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Old 11-29-2011, 08:16 PM   #1
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300 Cummins Diesel Battery Draining Problems

I have a 1990 Monaco Crown Royale with a 300 Cummins Diesel ... I just replaced the starting battery and both coach batteries ... my problem ... something somewhere is draining my starting battery in a matter of a hour on the road. I had the alternator and the starter checked out and they are fine .. anyone got any suggestions on where to start looking. My voltage meter on the dash shows that the starting battery is not being charged.
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Old 11-29-2011, 09:21 PM   #2
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Quote:
Originally Posted by Holdenried View Post
I have a 1990 Monaco Crown Royale with a 300 Cummins Diesel ... I just replaced the starting battery and both coach batteries ... my problem ... something somewhere is draining my starting battery in a matter of a hour on the road. I had the alternator and the starter checked out and they are fine .. anyone got any suggestions on where to start looking. My voltage meter on the dash shows that the starting battery is not being charged.
That's weird, could you have a bad connection to your alternator? What do you mean when you say "My voltage meter on the dash shows that the starting battery is not being charged."? Does your dash voltage meter show zero volts?

Somewhere you have a device called a battery isolator. This device has large wires, one connected to the alternator, one to the starting battery and a third to the house batteries. If you can locate that, check the voltage on each terminal. It should be more than 12V and as high as 14V on all three terminals. If not, you may have a bad isolator. Having said that, I have never heard of an isolator going bad. The purpose of this device is to charge the chassis and house batteries, from the alternator, without connecting the two battery banks together.

Jim E
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Old 11-29-2011, 09:28 PM   #3
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I have been going through a similar problem. My alternator died and I replaced it, but when you first start the engine it won't charge and after awhile it will charge. My engine is a 8.3 350 hp cummins similar to yours. Check to see if your alternator has the Duvac system, which is another circuit that goes to a battery isolator. If that circuit is compromised the alternator wont charge. The duvac system has 3 small wires that hook to the back of the alt. one for the isolator and one for the ignition cicuit and the other is for the tach. If either the ign. circuit or isolator cicuit is dead the alternator won't charge. Hope that helps.
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Old 11-29-2011, 10:34 PM   #4
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It is not a drain issue it is a charging issue. Isolators do go bad. They are just diodes and can fail.

Without some pictures of your battery compartment and a schematic of your system I can only guess. Since your dash meter is not showing a charging voltage that confirms that you are not getting charged and the issue is not a huge drain. A fully charged battery discharging in one hour will usually leave some great smoke signs.

If you have an isolator you should be able to connect the wire marked "alternator" to the post where the engine battery is tied. If things come alive your isloator is dead.

Again, you may or may not have one.
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Old 12-01-2011, 10:24 AM   #5
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This sounds like the alternator 'Field' voltage is missing. This voltage comes from a 16ga wire connected to a fuse usually under the dash and wired through a diode (a small series diode - not to be confused with the isolator if you have one). If your alternator was tested out of the vehicle that could account for it being judged as 'OK' since a tester would apply the 'Field' voltage. Without proper 'Field' voltage, the alternator output would just read battery voltage when the engine is running.

What causes this problem usually is a bad or corroded connection (usually near the alternator as that wiring is exposed to the elements), a bad diode, or bad fuse.

The purpose is to prevent the alternator from trying to charge the starting battery when you're cranking the engine. This prevents damage to the alternator if your battery is nearly dead or if the starting motor has a short or for other, related, reasons.

So, check the wiring to the 'F' (Field) terminal on the alternator. And the voltage there while it's running. Should be a solid 13.2 or so.
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