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Old 10-29-2021, 07:41 AM   #1
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Voltage drops when Under load - Alternator?

Voltage seems to drop when ISM engine is under load, e.g. going uphill. Until recently voltage showed around 13.5-13.6. Last trip it was around 13.2 to 13.3, dropping to a little below 12 under load. Dash gauge and VMSpc show pretty much the same. Engine batteries are 1.5 yrs old and the BigBoy was replaced about the same time. The alternator is a Leece Neville, probably original.

So I'm not sure if it's the alternator losing output, the BigBoy somehow going bad so soon or something else.
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Old 10-29-2021, 08:59 AM   #2
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The big boy only combines to charge secondary battery if bidirectional. If your always running the genny driving I could see loss of it boosting the chassis battery due to low alt. output.
Higher rpms could make a bad brushes chatter more or belt flap if tensioner dampening is bad. Serpentine belts don't slip much. Alternator pulley is small and gets a lot of grooved side contact ,not really suggesting its the issue.
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Old 10-29-2021, 10:28 AM   #3
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Anytime the alternator output drops below 12v would be a red flag for me (not an expert!) I cannot remember ever seeing fluctuations on the voltage gauge once the initial start up and running has occurred, but I don't watch the gauge constantly. Regardless, I would not trust the reliability of the alternator dropping to that level, even under load (load being the engine effort, not electrical demand.)

If this was my situation, I would have the alternator/regulator tested at the first opportunity.
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Old 10-29-2021, 10:40 AM   #4
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I had my L/N alternator rebuilt over the summer ; noisy bearings ; during the bench testing the regulator proved faulty , wouldn't allow full output , so new regulator installed .
In 11 years with the coach I'd never seen the volt meter go to 14 volts; I'd assumed that was because the coach is always plugged in and batteries fully charged ; voltmeter now goes to 14 volts after start up ; so the regulator had been acting up for a long time.
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Old 10-29-2021, 12:45 PM   #5
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OOPS! Typo

I should have said "dropping to a little below 13 under load"

Called an Alternator place, I'm bringing it in to them to load test. The tech there also suggested that cables could be corroded inside after 15+ years.
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Old 10-29-2021, 02:19 PM   #6
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Quote:
Originally Posted by rpasetto View Post
The tech there also suggested that cables could be corroded inside after 15+ years.
Amen to that ; in a pusher motor home , the air flow through the alternator is opposite to the way they were designed ( heat from exhaust going into the back of the alternator, instead of being blown away as in a front engine configuration ) it's actually a wonder they last as long as they do.
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Old 10-29-2021, 03:06 PM   #7
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Hi Rick P.; If you want to test the alternator get a multi meter and hook the leads to the +&- terminals on the back of the alternator. Use your cruise control to bring the RPM's up to al least 800. Get a voltage reading. Should be above 13.8 volts plus. Now add loads like dash heater blower, headlights and fog lights. If voltage drops more than .2 volts I am going to guess your alternator is not putting out voltage and needs to be rebuilt.

Read Not2bars posts here and you can see what he had to deal with in his alternator woes. Hope this info helps and good luck!
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Old 10-30-2021, 09:42 AM   #8
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My coach was ~12 years old when my alternator crapped out, Alt Fail light came on. Didn't bother doing any diagnostics, just pulled it and had it rebuilt cost ~$185 ~7 years ago.


If I were you I'd just have them go through and rebuild your alternator. The hard part is done >>> pulling it and putting it back in is the hard part.



My alternator puts out 14.2 on the Silverleaf VPMSpc display. Dash gauge lower.
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Old 10-30-2021, 02:44 PM   #9
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It sounds like your voltage regulator is not working correctly. When that happened to me, I replaced the regulator (it is attached to the top of the LN alternator and is a bit of a challenge to change). However, within a few months the diodes failed in the alternator and I had to remove the alternator in a campground and take it in to be rebuilt (the failing diodes may have been what caused the regulator to fail originally). So, my suggestion is to have the alternator rebuilt before it fails on the road somewhere. DO NOT let a rebuild shop exchange yours for one of their off-the-shelf rebuilt units. Have your original unit rebuilt. A little research will yield many horror stories of resulting problems when a supposedly correct substitute does NOT work correctly. Keep your original and rebuild it if at all possible. Just my opinion. Good luck.


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Old 11-02-2021, 07:45 PM   #10
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We have Leece Neville 200 amp alternator on our MH. Started out this trip showing 14+ volts on the dash gauge. Started fluctuating a week or so ago, pretty much the same symptoms as yours. Crapped out on us and resulted in a tow. (Thank you Passport America) Took the alternator to a rebuilder in Columbus, Ohio. Hite Parts Rebuilders, had it ready in under an hour. Now shows a steady 14+ at the gauge. Voltmeter at the isolator showing 14.2 volts on both legs. Might save yourself some trouble and get it rebuilt. God Bless and good luck, Ed & Sylvia
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