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Another alternator question


geordie40

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Ok ive got all of my new dash working with the exception of the alternator/battery light.

It all works fine with the old dash (escort cluster) however I can't make it work with the new LED light, it won't come on when I turn the key and start the car but comes on when the alternator begins to working ( volt gauge goes from 12v on tickover then jumps to 14v once I rev the engine).

If I measure voltage with ignition on but not started the I get 6v between the chassis and the connector on the alternator but if I measure from battery -ve terminal to alternator I get 12v.

Am I missing something obvious.

I have read about battery light not going off and to put a tungsten bulb in parallel but I can't get mine to come on until alternator begins charging.

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Alternator bulb needs to light both ways.

Ordinary bulb does not care about polarity LED does.

You can get LEDs that are actually 2 side by side in one package that work both ways but sounds like that this is not what you have.

Operation of ignition light is switch ignition on engine is stopped, 12V from ignition switch to one side of switch and current flows from switch through light to power rotor of alternator. Light glows.

Start engine and current through light kick starts alternator which brings alternator connection up to 12V so now both sides of light at 12V and light goes out.

Stop engine by switching ignition out and alternator still providing 12V to light but switch side now pulled to 0V by ignition load. bulb will not provide enough current to keep sparks going so engine stops and light glows until engine stops when it goes out.

It sounds like you have one side of LED to alternator which only starts on residual magantism in rotor when revs build and the other side still earthed. Check that switched live brings non alternator side of to 12V before engine starts.

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(crossed with IanS)

It sounds to me as though alternator light is wired incorrectly.   It should have one wire going to an ignition switched 12V supply and the other wire to the alternator.  In this way, before you turn on the ignition both sides of the lamp are 0V so it isn’t lit.  Then you turn on the ignition and now the light has 0V on the alternator side and 12V on the other side so it lights up.  Then you start the engine and when the alternator kicks in you get about 13.5V on both sides of the light so it goes out.  

What you are seeing tells me the light wires go to the alternator and ground instead of 12V.  So when you turn on the ignition the light still has 0V both sides so doesn’t come on.  Then when the alternator kicks in you have 13.5V on one side of the light and 0V on the other so the light goes on.

So if I am right you just need to change the wiring on the light so it goes to a 12V ignition switched wire instead of earth.

Also the fact that the alternator doesn’t kick in until you run the engine tends to indicate it is not self-exciting and needs a current flow through the rotors before engine start.  And as you have seen from previous posts, an LED does not produce enough current.  If you look at the back of the Escort cluster you will that there is a resistor in series with the LED.  So you either need to add a resistor or wire a ‘normal ‘ lamp across the LED (And hide it behind the dash as Bob recommends) to increase the current flow.

 

Make sense ?

Edited by alanrichey
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A question which should be asked. What alternator have you got or more specifically what does the connector on the alternator look like. Daniel Brookes post shows examples. https://www.rhocar.org/index.php?/forums/topic/47403-alternator-led/

Many modern non lucas alternators need an ignition live and an alternator/ignition lamp.

Edited by Longboarder
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17 hours ago, Longboarder said:

A question which should be asked. What alternator have you got or more specifically what does the connector on the alternator look like. Daniel Brookes post shows examples. https://www.rhocar.org/index.php?/forums/topic/47403-alternator-led/

Many modern non lucas alternators need an ignition live and an alternator/ignition lamp.

Its a type 6 on the picture

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Type six needs more excitatory current to terminal D+ than an LED can supply, even if the LED is the right way round! Thus as has been suggested you need to run a resistor in parallel with the LED. A suitable resistor is an old fashioned ignition light bulb, 12v 5w. Crude pic below.

Ign light.JPG

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