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Is my alternator knackered? #540782
1st Jul 2011 11:33am
1st Jul 2011 11:33am

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willo OP
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willo OP
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I start car, disconnect battery, switch on headlights, and engine dies.

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Re: Is my alternator knackered? [Re: ] #540798
1st Jul 2011 12:15pm
1st Jul 2011 12:15pm
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Wirralshire
StuyMac Offline
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Is the alternator light illuminated on the dash? Does it go out after starting the car?


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Re: Is my alternator knackered? [Re: ] #540806
1st Jul 2011 12:36pm
1st Jul 2011 12:36pm
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Birkenhead
BandyCoot Offline

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Learn something every day. Didn't know a car could run with the battery disconnected, thought it disabled the ignition.


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Re: Is my alternator knackered? [Re: StuyMac] #540814
1st Jul 2011 12:52pm
1st Jul 2011 12:52pm

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willo OP
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Originally Posted by StuyMac
Is the alternator light illuminated on the dash? Does it go out after starting the car?
yes light goes out, after starting, but when engine is running if i use eletric windows, with headlight on lights go dom

Re: Is my alternator knackered? [Re: ] #540818
1st Jul 2011 1:04pm
1st Jul 2011 1:04pm
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Posts: 12,002
Wirralshire
StuyMac Offline
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Once started the car should run with the battery disconnected, but if the alternator was dodgy you would see the alternator light coming on, or the battery wouldnt charge and it wouldnt start.

Get a volt meter on the battery - it should show ~12v without the engine running, and ~13.5v running and charging. If it doesnt show more volts running then there is an issue with the alternator.

The problem you describe could just be a safety feature on the car, without a battery to dampen the current in the system, it may detect a spike, and kill the car so as not to do any damage anywhere else.


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Re: Is my alternator knackered? [Re: ] #540860
1st Jul 2011 2:55pm
1st Jul 2011 2:55pm
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tranmere
chris_gilly Offline

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Originally Posted by willo
Originally Posted by StuyMac
Is the alternator light illuminated on the dash? Does it go out after starting the car?
yes light goes out, after starting, but when engine is running if i use eletric windows, with headlight on lights go dom


check the earth straps and points, also check for voltage drop between wiring

Re: Is my alternator knackered? [Re: chris_gilly] #540863
1st Jul 2011 3:08pm
1st Jul 2011 3:08pm

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willo OP
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battery is kicking out 12.4v,, and with engine running 14.6v
i have put new earth straps on, and fitted extra.

Re: Is my alternator knackered? [Re: ] #540873
1st Jul 2011 3:31pm
1st Jul 2011 3:31pm
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Wirralshire
StuyMac Offline
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Then its probably running as it should - normal running wont involve the battery being removed whilst the engine is running wink


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Re: Is my alternator knackered? [Re: ] #540980
1st Jul 2011 9:34pm
1st Jul 2011 9:34pm
Joined: May 2011
Posts: 1,304
Wallasey
TheComputerLab Offline
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As far as I was aware all electrics are run from the battery nit the apt, if you have fir example an electrOnic choke, lift and fuel pump then yes the engine will die through lack of juice and to too it the distribution cap will be electronic. In new cars fook knows but in old cars it is straight forward

Re: Is my alternator knackered? [Re: ] #540986
1st Jul 2011 9:46pm
1st Jul 2011 9:46pm
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Posts: 13,588
Birkenhead
diggingdeeper Offline

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The regulator circuits inside an alternator are designed to "assume" there is a battery connected which in itself acts a as a sort of regulator against peaks and troughs in the output voltage.

The alternator is a rectified AC generator not a DC generator and so does not produce a stable DC output without a battery to "smooth" the supply voltage.

Running without a battery is very dangerous as surge voltages may occur which could destroy the ECU etc.

Presumably when you switched the headlights on, the trough in the output voltage was insufficient to maintain the regulator circuit within the alternator and hence the alternator switched off.

Last edited by diggingdeeper; 1st Jul 2011 9:50pm.

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