Nitrogeno25
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| posted on 7/10/12 at 11:34 PM |
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Alternator not charging when hot.
I bought a daihatsu hijet alternator for my Duratec (inlet side) and at first it seems to work OK. Charging at 14.7v from when revved. The I noticed
that about 1500rpm and higher, lights starts dimming and voltage fluctuates from 14.3 to 14.7v.
Now when I let the engine idle for 15-20 minutes the alternator stops charging. The charging light on the dash is off (it is on before starting the
engine).
Any ideas? I suspect from the voltage regulator as the charging light doesn't come on when the alt stops charging 
Thank you very much!
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ElmrPhD
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| posted on 8/10/12 at 07:32 AM |
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Define "...stops charging..."
If it's done its job and the battery is fully charged, then, in fact, if SHOULD "stop charging"...until it detects a low enough
voltage to start charging again.
If the light on the dash stays off while in operation (and you've observed it passing the "test" of coming on when the ignition is
first switched on) then the lamp works and that exciter circuit is good. Then, dash light off during operation means that either side of that dash
light is not too far from 13 V.
Also, "light starts dimming" suggests that the light was already on fully and then started dimming at "1500 rpm and higher".
Is that what you mean? Cuz that would contradict "it seems to work ok". I'm confused. Oh, right, haven't had the morning
coffee, yet - ignore me...
Steve,
in the NLs
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pmc_3
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| posted on 8/10/12 at 07:36 AM |
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I've never known any car to detect its charged and stopped charging. As the OP said it will normally sit at around 14.5v and keep trickle
charging when running, voltage only dips a bit when light etc are turned on.
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Daddylonglegs
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| posted on 8/10/12 at 09:02 AM |
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Check the belt tension, might seem stupid, but I had a similar issue with varying voltage a while back (see previous posts) and it turned out to be a
slack alternator belt.
HTH
It looks like the Midget is winning at the moment......
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r1_pete
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| posted on 8/10/12 at 12:52 PM |
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Is your battery actually going flat?
I'm interested to know how you know it has stopped charging??
My Denso Alternater also from a Diahatsu, behaves much the same, with a voltmeter across the battery:
At idle 650 rpm there is barely 12v showing.
Lift the engine off idle and it rises to 14.5 - 7 v
Higher up the rev range it sits between 14.4 - 6
I thought my voltages were a bit high, but raising the question on here the denso alternators do run a bit higher voltage than most.
I have had no battery discharge problems.
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Nitrogeno25
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| posted on 8/10/12 at 05:37 PM |
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I think battery is OK, engine starts every time, measured 12.4v before starting the engine.
I know the alternator stopped charging because my volt gauge shows 12v and not 14.4 like it was showing in the first minutes after starting the car.
If can let the engine running, but after 15 minutes I tested with a DMM and the voltage at the battery is very slowly decreasing (fuel pump and light
on).
I also suspect that there is something wrong, because the charge light doesn't come one when I stop the engine and put the ignition on again.
Belt tension is OK, belt and tensioner are new.
Thanks again!
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pmc_3
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| posted on 8/10/12 at 05:50 PM |
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Connections all ok? Engine earth strap good?
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ElmrPhD
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| posted on 8/10/12 at 08:49 PM |
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Wow, that's some weird symptoms. Something is seriously screwy with your alternator and/or regulator.
I agree that you should check your earth/ground, especially of the alternator as that really needs to be a solid circuit and may explain the weirdness
with the light.
Otherwise, a broken winding inside the alternator that contacts enough when cold, but then separates with heat might explain your symptoms...a bit.
To check your battery, see if the voltage at its terminals stays above 11 while cranking the engine. If it goes below 10 V, then the battery is bad
and you're wasting your time trying to diagnose an electrical system with a bad battery.
Good luck!!
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