Alternator Will Not Charge During Idle But Charges At Higher Engine Speed?!?!?

[2007YukonXL]

Hey there,

I have a Yukon 1500 with a 5.3 and the alternator will not charge when the engine is at low RPM...however it will charge when the engine is at much higher RPM like 2000 and above.

When the engine is idling the voltage at the battery is at 12.45-ish volts and then I turn on the AC to full and turn on the headlights and the voltage slowly drops however if I increase the engine RPM to 2000 and above the volts will do to 13.5 and stay there...

Does anyone know why it would do this? Obviously the belt is on and not slipping, the "Littel" 175A Fuse is intact, and the battery terminals are clean and tight.

I am interested in a higher output unit because I do a bunch of towing (the no charging issue occurs when the trailer is disconnected so that is not the problem) and I really need the tow vehicle to charge the travel trailer battery because in the trailer I installed a "Renogy On Board Battery Charger" device and I also installed the 4 AWG welding cable from the starter battery to the trailer so the truck and trailer are set up and ready for some high juice power transfer but if the alternator will not even charge the truck's electrical system at idle then the trailer will no get any help.

Any adive is greatly appreciated and THANKS!!!

Bill

If I upgraded the alternator would I need to upgrade the alternator cable that runs to the fuse/ battery terminal and the "Littel" fuse?

 

[rockola1971]

Unless you have a base model you likely have the 145a alternator and as far as I can tell that is the largest factory alternator installed in the LT model. The Denali did offer a 160A alternator. You can have yours custom rebuilt or buy a custom model that is much larger amperage output since you arent limited much by how far the rear of the alternator can stick out. Id go ahead and go one gauge lower (bigger wire) for the alternator to battery + wire if you go with a higher output alternator.
But the likely reason your alternator is acting a fool is because either the voltage regulator inside it is getting ready to puke on you or your brushes are worn or the commutator that the brushes go up against are worn or both. Check your battery wire connections first for corrosion.

You have a 4awg wire running from your truck's battery + to the trailer? Thats not a good idea. What happens if your trailer breaks away and that 4awg wire is arc welding itself to the ground of your truck? With a trailer breaking away and causing the truck to get squirrely on you thats least convenient moment to have a truck shut down because the positive wire is shorted to the negative. Pucker factor x 10!

There should be a charging wire built into your trailer wiring harness already and there is a fused output factory under your hood already (unless you have a base model maybe) just for trailer electric brakes and charging circuit for your trailer's house battery.

You know there is a factory spot for another battery position so you can run dual batteries in your yukon, right? Its right under your passenger side hood hinge. The factory style battery tray is like $25-$35. You'll need a battery isolator so your start battery is never depleted because of the 2nd battery. Its a simple addition to any vehicle.

 

[gooffeyguy]

I'd make sure it isn't just the EPM (electrical power management) system doing its thing.

I believe if you press the tow/haul button it should disable this. Not positive though, just something to check.

 

[2007YukonXL]

THANKS for the quick response and the suggestions

>Unless you have a base model you likely have the 145a alternator and as far as I can tell that is the largest factory alternator installed in the LT model.

It is a base model with 4WD and a towing package.

>The Denali did offer a 160A alternator. You can have yours custom rebuilt or buy a custom model that is much larger amperage output since you arent limited much by how far the rear of the alternator can stick out.

I found this aftermarket alternator...any thoughts/ experience?

250A High Output Alternator for GMC Yukon, 2007 - 2012 5.3L V8 (323c.i.) - 8302-250-HD28-63 | PowerBastards.com

300A High Output Alternator for GMC Yukon. USA-made, 100% New, no cores needed. Fast Shipping. 1 Year Warranty. Ideal for car stereo, overlanding, power inverters, racing, law enforcement, snow plow vehicles and more.

www.powerbastards.com

If I go with this new alternator then I suppose I need to


>You have a 4awg wire running from your truck's battery + to the trailer? Thats not a good idea. What happens if your trailer breaks away and that 4awg wire is arc welding itself to the ground of your truck?

