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1976 TR 6 Ignition light on dim - brighter at accel

 My ignition light is on dim at idle. It gets brighter with acceleration. Everything electrical seems to work fine on this 2 1/2 year old restoration. The alternator charges well. I replaced the rectifier in the alternator. No change. Thoughts anyone? 

 

 

Edited by Pete in NY
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Hello Pete - welcome to the forum.  Two questions.

1 - Is this something which has suddenly started happening or has it always done that since the restoration ? 

2 - Is it a standard Lucas alternator ?

 

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Thank you both for your responses.

Rob - I don’t think the light was on initially following the restoration. I built the car and I’m quite observant. Having said that it’s a sunny weather car and driving with the top down (always) the light is not easy to see. I actually noticed it recently pulling the car into the garage one night after dark. A rare occurrence. 

The alternator is the original Lucas 18 ACR unit. As mentioned I just replaced the rectifier which had no effect. 

Mike - dumb question - is there a known listing of body earths? I mean all of the obvious dedicated ones are clear and solid. Beyond that - where would I start? Could/should there be an earth someplace just for the ignition light?As mentioned everything electrical works and well. Just this one isolated light. 

From what I have read the circuit can be effected by a faulty diode in the rectifier. That’s why I replaced that. However most reported the light would go out at acceleration. Mine gets brighter with acceleration.

I’m stuck.

 

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The ignition light connects between the feed to the alternator field coil (internally from the diode triplet*) and battery positive. On starting it provides a small current to the field which allows the alternator to wake up and start generating. As the alternator output increases with revs, the voltage rises until both ends of the bulb are at the same potential so of course the light goes dimmer then extinguishes.

 For the light to get brighter again the alternator would have to be generating excessive voltage at the triplet. That is usually due to a problem with the output diode stack (live side diode open circuit)  but you would have to be unlucky for the new one to have the same fault as the one you replaced. Do you have a suitable meter to check the diodes with?

A further possibility is a bad connection or fault inside the regulator itself which might cause the same thing to happen. 

*There are two sets of diodes in the alternator. A set of six which form the main rectifier stack and another smaller set known as the 'triplet'  or 'triple' which provides field current. The regulator compares the output from the main rectifier with the reference and adjusts the field current to maintain the correct voltage. If the main rectifier is faulty so the the output voltage is low,  or if the regulator for some reason does not see the correct output, the regulator will increase field current to try to bring the voltage up and in doing so will give excessive output at the triplet, resulting in the symptoms you have seen.

Edited by RobH
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