SteveWallace
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| posted on 14/5/14 at 02:55 PM |
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Alternator Problem
When I started the car to go to my IVA test the other day, the battery warning light came on and my volt meter on the dash was only showing 12V
instead of the usual 14V ish with the engine running. Luckily, I had enough juice in the battery to get to the test, pass it and get back home.
I tested the alternator and it wasn't putting out a charging voltage so I changed the slip rings and regulator (the usual suspects in such
cases), but no luck.
I know that alternators work by using the voltage from the warning light circuit to energise one of the coils to make an initial magnetic field so
that the alternator can start to generate when its spun up by the engine, so I tested the voltage between where the warning light circuit connects and
earth and got only 1 volt. I know that the wire is intact and not shorting somewhere because when I disconnect it from the alternator the battery
warning light goes out (i.e. no other route to earth).
I'm assuming that its the alternator that is no good, but I just wanted to check with the collective first in case there is another explanation.
E.g. is it right to see a voltage drop like that given the various diodes, rectifiers and regulators that live in the alternator.
BTW, I have a good battery that gives 12V to the rest of the car.
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19sac65
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| posted on 14/5/14 at 03:10 PM |
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What alternator is it
From my memory the warning light has power to it from ign switch and it earths via the alternator wire
When the alternator produces power the earth becomes live
A live to each side of the light puts the light out - if the alt stops producing power the bulb illuminates
If its a denso type alt i know they need a 12v ign feed to excite the coils in order for them to produce
However unless youve altered the wiring i cant see it suddenly stop charging
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DW100
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| posted on 14/5/14 at 03:19 PM |
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With the ignition on and the wire from the warning light to the alternator dis-connected you should have 12v on the end of the wire. What sort of bulb
are you using?
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owelly
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| posted on 14/5/14 at 07:11 PM |
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You may not have 12v at the alternator end of the field wire as it has lost some puff getting through the warning lamp. I would check that you have
12v at the feed side of the warning lamp and then check for continuity from the other side of the lamp through to the alternator.
http://www.ppcmag.co.uk
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avagolen
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| posted on 14/5/14 at 07:36 PM |
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Are you sure there are only 2 wires to the Alternator?
Should there not be 3? Has 1 fallen off......
1)Warning light,
2)+12v from the ignition switch - excitation,
3)Alternator output to the battery.
The Answer for everything, but never the last word....
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Dusty
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| posted on 14/5/14 at 07:43 PM |
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Easy enough to take a flying lead from battery positive to the alternator to provide the excitatory current, bypassing all the existing warning light
wiring. Doesn't have to have a bulb in for the test. Then run the car and check if you get 14+ volts out.
What alternator?
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SteveWallace
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| posted on 15/5/14 at 11:53 AM |
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quote:
Originally posted by DW100
With the ignition on and the wire from the warning light to the alternator dis-connected you should have 12v on the end of the wire. What sort of bulb
are you using?
Good point. I've just done that test and I have 12 volts at the end of the wire that connects to the alternator when its not connected, so that
means that the warning light/ excitation circuit is OK as far as the alternator.
However. when I reconnect the wire to the alternator and test the voltage at the same place, I only get 1 volt. Is that normal?
By the way, its is definitely only a two lead alternator, so basically the 12v volts from ignition run and the warning light are in the same
circuit.
Probably best at this stage to get another alternator from the scrappy and try again as I seem to have ruled out the other options.
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The Black Flash
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| posted on 15/5/14 at 02:23 PM |
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quote:
Originally posted by avagolen
Are you sure there are only 2 wires to the Alternator?
Should there not be 3? Has 1 fallen off......
1)Warning light,
2)+12v from the ignition switch - excitation,
3)Alternator output to the battery.
<apologies for hijack> I never got to the bottom of whether number 2 was required. I couldn't find a reliable source of info so
I've been running my (denso) without it, it's been fine for the few hundred miles I've covered so far. Am I missing something
important by not having one?
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DW100
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| posted on 15/5/14 at 03:06 PM |
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With only the current that a digital multi meter pulls through the bulb the voltage drop should be minimal.
with the wire disconnected you should get close to 12v
with it connected and engine not running it should be close to 0V (that is what puts the light on)
with the engine running you should have 14 ish volts on both sides of the warning lamp and it will be off.
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