Sorry I was not clear. I do have the Anderson SB120-04 connector plugs between the truck and the trailer. They will easily unplug (because they do not have a locking tab) if the trailer departs.

>There should be a charging wire built into your trailer wiring harness already and there is a fused output factory under your hood already (unless you have a base model maybe) just for trailer electric brakes and charging circuit for your trailer's house battery.

I used that wire for years however since I upgraded the travel trailer battery to a LifePo4 it needs a great deal more voltage and charging current so the recommendation is to install the heavy gauge cable to power the trailer battery since the "Renogy On Board Battery Charger" device will draw up to 50 Amps and the stock trailer "Pin7" wire will not be large enough.

Any thoughts on the aftermarket alternator and installation?

 

[rockola1971]

THANKS for the quick response and the suggestions

>Unless you have a base model you likely have the 145a alternator and as far as I can tell that is the largest factory alternator installed in the LT model.

It is a base model with 4WD and a towing package.

>The Denali did offer a 160A alternator. You can have yours custom rebuilt or buy a custom model that is much larger amperage output since you arent limited much by how far the rear of the alternator can stick out.

I found this aftermarket alternator...any thoughts/ experience?

250A High Output Alternator for GMC Yukon, 2007 - 2012 5.3L V8 (323c.i.) - 8302-250-HD28-63 | PowerBastards.com

300A High Output Alternator for GMC Yukon. USA-made, 100% New, no cores needed. Fast Shipping. 1 Year Warranty. Ideal for car stereo, overlanding, power inverters, racing, law enforcement, snow plow vehicles and more.

www.powerbastards.com

If I go with this new alternator then I suppose I need to


>You have a 4awg wire running from your truck's battery + to the trailer? Thats not a good idea. What happens if your trailer breaks away and that 4awg wire is arc welding itself to the ground of your truck?

Sorry I was not clear. I do have the Anderson SB120-04 connector plugs between the truck and the trailer. They will easily unplug (because they do not have a locking tab) if the trailer departs.

>There should be a charging wire built into your trailer wiring harness already and there is a fused output factory under your hood already (unless you have a base model maybe) just for trailer electric brakes and charging circuit for your trailer's house battery.

I used that wire for years however since I upgraded the travel trailer battery to a LifePo4 it needs a great deal more voltage and charging current so the recommendation is to install the heavy gauge cable to power the trailer battery since the "Renogy On Board Battery Charger" device will draw up to 50 Amps and the stock trailer "Pin7" wire will not be large enough.

Any thoughts on the aftermarket alternator and installation?

Assuming the mounting is the same for 2007+ and the same pinout for the plug that goes to the alternator then the alternator you noted will be ok but way overkill. Your renogy battery charger will determine what the charge rate to your LifePo4 batteries so you could put a 1000a alternator in there and the battery charger for your RV house batteries will only draw what the Rengoy charger allows them to draw if it is a smart charger.
You likely have a 145a alternator since you have the tow package. You can check without pullling the alternator. The alternator is marked on top of the rear case half towards the drivers side. It is not stamped but engraved with those stupid dot engravings. You might have to give the top of the alternator a good cleaning to read it. The circled in yellow areas is where you will find the rating.

You can pull the alternator at just about any auto parts store (some will pull it for you) and have them put it on their tester for free and it will rule out everything on your truck and isolate the alternator as good or bad.

My bet. Alternator is bad.

I just replaced mine a little over a week ago on one of my 2003 Tahoes. It would charge sometimes. Then sometimes when I started up the Tahoe it would show battery light on and when I revved it then it would start charging and stay charging about 50% of the time. This went on for 3 or 4 days before I got around to going to get an alternator for it and pulling it off. Reman alternator fixed the problem.

