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Alternator light goes out at circa 3000 rpm but no change in battery voltage

  • lohr500
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14 Dec 2018 17:21 #198953 by lohr500
Hi all, Newbie here, so please be gentle!!

We have a battered old 2004 Jimny which we use around the fields for my kids to learn to drive in.
The battery has stopped charging on the vehicle, but will hold a charge for a few weeks if charged up from a mains charger.
After mains charging the battery initially shows 13.5V and this drops to 12.5v after starting with the engine running.

Now here's the puzzling bit. The battery warning light on the dash is lit before starting and remains lit until around 3000 RPM. Above 3000 it goes out. But with a reliable voltmeter across the battery, I see no increase in voltage at the point when the warning light goes out.
The voltage remains constant at 12.5V. If I probe the main alternator output terminal I get the same voltage.

As the battery holds a charge after mains charging, I am thinking it must be the alternator that is shot, but why would the light go out above 3000RPM with no detectable change in voltage?

I don't want to waste time/money fitting a replacement alternator if this isn't the problem.

The main fuses are all OK and I presume the alternator regulator is built into the alternator and not a separate unit. Is this the case?
Drive belt is nice and tight with no slipping or squealing.

Any ideas. Does it sound like an alternator issue?

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14 Dec 2018 17:48 #198958 by Max Headroom
What happens if you load the system up with lights; heater-fan on max; radio; windscreen wipers on etc etc?

My neighbours car was fine (no warning lights, outputs reading correctly etc) until she had it under load as described above; then the alternator actually tried to stop - the belt squealed like a banshee (and it was a new and correctly adjusted belt).
She replaced the alternator and the problem was instantly fixed


IF IT AINT BROKE, KEEP FIXING IT UNTIL IT IS

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  • paradox1001
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14 Dec 2018 18:53 #198960 by paradox1001
It’s probably the brushes on their last legs or the regulator has gone.

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  • lohr500
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15 Dec 2018 10:20 #198983 by lohr500
Thanks for the replies.

Loading up the system seems to make no difference.
If it warms up outside a bit today, I'll remove the alternator and investigate further.

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  • Lambert
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  • The quickest Jimny in Harrogate...(that I own)
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15 Dec 2018 17:43 #198998 by Lambert
When running the alternator should be giving something in the order of 14v stable across the rev range. A new or reconditioned one sounds to be in your future.

Temeraire (2018 quasar grey automatic)
One of the last 200ish of the gen3s, probably.
ADOS Attention Deficit Ooooh Shiny!

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  • lohr500
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16 Dec 2018 12:36 #199043 by lohr500
I removed the old unit this morning.

Have ordered a cheapo replacement alternator from eBay today at £53.50 including a special eBay 10% discount valid for yesterday/today only.
UK based seller with delivery supposed to be by Tuesday.
At that price it isn't worth messing round stripping down the old unit to figure out if it can be fixed.

Will report back once the replacement is fitted, but I'm sure it is the root cause of the charging problem.

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19 Dec 2018 09:44 #199176 by lohr500
New alternator fitted this morning and problem resolved.

Battery warning light goes out as soon as engine is running and alternator is pushing out 14.5 volts as measured at the battery.

Only time will tell if the cheap eBay alternator is reliable!

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  • Lambert
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19 Dec 2018 11:46 #199180 by Lambert
Excellent.

Temeraire (2018 quasar grey automatic)
One of the last 200ish of the gen3s, probably.
ADOS Attention Deficit Ooooh Shiny!

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19 Dec 2018 12:57 #199181 by Max Headroom
Great result! :)


IF IT AINT BROKE, KEEP FIXING IT UNTIL IT IS

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