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Old 10-25-2015, 07:02 PM   #1
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Is my alternator bad?

Class A with Cummins 6.7 Diesel 2-12 volt and 4-6 volt batteries. I left home and as soon as I started driving the dashboard displayed warning "Battery-low voltage". After about 2 miles I found a place to safely stop on the side of the road. I crawled underneath the motor home and checked to make sure the belt was on the alternator. Everything appeared fine. I turned off the shut off switch and the motor died. Turned switch back on and drove a few more miles and then "check engine" light came on. I pulled over and shut off the motor and again looked for an obvious problem. Then the motor wouldn't start. Removed the 12-volt batteries and charged them. Next day went back, put the batteries in and drove home (same "Battery-low voltage" displayed). Tested the alternator and it checks out okay.
Any thoughts would be appreciated.
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Old 10-25-2015, 07:15 PM   #2
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How old are your batteries? Did you try to start the rv with the emergency house battery switch? One way to check the alternator is to disconnect the positive cable going to the start battery after staring the rv. . If the engine dies you have a bad alternator. If it keeps running you most likely have bad batteries.
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Old 10-25-2015, 07:47 PM   #3
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Sounds like it could be an alternator to me. Mine did the exact some thing when it died. In my case, I could start the Genny going doing the road, and it would push the voltage up and charge the batteries to get me home.

How did you test your alternator?
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Old 10-25-2015, 07:50 PM   #4
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Switched the switch for house batteries and motor started. I haven't disconnected the positive cable for start batteries but I turned off the main switch and the motor died.
Batteries are 2 years old.
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Old 10-25-2015, 07:53 PM   #5
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Switched the switch for house batteries and motor started. I haven't disconnected the positive cable for start batteries but I turned off the main switch and the motor died.
Batteries are 2 years old.
Well if your alt tested good, and you started the MH with house batteries, then you chassis batteries are bad, pull them and get them tested.
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Old 10-25-2015, 07:58 PM   #6
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l get all 6 batteries tested. Using multimeter one of the house batteries appears bad. If chassis batteries are bad, not sure how motor would start. Motor will run but dies when I turn off the main switch for the chassis batteries.
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Old 10-25-2015, 08:06 PM   #7
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I just had a similar problem. Low battery alarm sounded a few times. I thought the alternator was going out. Then early last Friday morning it overheated. Towed to shop. Belt tensioning pulley was freezing up. Replaced pulley and belt, drove over 600 miles home with no problems.
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Old 10-25-2015, 08:21 PM   #8
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How old are your batteries? Did you try to start the rv with the emergency house battery switch? One way to check the alternator is to disconnect the positive cable going to the start battery after staring the rv. . If the engine dies you have a bad alternator. If it keeps running you most likely have bad batteries.
DO NOT DO THIS! This is great advice to test old vehicles but not todays. If you take the positive wire off, with the engine running on a newer computer controlled engine, the alternator will put out max volts, this will send a voltage spike to the ECM and could fry it, (you may get away with it once or twice). To test the alternator check the voltage with a multimeter, it should be 13.5-14.5 in general, then to check the amp output use an amp clamp with your multimeter and put it around the positive leaving the alternator, rev the engine to 1500 RPM and get the reading. When checking the amps it should be with in 10% of the rating of the alternator. Or just take it to auto zone and have them test it for free in the parking lot. Good luck!
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Old 10-27-2015, 10:02 PM   #9
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To check your alternator output you need a multimeter set to check DC voltage. With the engine shut off , measure the voltage of your cranking battery, should be 12 to 13 volts DC, now start the engine, check the voltage again, with the alternator functioning the voltage across the cranking battery should be 13.8-14.5 volts DC. Most Motorhomes have a dash mounted volt meter, you should see a rise in the voltage when the engine is running vs having the ignition switch to the on position.
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Old 10-28-2015, 05:17 AM   #10
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Did test with multimeter. At the chassis batteries with engine off I had a 12 V reading. With engine running it rose to @13.5 V (analog tester).
I have no clue why the dashboard display show "Battery-low voltage" when engine is running and running the engine drains the chassis batteries. Tested the house batteries (4) and one is bad. Can this be the cause? I read I need to replace all 4 and not just the one that is bad.
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Old 10-28-2015, 07:42 AM   #11
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When the engine is running and the alternator is putting out current to charge the chassis batteries, a solenoid also closes which connects the house batteries to the chassis batteries/alternator so that they get charged as well.

You have a bad house battery, which is dragging down your chassis batteries.

You could try a test. Disconnect the house batteries completely (make sure the wires you disconnect can't move around and short out), and then go start your engine and take it for a drive. With the bad house batteries not connected, your low voltage light shouldn't come on. If it does, you have another problem in addition to the bad house batteries.
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Old 10-28-2015, 09:00 AM   #12
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I have house batteries completely out of the vehicle. I will go for a drive and then provide an update. Hopefully today after work.
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Old 10-28-2015, 11:41 AM   #13
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I had what I thought was a bad alternator once. An old trucker familiar with Cummins, told me to disconnect battery while running, if the alternator is working, the motor will continue to run. If the motor dies, the alternator is bad.
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Old 10-29-2015, 10:08 PM   #14
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House batteries disconnected. Went for a drive. Same result. Motor is running off of the chassis batteries.
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