 

[Doubeleive]

you could get a good quality mechman for around the same price 250amp, however they take about 3 to 4 weeks to build, ship and deliver.
that one you linked above does not seem to be the newer design with diodes, this one is designed to put out more amps at idle than stock units

250 Amp high output alternator for GM Truck 05-13

250 Amp high output alternator for GM Suburban Tahoe Silverado Escalade Silverado

www.mechman.com

 

[rockola1971]

you could get a good quality mechman for around the same price 250amp, however they take about 3 to 4 weeks to build, ship and deliver.
that one you linked above does not seem to be the newer design with diodes, this one is designed to put out more amps at idle than stock units

250 Amp high output alternator for GM Truck 05-13

250 Amp high output alternator for GM Suburban Tahoe Silverado Escalade Silverado

www.mechman.com

All alternators have diodes in them. An alternator without diodes is a generator. A generator outputs ac internally but that ac is rectified at the brushes and commutator and this is in old school vehicles. The generators were limited to around 20a and ended in 1963 for GM. Do you mean the "newer design" has more diodes than the standard 3 or 6? Wow I just looked at the specs...that 250A he noted has 12 diodes in it! Thats crazy!

OP needs to find out what his Renogy setup draws typically(plus full charge rate) and size the alternator accordingly. For instance if the Renogy system draws well less than 30a on top of the vehicles draw and he goes with say a 200a alternator then "most" of the time it will do but during times that the Trailer batteries are dead and the Renogy is drawing the full 30a while he is driving at night and all his running lights are on, headlights obviously on, A/C is on max because its hot out and jamming to 2live crew at 3/4+ volume while he is adjusting his power seat then he could be taxing the charge system or just running the alternator at or very near 100% duty cycle for quite awhile. That will kill an alternator soon enough. They run hot enough under normal conditions nested above the monster heat radiating dragon that the LS series engines are.

 

[Doubeleive]

All alternators have diodes in them. An alternator without diodes is a generator. A generator outputs ac internally but that ac is rectified at the brushes and commutator and this is in old school vehicles. The generators were limited to around 20a and ended in 1963 for GM. Do you mean the "newer design" has more diodes than the standard 3 or 6? Wow I just looked at the specs...that 250A he noted has 12 diodes in it! Thats crazy!

OP needs to find out what his Renogy setup draws typically(plus full charge rate) and size the alternator accordingly. For instance if the Renogy system draws well less than 30a on top of the vehicles draw and he goes with say a 200a alternator then "most" of the time it will do but during times that the Trailer batteries are dead and the Renogy is drawing the full 30a while he is driving at night and all his running lights are on, headlights obviously on, A/C is on max because its hot out and jamming to 2live crew at 3/4+ volume while he is adjusting his power seat then he could be taxing the charge system or just running the alternator at or very near 100% duty cycle for quite awhile. That will kill an alternator soon enough. They run hot enough under normal conditions nested above the monster heat radiating dragon that the LS series engines are.

the newer oem alternators also have 12 diodes and have the same X shaped casing, my 2018 alternator has 12 diodes and they can be swapped into anything from 05-20 as long as it is 2 pin, the only catch is the pulley, some of the newer oem alternators have a clutch pulley instead of a standard pulley, they also fit and can swap with 2500 & 3500 gas engines
oem option kw5 is a 220a

More Information for GM GENUINE 84143539

 

[MassHoe04]

There are a ton of affordable solar roof panel options out there that should handle more than your needs for charging the camper system. Especially, if the Renogy is able swap over to charge when plugged into shore power.

A couple of deep cycle marine/RV batteries in the camper with just a couple of regular flat solar roof panels came ordered from Airstream and my sister was able to "boondock" for long weekends with no shore power. They loved that set up! Worked great!!

If they needed more, they could have added more panels, but they went to camps with hookups for trips longer than a 3-day weekend.

That heavy wire going from your starter battery on the tow vehicle is just a disaster waiting to happen.

The added on-board second battery with the isolator is a good idea. But I think that might be better suited for a rig outfitted with a lot of on-board accessories, like: winch, small inverter for overland freezer, light bars/pods, etc.

For your situation...
I think isolating the systems on the camper would be much better than getting tangled up with the Tow vehicle.
The last thing you want is a big ball of fire leaving you stranded in the sticks...

 

[swathdiver]

Hey there,

I have a Yukon 1500 with a 5.3 and the alternator will not charge when the engine is at low RPM

What year Yukon is this?