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

[tom3]

Going by the user name it's a 2007? Did this have the control module controlling the output? Might get into a real can of worms with any mods if so. I agree with the other posters though, keep the tow vehicle's power supply as designed.

 

[gooffeyguy]

Going by user name that's what I thought also, but it's posted in the 2000-2006 forum, so good question

 

[Joseph Garcia]

Question to the OP....

Is the voltage at idle ALWAYS at ~12.5 volts? Or does it periodically fluctuate at idle between ~12.5 volts and ~14.5 volts?

 

[2007YukonXL]

Hi All,

THANKS for the quick responses and helpful advice.

Yes it is a 2007 (did not realize I posted on the -2006 thread).

The Renogy is the 50 Amp unit and I do want the heavy cables to charge the LifePo battery because it would be nice in the morning after boondocking all night to just start the truck and have it charge the camper...right now I have to disconnect the trailer and turn the truck around and use jumper cables.

The new 4 AWG cable is protected by a 65 amp fuse at the starter battery and also at the trailer end. The Renogy unit is a 50 amp unit so all it will draw is 50 amps through the new cable.

>Is the voltage at idle ALWAYS at ~12.5 volts? Or does it periodically fluctuate at idle between ~12.5 volts and ~14.5 volts?

At idle it is really close to 12.5 volts but does not go up to 14 volts even with the trailer tow mode on. It used to go up to 14-ish volts with the tow mode on and the trailer plugged in however now it does not with trailer connected or not.

NOTE: I just installed the new cable and the Renogy and was going to test it by first running the truck with the trailer disconnected and I was going to connect the trailer heavy plug and listen to hear if the truck engine lugged down...however when I started the truck I noticed that the volt meter was way lower than normal.

I have a second trailer with an onboard battery that does not have the fancy Renogy or LifePo so I will plug in that trailer to see what it is doing and report back.

Regardless I think I will go with the high output AC Delco alternator.

 

[Joseph Garcia]

Hi All,

THANKS for the quick responses and helpful advice.

Yes it is a 2007 (did not realize I posted on the -2006 thread).

The Renogy is the 50 Amp unit and I do want the heavy cables to charge the LifePo battery because it would be nice in the morning after boondocking all night to just start the truck and have it charge the camper...right now I have to disconnect the trailer and turn the truck around and use jumper cables.

The new 4 AWG cable is protected by a 65 amp fuse at the starter battery and also at the trailer end. The Renogy unit is a 50 amp unit so all it will draw is 50 amps through the new cable.

>Is the voltage at idle ALWAYS at ~12.5 volts? Or does it periodically fluctuate at idle between ~12.5 volts and ~14.5 volts?

At idle it is really close to 12.5 volts but does not go up to 14 volts even with the trailer tow mode on. It used to go up to 14-ish volts with the tow mode on and the trailer plugged in however now it does not with trailer connected or not.

NOTE: I just installed the new cable and the Renogy and was going to test it by first running the truck with the trailer disconnected and I was going to connect the trailer heavy plug and listen to hear if the truck engine lugged down...however when I started the truck I noticed that the volt meter was way lower than normal.

I have a second trailer with an onboard battery that does not have the fancy Renogy or LifePo so I will plug in that trailer to see what it is doing and report back.

Regardless I think I will go with the high output AC Delco alternator.

I'm thinking that you have an issue with the voltage regulator, which I believe it inside the alternator.

 

[swathdiver]

Hi All,

THANKS for the quick responses and helpful advice.

Yes it is a 2007 (did not realize I posted on the -2006 thread).

Ok, your truck is equipped with RVC and EPM, Regulated Voltage Control and Electronic Power Management. The computer controls the output of the alternator based upon set parameters which can be found in the Upfitter's Manuals. The file is too large for me to attach here.

Under the hood, not connected from the factory, is a power wire taped up that once connected to your fuse box will provide 12v power through your 7-pin connector to power the trailer and trailer battery. If you leave the 7-pin connected while not driving, it will draw down the battery.

There is the TP2 option for adding a second battery isolated from the truck to power the hotel services on the trailer and keep its battery charged up.

 

[2007YukonXL]

Swathdiver good info...THANKS.

That is definitely the smarter way of saying that the truck has a "Smart Alternator that varies the output based on conditions to increase fuel economy" like the advertisements did back in the day.

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 so it will therefore produce much less output at low engine/alternator speeds.

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 the alternator will 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.

Since the alternator pulley is smaller it will spin faster at idle and that means that the modification will need a smaller serpentine belt.

The existing belt is a 6K935 and that is 93.42 inches long

More Information for ACDELCO 6K935

I ordered a 6K923 belt that is 92.34 inches long so hopefully that will work

More Information for ACDELCO 6K923

I will report back.

THANKS AGAIN!!!

 

[houstontaylor]

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?

Based upon my experience with another car type, the alternator is bad. On that other car it was putting out about 13 volts instead of the usual approx. 14.5 volts at idle. The parts store checked it and said it was good, maybe because it was run at higher speed in their test or they didn't know it could be weak and still not work as it should. A fully charged battery should be about 12.7 volts and your bad alternator will not charge to that level. Even on my friend's car the 13 volt output would only likely have charged her battery very slowly. Replace it!

 

[JamesLond]

If you look at a RPM on a scantool, does it match what you're seeing on the tach?

 

[Z15]

fyi

Voltmeter Fluctuation Condition

Some customers may comment that the voltmeter is fluctuating between 12 and 14 volts on their full size pickup or utility vehicle. Starting with the 2005 model year, light duty full size pickups and utilities are equipped with a new Regulated Voltage Control (RVC) system. This system reduces the targeted output of the generator to 12.6-13.1 volts when in "Fuel Economy Mode" to improve fuel economy. The generator may exit "Fuel Economy Mode" if additional voltage is required. This will cause the voltmeter to fluctuate between 12 and 14 volts as opposed to non-regulated systems that usually maintain a more consistent reading of 14 volts. This fluctuation with the RVC system is normal system operation and NO repairs should be attempted.