Is my alternator bad?

[Kiawah]

Then, I would suspect you also have a bad chassis battery as well.....because you've removed the possibility that the bad house battery could draw down your chassis battery. If these batteries are about 5 years old, I would replace them.

Pull your chassis batteries out (heck, pull them all out), haul them to an autoparts store, and have them check them.

[dutchstr3884]

With a diesel engine you don't have spark plugs so once started a diesel engine can run without an alternator or battery as long as fuel is present. With modern diesel engines there are a lot of electronics that control throttle settings, fuel delivery, etc. so disconnecting the battery or turning off the disconnect switch shuts down the electronics which will also cause the fuel to be turned off (failsafe is closed). A couple of years ago while on a trip I started to get erratic readings on the dash panel, then the check engine light came on and shortly thereafter the transmission shifted down to 4th. I found a place to pull over and shut the engine off. Afterwards I couldn't get it started. Turns out the alternator was fried and the low battery voltage was causing havoc with the computer, thus the erratic dash panel readings.
Get the alternator checked, my bet is it is not working or not working properly. If it is not putting out at least 13.5 volts it is bad.

[cbilodeau]

I would suspect a bad battery connection or a bad ground connection.

I also would check the ground to the Battery Control Center. On my Fleetwood the BCC had a bad ground and also the +12V connectors were corroded. If the Battery Control Center has an electrical leak, you will have a lot of weird problems. In the BCC you have the Aux Start solenoids, etc.

[vstanz]

HARBOR FREIGHT SELLS A BATTERY CHARGING CIRCUIT TESTER FOR $19.99. GREAT ITEM TO CARRY IN THE TOOL KIT.

[cbilodeau]

Originally Posted by vstanz

HARBOR FREIGHT SELLS A BATTERY CHARGING CIRCUIT TESTER FOR $19.99. GREAT ITEM TO CARRY IN THE TOOL KIT.

I suppose this is it:

12 Volt Battery/Alternator Tester

[MJFZ]

Originally Posted by cbilodeau

I suppose this is it:

12 Volt Battery/Alternator Tester

I suggest this one:
100 Amp 6/12V Battery Load Tester

[cbilodeau]

Originally Posted by MJFZ

I suggest this one:
100 Amp 6/12V Battery Load Tester

The little one I showed would work to see if the battery is charging and if the battery is charged.

But this bigger one is not big enough in my opinion to test the Amperage of a motorhome. I am no expert at this kind of Load Tester but I am quite sure you have to connect it in SERIAL to be able to get the amperage reading. And 100A is enough probably for a motorcycle. I don't know how much the starter can draw on a Motorhome but it could be more than 500A. Probably between 400 and 800A but I don't know. But there is work around. In the first place I would test the charging and the battery state, then the wiring for the grounds, the +12V cables, etc.

[bluepill]

Originally Posted by cbilodeau

The little one I showed would work to see if the battery is charging and if the battery is charged.

But this bigger one is not big enough in my opinion to test the Amperage of a motorhome. I am no expert at this kind of Load Tester but I am quite sure you have to connect it in SERIAL to be able to get the amperage reading..

DO NOT CONNECT BATTERIES IN SERIAL UNLESS THEY ARE 6 Volt. When testing, always disconnect multiple batteries and test each one BY ITSELF. This is the only way to determine if any of the batteries is bad.

Easy, Cheap, and Accurate Battery Test

I am a 4-time ASE Certified Master Auto Technician. Not bragging, just qualifying. I was always frustrated by unreliable results from conventional battery tests. The older 15 second load test as well as the newer conductance testing both missed catching some batteries that would fail in actual use.
I devised a test that I found to be more reliable. The good news is that it only requires a cheap digital multi meter and a clock. If you have a battery charger, even better. Here is how it works:
1. Make sure that the battery is fully charged. Leaving a charger on it overnight is good.
If you don’t have a charger, test the voltage at the battery with the engine running. It should be between 13.5 and 14.5 volts. If so, drive the car for at least ½ hour with no lights or accessories on.

2. With the engine off, turn the headlights on for 3 minutes, then turn them off. Test the voltage at the battery terminals. It should be 12.7 to 13.0 volts if the battery is fully charged.

3. With the engine off, you will now leave the headlights on (HIGH BEAMS) for a period of time simply calculated below. The battery should maintain a voltage above 11.9 volts at the end of the time period. I usually check every 5 or 10 minutes during the test, just to see if the voltage drop is fairly consistent over time. I have seen cases where the voltage would drop slowly, and then suddenly start to drop much faster. These are the batteries that usually passed the “standard” tests.

Calculation of test time:
1. See if the low beams stay on when you turn on the high beams. This is common for systems with separate bulbs for low and high.
2. Find the “CCA” rating of your battery. It’s usually on the battery label, or look it up here: https://www.interstatebatteries.com/content/product_info/auto_interstate_f.asp
3. If all 4 bulbs stay on in the high beam position, divide the CCA by 20. If only the high bulbs light, divide by 10. This gives you the number of minutes to run the test.
Example: Battery rated at 600 CCA, 4 bulbs on = 30 minute test.

[TQ60]

X2!

Simple alternator or charger test is just a good volt meter.

Battery testing needs a measured load for a determined time to a target voltage to confirm capacity.

There are many ways to find a bad battery but only one to confirm a good battery.

I was involved with setting standards for a national carrier for battery testing and maintenance.

Folks in charge were hung up on conductance or impedance testing as low risk non intrusive but these require huge data bases and multiple testing cycles to get enough data to evaluate before you suspect a bad battery while the data can only indicate a battery "looks like a good one".

The headlight trick is a nice one as it is an easily repeatable load that can be done by anyone.

I have suggested using the inverter with a heater but not everyone has inverter or one large enough.

I developed a test using just the float charge to indicate suspect batteries that could be modified here.

Following above advice with headlights with either new battery or current battery place a charger on it for a day or 3 to insure fully charged.

Measure voltage and write it down.

Any voltage above 12.6 can be considered float charge.

Make note of light selection either high or low beams.

Disconnect charger and turn lights on and note the time.

When the voltage hits 12.5 note the time.

Leave lights on until voltage hits either 12.25 or 12.0 or some other chosen voltage between 12.0 and 12.5.

Turn lights off and recharge battery.

This does not indicate battery capacity as it only consumes a small portion of the capacity.
However it is an easily repeatable benchmark that can indicate failure as the times will be much shorter when battery wears or fsils.

My test had known loads and calculations against battery data sheets that would indicate time in seconds to drop from 54.4 volts floating to 50 volts (this was for a 48 volt system).

It did not specify batteries were good but it did indicate batteries bad.

[dwarne]

Just been there. 2009 Isnignia with ISC 360 Cummins. Going to take it to have AC unit charged and batteries were dead. Wouldn't start using jumper off house either. So put it on charger and got it going. Drove to shop and "low Voltage" warning came on. Kept going and "Check Engine" warning came on. Started looking for big tree! The shop was an electric repair as well. Guy came out and said it was probably alternator. I took it home after getting AC charges, and pulled off alternator. Leece N 170A, Took it back down and had it checked. It was bad. They had a 160A on the shelf for $250. Changed the pulley for me. Took it back home and installed it. Problem fixed. EXCEPT doesn't go over 12.5 V after starting. Then after a bit, 5 minutes maybe, it will go to 13.8V. Went for ride yesterday to make sure it was going to be OK and the voltage stayed up but never showed over 13.8. Guess that's OK.