likely have been enough to make even an OE alternator put out just enough extra juice.

Lol... ok I'm not ignoring you as a person, but likely wont do a custom tune for just this issue. Thanks tho!

 

[itschuck]

do you have an amp clamp to clip on the alt lead?

what kinda alternator did you install? I have a feeling it's not pulleyed or not putting out what you need at idle. you probably need at least 120amps at idle.. it might be time for the alternator upgrade iamdub did. newer style 6 pole.

No I sure don't. Its a tucson alternator unit that I installed.

 

[itschuck]

Did you upgrade the ground to the same size? A circuit it only as strong as its weakest link. You could use a 1,000A alternator and 4/0 for the charge cable and you'd still only be able to support what that factory 4 gauge ground will carry. I have 1/0 for my charging system- it'll fit through the RVC sensor with room to spare. Also, I have a ~1,000 watt Alpine that, according to dyno videos, should do 1,200-1,300 watts. A new battery (previous one was old and abused) greatly reduced my dimming lights and a 220A alternator showed further improvement. Now, the dimming is barely noticeable when I have the stereo cranked. A/C stays on max at all times here in Louisiana.

Having said all that, I think you have something wrong. I'd first upgrade the ground and verify all the terminations are clean so you can actually make full use of that new battery and 260A alternator.

I had not replaced the factory ground. Will do that today. I did go ahead and install a second battery and that has helped a lot. The volt meter on the dash will still dip when for example sitting at a light with the radio on and ac's both running, but the lights are not noticeably dimming anymore.

 

[itschuck]

You mention the radio, is it stock or customized?

With a stock truck none of these modifications are necessary. Alternator output does decrease when the battery has been charged to the satisfaction of the computer system. Look up the terms RVC for Regulated Voltage Control and EPM for Electronic Power Management with reference to GMC trucks.

https://www.gmupfitter.com/wp-content/uploads/2021/05/2011_LD_ElectricalPickupsChassisCabs_100813.pdf

The radio is an android tablet based touchscreen, that feeds a sundown audio audio 1200w amp, that provides a 1 ohm load to a set of dual 12's in the back.

I did go ahead and install a second battery and that has helped a lot. Next im replacing the factory grounds, make sure good surfaces, and match the 1/0 copper from the alternator. The new battery has 1/0 for + and - I think with this I should be completely resolved.

 

[swathdiver]

The radio is an android tablet based touchscreen, that feeds a sundown audio audio 1200w amp, that provides a 1 ohm load to a set of dual 12's in the back.

I did go ahead and install a second battery and that has helped a lot. Next im replacing the factory grounds, make sure good surfaces, and match the 1/0 copper from the alternator. The new battery has 1/0 for + and - I think with this I should be completely resolved.

Remember to replace the batteries in pairs. An old one will drag down the newer one.

 

[j91z28d1]

I found one of my trucks that I added electric fans to, that if you let it drop down to battery voltage at idle. I shorten the life of the battery a lot, like once a year. till I put a much smaller pulley on it and got the voltage up, the battery went back to 5 years ish. of course after that the alt died from it, but found a diesel one rust *** a direct swap. it was a larger case and charged at idle without over speeding. it's still on the truck all these years later.

the 2nd battery will help, but it's best to get it running on the alt at full load. a volt/dc amp clamp will help you know what's going on. a lot of these aftermarket alternators use the same size case/armature and just change diodes to up the current at high rpm, but they aren't a lot higher output at the low idle sleeps.