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          My car was fitted with a small alternator when I bought it and it has worked perfectly until yesterday. Now the ignition light stays on permanently whatever the revs but the ammeter shows a good charge. I have checked the wiring and connections and all appears fine. Is this an internal alternator problem and could I bypass the ignition light and just keep a eye on the ammeter?

          Cheers Richard

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It does sound to me like an internal alternator problem, either the regulator, or one or more of the diodes which feed the rotor.

Cant see how anything else would leave the ign light on.

Bob

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include replacing the ignition light bulb on the list.

had friend with similar problem in non TR car and the ancient, failing ignition light bulb was the problem. When the clever electrics man replaced the bulb the problem went away.

Edited by littlejim
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          Thanks for the advice Bob and Jim. Ever the optimist I will use the car and see if it goes out as mysteriously as it came on. If not  I will try changing the bulb and if that doesn't work I will bypass it.

          Cheers Richard

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Had a similar 'fault' that turned out to be a bad connection.  Unplug/disconnect all external cables related to the alternator, clean and reconnect.  That method fixed my problem.

Cheers

Peter W

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In the case of the ACR type, then yes, the bulb is part of the circuit to initialy energise the rotor, once it starts to generate its own power the alternator provides its own energising current, & that is when the bulb should go out, as it has 12V (nom) on both sides of it. if one (or more) of the smaller diodes which provide this energisation is blown, then the bulb will continue to energise the alternator, & so will remain alight.

Richard. I would check the on-charge voltage to make sure it is not exceeding say 14.8V at highish revs on a charged battery, because if the regulator is damaged it could cook the battery.

Bob.

Edited by Lebro
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  • 2 years later...

Thanks "littlejim" from Canberra, for shining a light on this problem!

I have just replaced my 20th century glowing ignition light with 21st century one & hey presto it goes out when it should & of course comes on at start up! So now have a spare bulb?

A 18 ACR/45amp conversion on a TR2, charging a 10 year old boat leisure battery as my rebuild nears completion, when new tyres & battery will be on the list of must haves!

Well done Forum!

Ian

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Apologies for the slight thread drift, but my ignition warning light has recently stopped working but the charging circuit still seems to be functioning.

The car has been converted to negative earth and has an alternator fitted.

Is that likely to be just a blown bulb and if so, how can the charging circuit still work without the small load provided by the ignition bulb?

Rgds Ian

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The current through the ignition bulb just helps the alternator to start up, Ian.  It does that by passing a small current through the field winding in the rotor which produces a slight magnetic field to start the generation off. There will also be residual magnetism in the rotor from the last time it was run, and in some cases that will be enough to do the job but often the extra boost is needed.  Once generating, the alternator feeds itself with current through a set of diodes called the 'triplet' , so from then on the bulb is not important except as an indicator.   If the voltage coming from the triplet equals or exceeds the battery voltage, the light goes out. 

It sounds as though your alternator has enough residual to start up on its own.  You say it charges properly so obviously it is working, meaning the diodes inside must be OK.  In that case your lack of light is probably either a blown bulb or a bad contact. 

 

 

 

Edited by RobH
typo
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17 hours ago, RobH said:

The current through the ignition bulb just helps the alternator to start up, Ian.  It does that by passing a small current through the field winding in the rotor which produces a slight magnetic field to start the generation off. There will also be residual magnetism in the rotor from the last time it was run, and in some cases that will be enough to do the job but often the extra boost is needed.  Once generating, the alternator feeds itself with current through a set of diodes called the 'triplet' , so from then on the bulb is not important except as an indicator.   If the voltage coming from the triplet equals or exceeds the battery voltage, the light goes out. 

It sounds as though your alternator has enough residual to start up on its own.  You say it charges properly so obviously it is working, meaning the diodes inside must be OK.  In that case your lack of light is probably either a blown bulb or a bad contact. 

 

 

 

Hi Rob, I was thinking of putting a brighter bulb in my ignition warning light as present illumination is very dim. This would draw more current, so would there be an issue for the alternator field winding? I guess that once energized the voltage on either side of the bulb equates and hence it wiould go out - correct?

Thanks, Colin.

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It shouldn't be too dim Colin - have you tried a new bulb?   They do dim after many years of use because the glass blackens.  

If you want more brightness, you could use a 12v LED lamp in parallel with a 68 Ohm resistor.  The LED draws a low current but the resistor increases that to the same as a normal bulb. 

Yes the current for the bulb reduces as the voltage rises and eventually goes to zero. 

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15 minutes ago, RobH said:

It shouldn't be too dim Colin - have you tried a new bulb?   They do dim after many years of use because the glass blackens.  

If you want more brightness, you could use a 12v LED lamp in parallel with a 68 Ohm resistor.  The LED draws a low current but the resistor increases that to the same as a normal bulb. 

Yes the current for the bulb reduces as the voltage rises and eventually goes to zero. 

Thanks Bob and Rob. No I hadn't thought of an LED. I have lots of LED`s.

So assume I calculate a suitable series resistor for the LED at 12V dc, and then simply put a 68 Ohm across the the LED/resistor combination?

Colin. 

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6 minutes ago, RogerH said:

Hi Colin,

 do as Rob suggests first - replace the bulb

 

Roger

Roger, yes I was going to try that, but just wanted Robs feedback on the LED, in case it didn't work.

PS, its nice to see you back in the Forum

Colin.

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1 hour ago, c.hydes said:

So assume I calculate a suitable series resistor for the LED at 12V dc, and then simply put a 68 Ohm across the the LED/resistor combination?

Yes that's it. 

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Any commercial led replacement bulb will not need a current setting resistor, but you would still need the 68 ohm one to start up the alternator. I have that setup, & actually I would not bother now, I would just keep the old style bulb.

Bob

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3 hours ago, Lebro said:

Any commercial led replacement bulb will not need a current setting resistor, but you would still need the 68 ohm one to start up the alternator. I have that setup, & actually I would not bother now, I would just keep the old style bulb.

Bob

Understood Bob & Rob, thanks, Colin.

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