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Old 05-24-2022, 10:29 PM   #1
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This is Alternator Will Not Charge During Idle But Charges At Higher Engine Speed?!?!?

Hey there...this is strange...

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. Pushing the Tow switch does not help.

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 go to 13.5 then 14.0-ish and stay there...so the alternator is charging but not at idle and this is a new "feature"

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.

The no charging issue occurs when the trailer is disconnected so that is not the problem.

I did install 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 so I am not able to test it at this point.

I have found this alternator online...any experience/advice?

https://www.powerbastards.com/prodde...02-250-HD28-63


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

THANKS for the help!!!
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Old 05-25-2022, 08:21 AM   #2
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Originally Posted by Vibe101 View Post
Hey there...this is strange...

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. Pushing the Tow switch does not help.

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 go to 13.5 then 14.0-ish and stay there...so the alternator is charging but not at idle and this is a new "feature"

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.

The no charging issue occurs when the trailer is disconnected so that is not the problem.

I did install 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 so I am not able to test it at this point.

I have found this alternator online...any experience/advice?

https://www.powerbastards.com/prodde...02-250-HD28-63


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

THANKS for the help!!!


What year is your vehicle?

it may well just be a bad alternator. Before spending money on a new Alternator though it would be a good idea to have it tested. Many Auto Parts stores can test it if you remove it and take it in. Usually no charge.

Depending on year, it may be a "Smart Alternator" and they do reduce output when the starting battery is fully charger (~12.6- 12.7 volts).

If it is a Smart Alternator you'll probably have to run two wires from trailer to starting battery, connecting to both pos and neg terminals. Using chassis ground for the negative return can interfere with alternator voltage controls with the smart alternator.
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Old 05-25-2022, 08:31 AM   #3
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Is this a new issue or do you believe this has been going on for some time?

As Mike said, it may be a bad alternator but you've added some things to the charging system side of the truck so it would pay to do some thorough testing before just throwing parts at it.
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Old 05-25-2022, 09:00 AM   #4
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not normal for smart alternator either. The GM smart alternator is bypassed when in tow mode which he said he tried.

I would suspect a failing alternator... But thats a guess without seeing the vehicle
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Old 05-25-2022, 10:35 AM   #5
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Wow...always amazed at how knowledgeable/ responsive/ helpful people are here on this forum...THANKS!!!

So it is a 2007 GMC and it does have the smart alternator.

I waited until it got dark and ran the engine with the trailer disconnected with AC on and headlights on and I could see the interior dome light do what would not be described as a "flicker" but a slowly changing of brightness at idle and the trailer tow mode had no effect. When I raised the idle up to 2000 RPM I could certainly see the headlights get brighter so this confirms what the volt meter was saying.

>Is this a new issue or do you believe this has been going on for some time?

It has got to be new because I have always seen the volt meter on the instrument panel about 13 volts when in normal mode and 14 volts when trailer mode is on.

I have not really added anything to the charging system just a cable from the battery positive terminal through a fuse to the back of the truck so none of this stuff when disconnected would affect the charging system.

I will take the alternator in to get it checked...

What about the high output alternator...does anyone have any recommendations for a good source?

It would be good to have one that produces more output because I added the Renogy and the LifePo.

I did replace the alternator about two years ago because it was doing a fast flicker at idle.
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Old 05-25-2022, 11:11 AM   #6
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Quote:
Originally Posted by Vibe101 View Post
Wow...always amazed at how knowledgeable/ responsive/ helpful people are here on this forum...THANKS!!!

So it is a 2007 GMC and it does have the smart alternator.

I waited until it got dark and ran the engine with the trailer disconnected with AC on and headlights on and I could see the interior dome light do what would not be described as a "flicker" but a slowly changing of brightness at idle and the trailer tow mode had no effect. When I raised the idle up to 2000 RPM I could certainly see the headlights get brighter so this confirms what the volt meter was saying.

>Is this a new issue or do you believe this has been going on for some time?

It has got to be new because I have always seen the volt meter on the instrument panel about 13 volts when in normal mode and 14 volts when trailer mode is on.

I have not really added anything to the charging system just a cable from the battery positive terminal through a fuse to the back of the truck so none of this stuff when disconnected would affect the charging system.

I will take the alternator in to get it checked...

What about the high output alternator...does anyone have any recommendations for a good source?

It would be good to have one that produces more output because I added the Renogy and the LifePo.

I did replace the alternator about two years ago because it was doing a fast flicker at idle.

Be careful with buying a "high output" alternator, many produce more at high rpm but even less than stock at idle.

I'd say look and see if there is a police package alternator you could install. I did that on my 2003 Ram years ago, put a police package alternator out of a related vehicle with the same engine.

