searover
regular
Reged: 03/02/2004
Posts: 219
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Hi. all ,I have a problem with my red light coming on. obiousely not charging . this only happens at 1100 revs or less the belt is not loose, could it be the altenater,as the light goes off as i inrease revs. this is a new fault as it has never happened before. any lecy,s out there might be able to help to tell me what to test. cheers ..........
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pampas
regular
Reged: 17/07/2003
Posts: 606
Loc: Falmouth
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If your w/l comes on before you start and goes out after starting then the alternator will remain charging throught its rev range,fist thing that i would do if it were mine would change the belt unless it was a new one fitted during layup. 2nd thing would be to tighten the belt as I suspect that at that critical speed its whipping ,increase the revs and the whipping stops and all appears to be ok. 2nd option is the cheapest so try that first.
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VicS
regular
Reged: 13/07/2002
Posts: 9283
Loc: Home: Kent. Boat: Chichester
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If the belt is not slipping and this is not its normal behaviour then something is wrong, although it may not be serious or significant especially if you are achieving normal output and charging rates at normal operating revs and in particular if this is a case of the light staying on initially but not coming on again when the revs fall after increasing briefly.
Check all connections if you have an external regulator (unlikely unless you have something fancy fitted)
Check what volts you are getting and compare with the spec for the alternator, if you can, check the charging current but unless you know what you are up to don't!
If you have any doubts about the alternator output get it checked by an auto electrician or at a Lucas depot.
It is possible that it is a poor connection away from the alternator but in the circuits that feed the power to the warning light, but I think that would result in the light glowing dimly rather than coming on at full brightness.
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Old Chemists never die, they just fail to react
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William_H
regular
Reged: 28/07/2003
Posts: 3613
Loc: West Australia
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The warning light in a simple system provides excitation current to the alternator until such time as the alternator itself provides excitation current. The light glow indicates current is coming from the battery. The light going out indicates alternator taking over.
In a simple engine manufacturer supplied system most critical connections are internal to the alternator if it has an inbuilt regulator.
You could check the battery voltage to see if it rises to 13.75 volts when the engine is running (idle) or if it doesn't rise until the engine revs up. This will confirm you have a fault.
I doubt a loose belt would give the symptoms. I think the best bet is to remove the alternator for bench check of all the diodes especially the excitation diodes. good luck olewill
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moody1
regular
Reged: 10/12/2005
Posts: 414
Loc: Anywhere in N.Europe / Baltic ...
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We had same and hubby took alternator to auto-electrics service who found the regulator on back of alternator was faulty. He replaced this and alternator output was then better than ever seen before. Total cost was about 20 quid, all in one day.
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Some of us Ladies may surprise you.... Wife of Refueler
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Mal113
regular
Reged: 10/02/2006
Posts: 58
Loc: The Netherlands
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This is no doubt a simple case of the two carbon brushes being worn and therefore insufficient pressure on the slip-rings providing excitation to the alternator rotor winding. Check and fit new brushes and the problem should go away, this is not an unusual problem.
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alan17
regular
Reged: 12/03/2006
Posts: 278
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Re Mal 113
Replacing brushes on some alternators is very simple, on others it requires the alternator to be stripped and is quite trticky. Unless you know what you are doing take it to an electical repair shop.
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Mal113
regular
Reged: 10/02/2006
Posts: 58
Loc: The Netherlands
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I have not found one yet that is difficult but being an Electrical Engineering I maybe a little biased
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wiggy
regular
Reged: 13/06/2001
Posts: 488
Loc: Itchenor
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had same problem, got progressively worse until I had no charge at all (was on my diesel car) turned out to be faulty regulator.
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Have a look at the new MG Spring 25 web site here http://www.freewebs.com/mgspring25/
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VicS
regular
Reged: 13/07/2002
Posts: 9283
Loc: Home: Kent. Boat: Chichester
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Quote:
This is no doubt a simple case of the two carbon brushes being worn
It is a possibility but in my opinion and experience the symptoms described would not lead me to say that is it the problem without doubt! Far more likely to be a defective regulator as others have suggested. In some alternators brushes and regulators are integrated in others they are separate.
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Old Chemists never die, they just fail to react
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William_H
regular
Reged: 28/07/2003
Posts: 3613
Loc: West Australia
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I tend to agree with Vic as in many simple systems the current for the light actually goes through the brushes. However for the cost of replacement brushes (with regulator) it may be worth doing just to see. This is assuming your alternator has a popular car type origin.
if it is as I am familiar the regulator is a little black plastic block about 20mm across on the back of the alternator (opposite end to the pulley) with 2 screws holding it in. It has wire push on connections or a plug. When you remove it you will also remove the brushes and be able to see the slip rings. (hopefully) the rings should be clean and bright not too burnt.
It is worth having a check olewill
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DuncanMack
regular
Reged: 02/10/2005
Posts: 1665
Loc: Dunno, no GPS signal...
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Quick test for worn brushes....
Whack alternator with a hammer - not hard enough to damage it tho 
This usually helps the springs to shove the brushes out the last little bit.
If this gives a (temporary) fix then replace brushes.
Car and truck tip!
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Duncan
The march of progress - but in which direction?
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searover
regular
Reged: 03/02/2004
Posts: 219
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Thanks to you all ,I knew you wouldnt let me down, the experience on here is unbielevable. Ive taken the altenater off now and will have it serviced, or repaired, ill let you know the outcome.Thanks again chaps... 
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