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Old 08-23-2005, 05:48 PM   #1
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Alternator issue

Hi all,
I have been having problems with a run down chassis battery in my 1984 310 MH. Battery is 6 mo old and seems to hold a charge if I charge it with an external charger. But if I rely on the MH charging system, the battery voltage after a day of driving will be between 12.2 and 12.4. So I concluded that I have a problem with my charging system and took it in to have it looked at. They said the alternator was charging but that the belts were in bad shape and might be slipping and should be replaced. So I had them do that and told them also to inspect the alternator while they had the belts off because I thought it sounded noisy. They did and concluded it had dry bad bearing and should be replaced. They put in a 125 amp alternator. The bill for the new alternator, and two belts installed was $235. Pretty good price. These guys mostly work on diesel trucks and I have never had any problem with any work they have done. I not sure how much they know about the electrical system of a motorhome once you get beyond the powerplant.

When I go it back I made the following measurements:

Center terminal of isolator - 13.62 V
Left " " 12.87 V
Right " " 12.88 V

Terminals of chassis battery - 12.66 V
Terminals of coach batteries - 12.88 V

The values were the same if the engine was idling or being run at ~ 2000 rpm.

Dashboard volt meter reads approximately 11 V.

I am wondering if the charge at the terminals of the chassis is as high as it should be? Also, whether there is anything about these values which doesn't seem proper?
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Old 08-23-2005, 06:26 PM   #2
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Quote:
Originally Posted by guy99
When I go it back I made the following measurements:

Center terminal of isolator - 13.62 V
Left " " 12.87 V
Right " " 12.88 V

Terminals of chassis battery - 12.66 V
Terminals of coach batteries - 12.88 V

The values were the same if the engine was idling or being run at ~ 2000 rpm.

Dashboard volt meter reads approximately 11 V.

I am wondering if the charge at the terminals of the chassis is as high as it should be? Also, whether there is anything about these values which doesn't seem proper?
If your voltage at the battery terminals is 12.66 volts, you are discharging the battery. Fully charged, a battery should have 12.75+ volts. You should have a minimum of 13 volts at the terminals when running. It sounds like you have a wiring issue.
Depending on how mechanically inclined you are, you can run a new charge wire direct from the bolt-on terminal of the alternator to the positive terminal of the chassis battery. After you do this, start the engine, and recheck your voltage readings. It should be at least 13 volts, preferably a tad more.
If you want to hash it over with a bunch more people, you are welcome to join us in the chat area of the forum this evening starting at 9 pm eastern time.
several mechanics and vintage motor home owners will be there for you to pick our brains.
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Old 08-23-2005, 06:56 PM   #3
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Are all the batteries fully charged? 13.62v at the alternator is on the low side of ok. If the batteries aren't charged it will come up when they are charged.

Too much drop across the isolator for me. They are nothing but diodes and should only loose about half a volt. Also seems like a lot of drop to the chassis battery (if it is charged, if not charge and check again, there will be a larger drop with higher current flow).

Before replacing anything I would make sure all connections (including the engine and all battery grounds) are clean and tight. With charged batteries and clean connections check it all again. If the isolator is still loosing .8v it probably is going.

