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Old 06-15-2009, 12:45 PM   #1
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Electrical Problem

Would anyone have any ideas why my motorhome is not charging the battery? It has a brand new alternator and battery. There is a battery cutout switch that is switched on. I took the alternator out and had it bench tested and it is working satisfactorily. So why isn't it charging the battery? What am I overlooking? I checked to see if the aux battery switch was on the main battery which it is. I'm at a loss.
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Old 06-15-2009, 01:20 PM   #2
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Does your alternator have an internal regulator, or is it external?

And, by what method have you determined the battery is not charging?
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Old 06-15-2009, 02:14 PM   #3
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Which battery?

If the truck battery is fully charged the alternator sees a fully charged battery and will cease charging and the coach battery will not get a charge.

Again if the coach battery is fully charged and wired directly to the truck battery the truck battery will not get a charge.

The alternator charges until the regulator sees a charged situation and then drops back.
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Old 06-15-2009, 02:36 PM   #4
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Quiet Water- Had a similar problem with our MH. Like yours, the alternator bench tested OK. Mine would charge intermittenly. The problem turned out to be a loose/faulty connection of the battery ground cable,where it connects to the frame, not at the battery.

I think these are famous for faulty grounds and it's an inexpensive fix.
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Old 06-15-2009, 05:11 PM   #5
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Thanks guys, The non-charging battery is the truck battery. I discovered it was not charging when the motor started stuttering and then quit. I tried restarting but there was obviously no charge. I put in a new battery to start the engine which it did with no problem. An ammeter showed a discharge of the battery. I ran it some more with the new battery but after awhile it, too, started to die. Now I'm thinking it is what 345bill suggested in that there is a loose ground. Since I am not near the MH at the moment, I'll have to wait till tomorrow to check this out. Hope that is all it is. Cheap fix.
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Old 06-15-2009, 05:21 PM   #6
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I think if you had a bad ground on the truck battery, it would not start. Assuming the RV battery IS charging, I would check for a bad diode in the battery isolator.
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Old 06-15-2009, 05:32 PM   #7
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Would it be advantageous to disconnect the house batteries so the battery isolator would no longer see it? I'm not talking permanently, just long enough to get it to a shop that can look into this further.
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Old 06-15-2009, 06:08 PM   #8
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I'm assuming you have a battery isolator, which is basically two diodes and a heat sink. If you do, you can swap the two battery leads, and if your truck battery then charges, that's the problem. Just one caution...disconnect the positive leads from both batteries before doing this to prevent any unintended "welding".
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Old 06-15-2009, 06:15 PM   #9
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Thanks, I'll give this a try. It sounds logical.
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Old 06-18-2009, 09:01 AM   #10
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Well, I crawled under the motorhome and cleaned, tightened, and inspected all electrical lines. Even though I had put a brand new battery in before I left it a couple of days ago, there was no charge when I tried to start the motor. I jumped started it off my truck and the motor ran just fine. Went for a test drive and all went well for about 4 miles. Then the motor started to stutter and pop. I limped back to the drive way and almost made it back to the starting point before it died. My analysis is that the alternator is not working properly. Even though it bench tests satisfactorily, it is not putting juice to the engine. I've checked the wiring on the alternator and I can't find anything amiss. This is over my head so I'm going to try to charge up the battery again and try to make it to the RV service center. Hope they can troubleshoot it. The fact that the battery keeps losing it's charge indicates a short somewhere in the system. Very frustrating!
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Old 06-18-2009, 11:05 AM   #11
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Before you head to the shop.
Take a battery jumper cable and attach it to the body of the alternator and the ground terminal of the battery. Start the RV and measure the voltage from the output post of the alternator to the battery ground post. If you see more than 13 volts here then measure the voltage across the battery itself. If you have voltage from the alternator but not at the battery the cable from the alternator is open. That cable may have a fuseable link in it that has opened.
If you do not have voltage from the alternator to the battery ground while the jumper cable is attached the alternator is not working.
If you have good voltage from both test remove the jumper cable and measure from the output post of the alternator to battery ground again. If this voltage is low or not at all the engine is not grounded to the frame.

If all tests good and the battery is still going dead in a day or so you have something on when the RV is turned off.

I am assuming your RV is not a diesel. If it is and has glow plugs there may be a 40 second delay after starting before you will see charging voltage. This is because some diesels leave the glow plugs on after starting and this draws the voltage down for that peroid. If so equipped just wait a minuet before testing.
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Old 06-22-2009, 12:06 AM   #12
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well, the verdict is in. I don't know enough about electrical systems to give a full explanation but it had something to do with a short in the aux battery switch. Apparently, some wire in there was grounding out the alternator. I don't know how that works but, anyway, it is now fixed and all is well.
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