Alternator Light on

[tmw188]

Lately but not constant I get a Alt indicator light on my dash, it usually always goes off. My ScanGaugeD doesn’t display any abnormal numbers during this time unless ~14.3 is too high- I don’t think so. It’s never too low and the belt is fine. Unless the regulator is saying it’s putting out a more than normal charge at that time?

[96 EVO]

I wouldn't think 14.3v would be too high.

I often see 14.0-14.1v with the engine running.

[Rocketman3]

I had that happen a couple of times. 
 

in my case, I had installed an 800w solar array, I had been driving for a while - so all batteries were full. And I figure the battery maintainer gave enough power to run the engine - so the alternator decided it was no longer needed. 
The alternator turned off and the light went on. In my rig the RPM is tied to the alternator- so my RPM’s went to zero - while the engine was running nicely. 
 

After researching it- I just turn on my headlights, so my alternator feels needed and  the alt fail light goes off - my RPM needle returns. No issues. 
 

Not saying that’s your issue…
 

[Michael Powell]

Mine does that but when I turn the headlights on it goes off.. Monaco had a bulletin on installing a relay in the rear electrical box never did it to mine. just when it comes on I pull the headlight switch out and it goes off. Chech your voltage with a good volt meter make sure you don't have a problem...

[tmw188]

Good hear it’s somewhat common, although I haven’t experienced it until recently. 

[Rikadoo]

So with Alternators that have “factory” regulators installed the light has two reasons to operate. The first is when it is not outputting, the other is when the output voltage exceeds the limits of the voltage regulaor setting. Its best to check the voltage in two places 1st directly on the Alteernator with the coach NOT plugged in, 2nd on the coach batteries. You are looking to see similar voltage or at least within 1/2 volt. Next Leese Nevel alternators do have a adjustment point where you can lower or raise the output, (procedure we had to do with ambulances) it requires a plastic screwdriver and a voltmeter. I believe you can go on line to the company and it has procedures on how to compleate the operation. Either way if it was me i would perform the voltage check without turning anything on, measure your voltage rpm over time to ensure voltage does NOT continue to climb, as over voltage can ruin voltage sensitive electronics! Connect your voltmeter (i perfer to use two meters, one on the alternator the other on the batteries) start the engine, then hit cruise “set” button which should increase the rpm to like 1300, now just let it run for 10 minutes, watch your voltage. It should not continue to climb past 14. Ish volts, lets say it starts getting to 15 and continues to climb during that time you have a regulator issue that needs to be  addressed. If it does not exceed the 14 ish volts and remains fixed then click on your headlights an see what the voltage drops to. Cause though the headlights may turn off the Alt light, if the voltage remains too high 14.9 and above you may start to notice premature bulb burn out. Most charging systems use a sweet spot ranging from 13.8 to 14.5 volts, and regulators are  temperature sensitive in that as they get to operating temps usually the output will drop slightly to reduce battery gassing, and or battery damage.  Suffice it to say overcharging of any amount can be $$$ from bulbs, batteries, engine / transmission computers and everything in between. I can go on an on about the reasons why its important to test and know the reasons why your indicator light is coming on, just know its a tatteltale, something IS wrong in how its  perimeters were initially set and being broke down somewhere in nowhere USA is not fun, plus expensive.

[Steven P]

Mine will do this at times.  I've noticed it's more common when I first run the genny so the Magnum is in bulk charging at this time. Also, in cold weather my house batteries charge at a higher rate.  I have an Amp L Start connected and feel that is why what the house batteries are doing is relevant. 

[Tom Cherry]

There were some other topics and posts as well as comments and theories on this and also some “revisions” in the electronics and wiring to eliminate the “Uh-Oh” false positive light.

@Frank McElroy explained that to me…he might chime in….as I would probably misspeak and he would have to edit.  LOL!

One thing that I know for sure, the later models don’t show the FALSE POSITIVE light.  Also comments about high voltages are relevant, but one needs to also consider that if one chooses to “equalize” (as Magnum suggests and provides a means to do…BUT..TROJAN says WE DO NOT RECOMMEND EQUALIZATION), that you do subject the entire HOUSE SYSTEM to much higher voltages.  I THINK that my BIRD/BIG BOY charging system might stay connected…or not?  So, the Chassis would be charging at a higher voltage.

The one thing I can contribute with 100% certainty…..if you have a full fledged BIRD SYSTEM, you can often see higher voltages on the Chassis system.  If you leave a campground or start to drive with a partially deleted House Bank…..which is normal if your slides are hydraulic and House Bank powered, then the BIRD will supply Chassis voltage (13.8/14.0 range) to charge both banks.  Now, if it is hot, I turn on the Genny and run the roof top AC’s.  BINGO, once the Magnum has AC, IT TAKES OVER….the Chassis Voltage jumps up into the mid 14.X range.  This is usually about 0.5 VDC HIGHER than the Chassis. I then know, by watching the Chassis voltmeter, when the house bank reaches FLOAT.  So that is my “quick” check that the BIRD/BIG BOY is working and engaged.  I have had my wife read the Magnum remote and the voltages that I see on the dash voltmeter are the same.

Also bear in mind, if you read the Magnum remote meter and then quickly read your VOM on the House Bank….the VOM will be 0.15/0.20 VDC higher due to line loss.  YES….MAGNUM says this is normal…LIVE WITH IT…

BUT I have never, in 65K miles, got “the light”….

[Frank McElroy]

On newer coaches, a signal for the alt light on the dash comes from the engine ECM along the databus to the dash cluster.  As long as the engine ECM is seeing that the batteries are being charged, either by the engine alternator OR the generator through the Magnum inverter/charger through the big boy to the chassis batteries, the alt light will stay off.

[tmw188]

The light just kinda dies it’s own thing but eventually for off, I’m not going to be concerned about it seems somewhat common. I hav a ScanGauge that I monitor. Thanks everyone for the input.

[MyronTruex]

Toms's's explanation is excellent.

New batteries most likely have a higher resistance and charge up quickly so your alternator does not have to work so hard and the lack of (load) allows that voltage to creep up.

As mentioned, this is most likely happening when your generator or shoreline, or solar art outputting some. Simply turning on the headlights often soothes the beast.