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Old 10-06-2022, 10:15 AM   #1
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Alternator Output at Idle

I have a 1997 Monaco Executive with the Cummins M-11. What should my alternator output be at idle?
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Old 10-06-2022, 10:25 AM   #2
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What measurements are you taking? Amps or volts? Both are affected by the load.

Assuming your batteries are already charged, no other heavy draws on the system, it should produce 13.8v

You would need to know the specific alternator installed on your engine to know theoretically it’s max amp output, previous owners could have changed the unit from what was originally installed.

Also what is your idle speed? 600? 800? 1000?
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Old 10-06-2022, 11:45 AM   #3
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Old 10-06-2022, 12:38 PM   #4
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After an hour of searching, the only M11 alternator output I found said 130A maximum at max. RPM of 2100.. Some alternators are designed for high output at idle speed, which is 700 for Cummins engines.
Interestingly, I stumbled across this information about the M11 engine, which is unrelated to your question..
2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;Good Sam Life member,FMCA." My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy
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Old 10-06-2022, 01:03 PM   #5
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I have a 340 amp alternator and at idle around 700ish rpm I get 14v with normal loads.

I can get around 150 amps at idle at 13.8v but I don’t do that for long.

It also has to do with the size pulley on the alternator, smaller = more at idle.
1998 HR Endeavor Cummins ISB 275 / Banks Allison 3060
20014Jeep Wrangler JKUR with M&G air brake with breakaway
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Old 10-06-2022, 01:14 PM   #6
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Originally Posted by dblong View Post
I have a 1997 Monaco Executive with the Cummins M-11. What should my alternator output be at idle?

While I am sure there will be answers posted, even for your specific coach there is no single answer.

First, alternators put out less amps at engine idle speed than at higher RPM.

If the battery banks are discharged, that alternator output may be only enough to bring battery voltage up a little over what it was before starting.

Later/when the battery banks are charged, so not many amps are required (similar to float voltage for a smart charger) it should read around 14 VDC.

Basically, your battery bank demand will have a lot to do with your voltage reading, particularly at idle speed.
Brett Wolfe
Ex: 2003 Alpine 38FDDS. Ex: 1997 Safari Sahara. Ex: 1993 Foretravel U240
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