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Tell tale dash lights


zhap135
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As I understand it, one side of the bulb is wired to the 12V ignition supply and the other side to the alternator.  So when you turn on the ignition the bulb as 12V on one terminal and 0V on the alternator side do it lights up.  At the same time the current flow goes through the alternator exciting coils to earth.  Once the engine starts the alternator starts up and puts 13V+ onto the other side of the bulb, so with only 1V across the bulb it goes out (probably also has diode in the circuit to eliminate any possible flickering at the low voltage).

So if your light is not coming on when you switch on the ignition then either the bulb is not getting a 12V feed (which I think you have checked) or the feed from the alternator is not 0V.  If it is wired to the correct terminal on the alternator I would think it is an internal problem with the alternator.

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4 hours ago, alanrichey said:

As I understand it, one side of the bulb is wired to the 12V ignition supply and the other side to the alternator.  So when you turn on the ignition the bulb as 12V on one terminal and 0V on the alternator side do it lights up.  At the same time the current flow goes through the alternator exciting coils to earth.  Once the engine starts the alternator starts up and puts 13V+ onto the other side of the bulb, so with only 1V across the bulb it goes out (probably also has diode in the circuit to eliminate any possible flickering at the low voltage).

So if your light is not coming on when you switch on the ignition then either the bulb is not getting a 12V feed (which I think you have checked) or the feed from the alternator is not 0V.  If it is wired to the correct terminal on the alternator I would think it is an internal problem with the alternator.

Thanks, that sounds like that explanation makes sense - will try that.

(couple of hours later...) Having tried, the light goes on on ignition, off whilst turning over, and then remains on when the engine has started. That would suggest that there is an issue with the alt, and as it happens when placing a vm over the battery terminals when running there is no change in the reading to when not running.

So looks like I need to whip teh alt out and bench test and maybe replace

 

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Yes, with the engine not running the battery voltage is about 12V, once the alternator kicks in the voltage should be 13.5.-14V.

Going off while cranking is normal, the system will switch non-essential stuff off to ensure full voltage to the starter motor.

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3 minutes ago, alanrichey said:

Yes, with the engine not running the battery voltage is about 12V, once the alternator kicks in the voltage should be 13.5.-14V.

Going off while cranking is normal, the system will switch non-essential stuff off to ensure full voltage to the starter motor.

Thant's fantastic, thank for that Alan, and for the tip on the correct wiring!

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Try bigger wattage bulb in alternator feed wire temporary before stripping alternator off, I had same problem with ignition on - blue thin wire to alternator is +ve to one terminal of warning light and other terminal of warning light is to -ve, when the engine starts and alternator running the bulb 'excites' the alternator and the blue wire changes to -ve ,hence the light goes out, if the alternator belt snaps etc the alternator stops charging and the blue wire returns to a +ve condition illuminating ignition light, sounds complicated but really easy if you break it down

 

Edited by Pintogogo
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19 hours ago, Pintogogo said:

Try bigger wattage bulb in alternator feed wire temporary before stripping alternator off, I had same problem with ignition on - blue thin wire to alternator is +ve to one terminal of warning light and other terminal of warning light is to -ve, when the engine starts and alternator running the bulb 'excites' the alternator and the blue wire changes to -ve ,hence the light goes out, if the alternator belt snaps etc the alternator stops charging and the blue wire returns to a +ve condition illuminating ignition light, sounds complicated but really easy if you break it down

 

Thanks Pinto, will have a look

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23 hours ago, Pintogogo said:

Try bigger wattage bulb in alternator feed wire temporary before stripping alternator off, I had same problem with ignition on - blue thin wire to alternator is +ve to one terminal of warning light and other terminal of warning light is to -ve, when the engine starts and alternator running the bulb 'excites' the alternator and the blue wire changes to -ve ,hence the light goes out, if the alternator belt snaps etc the alternator stops charging and the blue wire returns to a +ve condition illuminating ignition light, sounds complicated but really easy if you break it down

 

Hi Pinto, sorry can I just run that again. Now I understood from some of the other responses that the thin blue (+ve) is goes to one side of warning ( a LED in my case), and the +ve from the ignition switch goes to the other When then the alt is working then the potential difference is going the other way, and as the telltale is a diode nothing lights. If the Alt is not working or working suboptimal, then the PD is lower at the alternator end allowing the light to illuminate. This seems to make sense to me, but doesn't marry with what I understood that you have just said - have I mixed up what you were saying. I understood that you say the other terminal to the ALt light should go to earth?

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Posted (edited)
On 4/25/2021 at 12:23 PM, alanrichey said:

As I understand it, one side of the bulb is wired to the 12V ignition supply and the other side to the alternator.  So when you turn on the ignition the bulb as 12V on one terminal and 0V on the alternator side do it lights up.  At the same time the current flow goes through the alternator exciting coils to earth.  Once the engine starts the alternator starts up and puts 13V+ onto the other side of the bulb, so with only 1V across the bulb it goes out (probably also has diode in the circuit to eliminate any possible flickering at the low voltage).

So if your light is not coming on when you switch on the ignition then either the bulb is not getting a 12V feed (which I think you have checked) or the feed from the alternator is not 0V.  If it is wired to the correct terminal on the alternator I would think it is an internal problem with the alternator.

Have jsut bench tested the Alt, and I seem to be getting about 15-16v accross the battery when running with a drill. When the drill stops teh voltage someimes drops back to 12, but sometimes sustains at this level. Thios seems bvery odd to me, 

  1.  because i understand that the V should be about 13-14
  2. because the voltage sometimes stays high

Does this seem familiar to you?

Edited by zhap135
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Does seem to be a bit excessive, maybe the voltage regulator (which also converts the AC to DC) is playing up.  Can’t explain the voltage not dropping off.

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12 hours ago, alanrichey said:

Does seem to be a bit excessive, maybe the voltage regulator (which also converts the AC to DC) is playing up.  Can’t explain the voltage not dropping off.

Mmm, yes. I guess it's a replacement if that's the case. The battery is new. However the volt meter is old, proper old one with a needle which seems to be moving freely. Wonder if I should invest in a new one.....

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9 hours ago, Pintogogo said:

Hi sorry its my fat fingers late at night, other side of lamp should go to Ign switch +ve ....sorry

Brilliant , Thanks for the clarification Pinto👍

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