A cheaper and better isolator is a continuous duty solenoid. Wire the chassis battery off the alternator, put the solenoid contactors in series with the alternator and house batteries, run the solenoid coil off the ignition circuit. When the engine is running it will charge the chassis batteries at full alternator voltage without the drop from the diode isolator, when the ignition is off the contactors open and the batteries are isolated.
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Old 08-23-2005, 07:07 PM   #4
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Also check where the alternator sense wire is attached. If it is before the isolator there will be a lower voltage at the batteries because of the isolator voltage loss. It has to be after the isolator to get a good battery voltage.
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Old 08-23-2005, 07:11 PM   #5
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Originally Posted by 74Argosy24MH
Also check where the alternator sense wire is attached. If it is before the isolator there will be a lower voltage at the batteries because of the isolator voltage loss. It has to be after the isolator to get a good battery voltage.
That is why I suggested direct-wiring the charge wire to the battery. If it still gave problems, that would eliminate a whole bunch of stuff as potential problem areas.
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Old 08-23-2005, 10:14 PM   #6
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Wild guess, a direct wire to the battery will give 13.62v at the battery; I don't understand how that will help. The losses are pretty well identified, the question is what is causing them.
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Old 08-24-2005, 06:25 AM   #7
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could it be the isolator itself? When I changed to a higher powered alt I also had to change out the isolator. The original one on my AS was too low in capacity to handle the new alt.
Also when I had intermittent wiring/charging problems, it came from corrosion inside the wires to the starter, from heat exposure. I changed these out and wrapped with heat wrap. Since changing those wires and the alt and isolator two or three yrs ago i have had consistent charging rates.
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Old 08-24-2005, 09:44 AM   #8
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OK. Here's my summary of what is/might be wrong.
1. The alternator sense wire needs to be connected to the battery lead AFTER the isolator. If this is not done, the alternator will use the voltage before the the voltage drop which occurs at the isolator and think the battery is being charged when it isn't. If I connect the sense wire to the battery after the isolator and the voltage goes up to 13.6 or so does this fix the problem? Or is number two still a problem?
2. The voltage drop introduced by the the isolator seems too high. I assume that this can only be caused by excess resistance in the isolator - the isolator has big cooling fins which would seem to indicate that it is expected (designed?) to generate heat - i.e. have sufficient resistance that a goodly amount of heat is generated. What is the proper resistance?

Thanks for you help on this and I'm sorry I didn't see the post about the chat until too late.
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Old 08-24-2005, 01:04 PM   #9
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Make sure the chassis battery is fully charged, it will make the alternator output (and other voltages) lower. That could be why there is a .2v drop from the isolator to the battery and will also increase the isolator drop.

If you connect the sense wire after the isolator and the voltage goes up you are good, but I would still be concerned about the loss in the isolator. They are nothing but a couple of diodes, resistance in one direction will be infinite, the other should be only a few ohms.

I have an old 95 amp Surepower I just checked, one side is 1.704, the other is 1.544. Hooked a battery to it, 12.68v. One terminal is 12.44v, the other is 12.49v. If your battery is charged and you are seeing .8v drop it seems high.
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Old 08-24-2005, 02:34 PM   #10
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More data and a resolution. This morning after having the chassis battery on a trickle charger all night the output of the isolator was above 13V but there was still a big drop from the alternator output.

So I decided to change the isolator. The shop who replaced the alternator didn't have a source for the isolator so they sent me to an auto (and industrial and farm) electric shop. They suggested that I replace the isolator with a continuous duty relay. So since this was also suggested here, I had them go ahead. Half an hour and $100 (including parts) later everything is working fine.

One question, the relay is powered off of the ignition switch so that when the engine is running, the alternator is connected to both sets of batteries. When the ignition is off, the relay is open and the two batteries banks are not connected.

So far so good. What happens when the engine is running AND the generator is running. The alternator will supply current to both batteries AND the intellipower converter will send current to the house batteries (which will be connected via the relay to the chassis battery). So both house and chassis batteries will see voltage from the alternator AND the intellipower converter.

Is this a problem?
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Old 08-24-2005, 03:24 PM   #11
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Put an on/off switch in the coil circuit (or automate it- a normally closed 120v coil relay, contacts in series with the new solenoid coil, coil powered from any ac). The biggest problem I see would be if the alternator failed and output some weird voltages/frequencies and fed back into the converter. It would kind of bite to loose the alternator and converter at once.
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Old 08-24-2005, 03:55 PM   #12
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I just called the guy who installed the relay. He says the converter will be diode protected and not to worry.

Does that sound right?
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Old 08-24-2005, 04:10 PM   #13
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It really probably isn't going to be a problem, it is fairly common to both run with the generator on for the ac and use the solenoid to couple the battery banks. I have never heard of a failure from either back feeding into the other.
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