In your case look at what is available from the Chevy Tahoe PPV.
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Old 05-25-2022, 11:21 AM   #7
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unless funds are tight, get a new one and not a remanufactured one. from personal experience you will only have headaches with anything but new. look for one from a police or ambulance package too.
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Old 05-25-2022, 11:23 AM   #8
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Originally Posted by Vibe101 View Post
Wow...always amazed at how knowledgeable/ responsive/ helpful people are here on this forum...THANKS!!!



So it is a 2007 GMC and it does have the smart alternator.



I waited until it got dark and ran the engine with the trailer disconnected with AC on and headlights on and I could see the interior dome light do what would not be described as a "flicker" but a slowly changing of brightness at idle and the trailer tow mode had no effect. When I raised the idle up to 2000 RPM I could certainly see the headlights get brighter so this confirms what the volt meter was saying.



>Is this a new issue or do you believe this has been going on for some time?



It has got to be new because I have always seen the volt meter on the instrument panel about 13 volts when in normal mode and 14 volts when trailer mode is on.



I have not really added anything to the charging system just a cable from the battery positive terminal through a fuse to the back of the truck so none of this stuff when disconnected would affect the charging system.



I will take the alternator in to get it checked...



What about the high output alternator...does anyone have any recommendations for a good source?



It would be good to have one that produces more output because I added the Renogy and the LifePo.



I did replace the alternator about two years ago because it was doing a fast flicker at idle.
Don't overlook the battery as a potential cause. How old is it and have you recently had it tested?

As for larger aftermarket alternators, I'd first determine the exact problem. Fixing it may be all that's necessary.

Size-wise the stock alternator should handle even the extra load of a DC-DC charger. My 2004 truck with only a 130 amp alternator handles lights, heater/Air Conditioning, and an extra 30 amps feeding my Renogy DC-DC charger.

Many "upgraded) alternators require some modifications like new pulleys, larger wire kits, and different drive belts.

I like to stay "stock" as much as possible so I don't find myself stranded with no off the shelf parts readilly available.
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Old 05-25-2022, 03:44 PM   #9
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FYI: "IsleDog"'s comment on buying new instead of reman- -I worked at an alternator plant for 20 years. We ran alternators only for new car factory installation. Unless we got an order for "reman" alternators. We did not take in used alternators to rebuild we just ran the same alternators down the line as we did every day. New car application alternators were placed in layer in a large metal wire basket to be shipped to various assembly plants. The so called "reman" alternators were placed into individual cardboard boxes with REMAN clearly printed on them. There were absolutely no differences between either alternator as manufactured by us. Strange world ain't it, , ,
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Old 05-25-2022, 04:22 PM   #10
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Quote:
Originally Posted by Vibe101 View Post
Hey there...this is strange...

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. Pushing the Tow switch does not help.

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 go to 13.5 then 14.0-ish and stay there...so the alternator is charging but not at idle and this is a new "feature"

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.

The no charging issue occurs when the trailer is disconnected so that is not the problem.

I did install 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 so I am not able to test it at this point.

I have found this alternator online...any experience/advice?

https://www.powerbastards.com/prodde...02-250-HD28-63


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

THANKS for the help!!!
I am thinking that your voltage regulator is slowly dying....
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Old 05-25-2022, 04:32 PM   #11
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That is almost always caused by the alternator belt being loose. The high current draw at the low engine speed (idle) causes the alternator to slip on the belt but the higher engine speed compensates somewhat.

With the engine not running, grab the fan on the front of the alternator and try to turn it by hand. If you can turn it at all the belt is loose.

I saved a coworker a bunch of money when the dealer told him his alternator was bad. After they would have replaced the alternator the belt would be tight so he wouldn't know the difference.

Ray
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Old 05-25-2022, 04:54 PM   #12
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My prediction: Have the unit tested, but it has the hallmark of a bad diode in the bridge - especially the flicker. If one of the usual 6 shorts out, idle voltage drops significantly.
You can use a simple VOM to look for AC voltage at the battery. The following link is fairly complete on alternator diagnostic problems.
https://axleaddict.com/auto-repair/A...g-System-Tests
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Old 05-25-2022, 05:16 PM   #13
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That is almost always caused by the alternator belt being loose. The high current draw at the low engine speed (idle) causes the alternator to slip on the belt but the higher engine speed compensates somewhat.

With the engine not running, grab the fan on the front of the alternator and try to turn it by hand. If you can turn it at all the belt is loose.

I saved a coworker a bunch of money when the dealer told him his alternator was bad. After they would have replaced the alternator the belt would be tight so he wouldn't know the difference.

