Lucas W3 IGN warning light

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GaryCullen
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Joined: Wed Nov 20, 2019 1:34 am
Location: New Zealand

Lucas W3 IGN warning light

Post by GaryCullen » Wed Jan 19, 2022 10:59 pm

I need to repair a couple of these W3 IGN warning lights.
The ones I have, have damaged insulated damping resistor wire so I cant establish the correct ohmage.
From memory I think about 26 Ohms.
For authenticity sake I would like to replace with new resistance wire, I can get some measuring 63 Ohms/ per meter so just under half a meter seems a reasonable amount to rewind onto the bulb holder.
Anyone know the original resistance?
cheers

Gary
Auckland

6350_William_French
Posts: 96
Joined: Wed Nov 20, 2019 10:27 am

Re: Lucas W3 IGN warning light

Post by 6350_William_French » Thu Jan 20, 2022 11:34 am

I think the original ignition warning light bulbs were 2.5v and carried 0.2 amps.
If you used these bulbs the resistance should be 47.5 ohms. However finding these bulbs now is not easy and Holdens do an ignition light replacement which looks the same but uses an LED. Might not be original but it looks correct
That is assuming your car is a 12v?
from Bill

Rod
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Location: New Zealand

Re: Lucas W3 IGN warning light

Post by Rod » Thu Jan 20, 2022 10:03 pm

You’re 100% correct Bill.
2.5v 0.2A Lucas C252A globe.
Gary-Jaycar have MES 2.5v 300mA globes in stock (p/n SL3210), if you're fabricating your own ballast resistors anyway, you can alter the value to suit the increased wattage.
Last edited by Rod on Thu Jan 20, 2022 11:36 pm, edited 1 time in total.

GaryCullen
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Location: New Zealand

Re: Lucas W3 IGN warning light

Post by GaryCullen » Sun Jan 23, 2022 1:48 am

Thanks Rod and Bill.
I have a 6v and a 12 v one to fix up.
The bulb 💡 that was in the 6 v lamp is labelled 6v!
With a 30 ohm resistor on a 6 v supply it is way too bright.
I will experiment with the resistance value and get to run a bit dimmer.
Cheers
Gary

Rod
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Location: New Zealand

Re: Lucas W3 IGN warning light

Post by Rod » Sun Jan 23, 2022 3:53 am

The original values should be something like this:
For a 6v system and 2.5v 0.2A globe:
* resistor needs to drop 3.5v (6 - 2.5v) @ 0.2A *
R (value of resistor in Ohms) = 3.5/0.2
R = 17.5 Ohms

For a 12v system and 2.5v 0.2A globe
* resistor needs to drop 9.5v (12 - 2.5v) @ 0.2A *
R = 9.5/0.2
R = 47.5 Ohms (as Bill correctly stated)

For 300mA globes I calculate the R values as 11.7 and 31.7 Ohms respectively (rounded).

I am surprised a 2.5v globe (I assume this is what you're using) is brighter on a 6v supply with nearly twice the resistance in circuit than what Mr Lucas specified- unless perchance you've inadvertently got it in parallel with the globe? Its been a while since I've messed with these things so I'm not 100% certain this is even possible?
Also remember, (if by chance you're measuring the resistance of the globe itself) that the filament itself has a positive temperature coefficient and if you're looking for accurate results you need to put it in circuit, measure the current and use the old V=IxR to get an accurate R value.

GaryCullen
Posts: 54
Joined: Wed Nov 20, 2019 1:34 am
Location: New Zealand

Re: Lucas W3 IGN warning light

Post by GaryCullen » Sun Jan 23, 2022 6:27 pm

Thanks Rod, great explanation.
The bulb in the 6 v lamp was a 6v labelled one.
I am wondering now if a PO re wired it wrong. There is evidence of it being repaired previously.
Cheers
Gary

6350_William_French
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Re: Lucas W3 IGN warning light

Post by 6350_William_French » Tue Jan 25, 2022 7:32 pm

Hi
Just out of interest why did Austin originally go to the trouble of putting a 2.5v bulb in the ignition light when a 12v one would do the job perfectly adequately without the need of the resistance? I am sure someone will tell me!
from Bill

GaryCullen
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Location: New Zealand

Re: Lucas W3 IGN warning light

Post by GaryCullen » Tue Jan 25, 2022 10:19 pm

My assumption is that they ( Lucas ) did not want a too bright 12v or 6v lamp on the instrument panel... too distracting. But bearing in mind once the engine is running and generating volts the IGN warning lamp goes out.
Running a 2.5v with a dropping resistance ( wire wound around the Lamp holder ) gets you a low watt and fairly dim light.
Enough to be seen in daylight and dim enough at night not to be distracting..
My other thought is that they also wanted to use a small lamp holder in the instrument panel and make it easier to change the bulb when it blows without having to take the instrument panel apart.
The W3 type ign lamp holder uses a press in bulb base and I have seen both screw and bayonet type 2.5V bulbs used. The red plastic lens pushes against the top of the bulb to hold it in after you have screwed the bevel into the holder. Running a higher wattage bulb would melt the lens... i have seen this too.

just my thoughts.
cheers

Gar

Rod
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Location: New Zealand

Re: Lucas W3 IGN warning light

Post by Rod » Wed Jan 26, 2022 8:06 am

The most common explanation I've seen is that Lucas wanted to make the warning light as small as they could, and at the time there was no automotive globe suitable for the application. Instead they used a 2.5v torch globe (i.e. for a 2 x D cell torch) which combined small size, availability, low cost, robustness (by virtue of the low voltage filament) and low power. As Gary points out the globe is retained solely by the lens which can, and does melt if the wrong wattage is used. The correct 2.5/0.2A globe operates at its rated voltage thanks to the ballast resistor. The same body can be fitted with different colour lenses for different applications of course.
Its always described as an "ignition warning light" in this application. As ammeters are almost without exception fitted in conjunction with the light I surmise the ignition warning is its primary function. I suspect its origin goes back to the early days of Delco system ignition where the manufacturers were concerned about the consequences of leaving the ignition on if the points happened to be closed. The flat battery would've been of little consequence compared to the inconvenience and expense (at the time) of replacing a burnt out coil.

peter_christie
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Location: Newton Mearns, Glasgow

Re: Lucas W3 IGN warning light

Post by peter_christie » Wed Jan 26, 2022 11:34 am

American cars of the period described the purpose of the warning lamp as [Generator Not Charging] which really describes its function.
Peter C. [6515]

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