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Old 06-18-2015, 12:44 PM   #1
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alternator voltage problem

Hello. We have an 01 Monaco Signature with a Cummins ISM 500 engine. On the final leg of a 2000 mi round trip we had a charging issue. We had been running the tv, the fridge, lights, etc off the alternator for many hours so it was getting a workout. Despite this, the dash gauge and the digital house batt gauge showed a strong 14+ volts. My wife turned on the microwave to heat some lunch and that was it. The voltage spiked to 17+ on the house batt gauge. This caused the inverter to shut down due to an over voltage condition. Then, the volts fell to12 as though the alternator was no longer working. Both the digital house batt gauge and dash gauge showed about 12 volts. After a stop to check the alt and wiring, and about an hour, the gauges slowly started to creap up. They are showing about 13 volts now, but not the 14+ volts they were showing. Any ideas? Did I cook my voltage regulator, alternator, both? Does anyone know if the voltage regulator is separate from the alternator on this series coach? If so... Where might I find the voltage regulator? Any other ideas? Thanks a bunch for any help!
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Old 06-18-2015, 02:31 PM   #2
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An 01 should be a Leece Neville ( now prestolite) 200 or 250 amp DUVAC alternator. That alternator has a built in regulator. Since your ALT Fail light is not on I would check the diode isolator first. That is a big finned aluminum unit , probably Culter Hammer. It has three terminals. The center terminal goes to the alternator output, The end ones go the the house batteries and the chassis battery. The theory is the alternator through the diodes can charge both banks while still maintaining isolation between the two banks. With the engine running chack the voltage on all three terminals. If all is working the center will be about 14.1v and the other two about 0.3v less or 13.8v. Good luck
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Old 06-18-2015, 03:39 PM   #3
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I have always started the gen to use the microwave, that thing will pull a new set of battery's down fast. I have found over the years it cost about the same to run my 7.5 gen as it does to run the dash AC in fuel usage, but not sure on your 10 or 12 gen on the fuel usage.
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Old 06-18-2015, 08:52 PM   #4
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My coach has a Leese Neville 200amp alternator. It is not original... the date on it is 2006. I will try checking the three poles on the isolator for volts. I replaced the isolator a few years ago. Do they have a high failure rate? As I look at it, I notice there is some black goo dripping down from beneath the top and bottom poles... perhaps from heat? Any ideas what would have caused the voltage spike to 17+ volts? Thanks for the help!
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Old 06-19-2015, 03:35 AM   #5
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The 17 volt output would be due to two things.

1, A failed voltage regulator causing full output.

2, If the system has a diode type isolator, the alternator needs
feedback from a separate wire, running from the battery to the alternator. It goes to the sense terminal, on the alt. If the "sense voltage" is lost, the internal regulator goes to full output. This is happens, because diode isolators only allow power to run 1 way.

If both of your batt. banks are overcharging, jump the alt post to a batt post on the isolator. If things calm down, you have lost the sense signal. If it continues to overcharge, then it could be the internal regulator.

Sometimes the sense wire is on the isolator, with the start batt, charging wire. If the connection is corroded and loose, it will cause random charging issues.
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Old 06-19-2015, 09:20 AM   #6
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Thanks. This is the first time this has ever happened. The over voltage event correlated to the microwave being started. This was after 8+ hours on the road with several ac loads on the inverter. After about 30 min with no inverter, including an engine shutdown, the volts went to 12 as though the alt was putting out no power. Then, slowly, the volts crept back to 13+. This indicated some alt activity, but not the solid 14+ volts it used to show when the engine is running.
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Old 06-19-2015, 09:27 AM   #7
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The overvoltage charging, could have blown a diode in the alternator.

Have the alt checked.

The microwave current draw, would tax the charging system. That when bad connections show up.
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Old 06-20-2015, 08:35 AM   #8
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I checked the sense wire and its voltage corresponds to the batt bank. I also checked the voltage isolator and the center terminal is hot... ie it's cooked. Any suggestions on what brand of alternator to get? Considering cost, the Delco 28si caught my eye. I'm not sure if another Leece Neeson is the way to go since my coach has already eaten at least two of them. Also, do you happen to know the model number of the voltage isolator? Thanks again.
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Old 06-20-2015, 11:32 AM   #9
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I would do away with the isolator. Connect the alt. wire to the batt. wire. Tape up the other.

Get a BIRD setup, to hook between your batteries, with battery cables.

That gives you by directional charging, with no .5 volt loss, from the diodes.

Another option,

Look at "Yandina 160 amp combiner".
Put alt and engine battery on one post. House battery on other.
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