Ray
Agree that loose belts can cause this issue but with the advent of the serpentine belt it's less of an issue. A serpentine belt drives other essential accessories like water pump, power steering, and A/C compressor. It's well tensioned by a spring loaded pulley and if it loosens it's tension all those accessories will be affected, often with alarming noise.
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Old 05-25-2022, 06:45 PM   #14
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It is not the belt being loose. The serpentine belt is new-ish and no squeel. No fan because the truck has electric fans.

So here is the deal...the volt meter does indeed show low-ish voltage at idle (AC on/ lights on/ etc) however when the engine speed is increased to about 2000 RPM the volts go up to 14-ish as expected.

Here is the interesting part...

When I plug in the Anderson connectors of the 4 AWG cable that run back to the trailer and the engine is at idle there is no current flowing to the LifePo battery through the Renogy 50 Amp charge controller...HOWEVER...when I increase the engine speed to about 2000 RPM the moment the truck volt meter shows 14-ish volts I can hear the engine lug down as the Renogy calls for power so at least I know that part is working.

So I now know the Renogy system is working and the truck alternator is working but is weak at idle so I agree that perhaps a diode(s) burned out and it is now working on less than 12 diodes as evident by the flickering.

Since I want to be at the boondocking camp site and just remote start the truck in the morning and have it idle and charge the LifePo/ and/or run the trailer fridge on AC, I think the best option is to get a "High Output" alternator with the smaller diameter pulley as used on ice-cream trucks and ambulance type vehicles.

I dont want to go with a dual alternator at this point but I think this alternator will do:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

They claim it should put out 150 Amps at idle and 300 Amps at cruise. I do not have anywhere near 300 amps of stuff on the truck and the trailer combined and even if I tried to crank up the trailer AC the Renogy will limit the amount of current flow to the trailer to 50 Amps so I hope it can put out a sufficient amount of current at idle and way more than what could be drawn at cruise. They say it has better fans and upgraded diodes and brushes so if it was designed for 300 Amps then hopefully my 150 AMP max will make for a longer life.

I will report back.

THANKS AGAIN!!!
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Old 05-25-2022, 07:03 PM   #15
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If you want a high output alternator you might checkout mechman.com they have 6 phase high output units. It doesn’t keep you from using a stock replacement if you are stranded somewhere. Second is the regulator is part of one of three modules, ECM,BCM, or PCM. At some point they started using a ground cable induction loop to sense loads before the voltage drop occurs, if you have one anything you add must have the ground run through that loop. Hope this confusion helps.
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Old 05-25-2022, 08:35 PM   #16
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My former department purchased Sonomas for patrol vehicles, they idle most of their lives. The manual for these clearly state that the alternator doesn't provide adequate battery charging at idle. We had to order auxiliary battery kits and place a second battery in the bed for them to survive all the idling and low speed patrols. Check your manual first. You do NOT have to remove the alternator to have it tested. Modern testing devices test the alternator at idle and 2000 rpm while still in the vehicle. It will check your battery first and then have you start the vehicle to test the alternator. Best of all, it's free (I know, it's my retirement gig). Please, get a stock alternator, I see all sorts of issues when people upgrade part of a computer controlled vehicle without telling it!
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Old 06-09-2022, 07:43 PM   #17
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Originally Posted by Vibe101 View Post
Hey there...this is strange...

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. Pushing the Tow switch does not help.

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 go to 13.5 then 14.0-ish and stay there...so the alternator is charging but not at idle and this is a new "feature"

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.

The no charging issue occurs when the trailer is disconnected so that is not the problem.

I did install 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 so I am not able to test it at this point.

I have found this alternator online...any experience/advice?

https://www.powerbastards.com/prodde...02-250-HD28-63


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

THANKS for the help!!!
Go to your dealer and have it all checked out! You might have a problem they can fix, but often it is as it is and it will only charge when travelling.
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Old 07-01-2022, 03:12 PM   #18
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Finally got a chance to post back.

It was the alternator that was bad.

I installed the new 300 AMP alternator with the smaller drive pulley and it works great...produces lots of power at idle. Left the camp with a 80 percent battery and it charged it back up to 100 percent during the drive. I will see if it recharges the batteries at idle and report back.

So if someone is looking to recharge their LifePo4 batteries during a drive then I can confirm that the Renogy + high output alternator + welding cables run from the TV battery to the trailer works good.
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Old 07-01-2022, 07:57 PM   #19
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Originally Posted by Vibe101 View Post
Finally got a chance to post back.



It was the alternator that was bad.



I installed the new 300 AMP alternator with the smaller drive pulley and it works great...produces lots of power at idle. Left the camp with a 80 percent battery and it charged it back up to 100 percent during the drive. I will see if it recharges the batteries at idle and report back.



So if someone is looking to recharge their LifePo4 batteries during a drive then I can confirm that the Renogy + high output alternator + welding cables run from the TV battery to the trailer works good.
Great!
Thanks for letting us know.
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