Indicator warning light led

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Jonathan Kay
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Have you changed the ignition warming light to an LED?

But it’s soluble either way. 

Jonathan

randybugger
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Yes, I've done some reading and it seems I need a resistor in parallel. 
I tried wiring the original bulb in parallel to see if it would work, the LED now goes out but flickers on and off at tick over. The original bulb doesn't do this. Will a resistor cure this?

Jonathan Kay
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Andrew's notes on the subject.

It would be nice to know how many wires are connected to how many terminals on the alternator, but this should work regardless of that.

Jonathan

randybugger
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Thanks Jonathan

read through that post but can't seem to find a resistor the right value anywhere.

3 wires on the alternator.

 

Jonathan Kay
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... you've got a spec but can't find the product... or you can't find the spec in the archives?

Jonathan

randybugger
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It says a  40Ω resistor but I can only find much lower numbers? 

revilla
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Hi randybugger,

If you want to replace the alternator warning light with a regular LED properly you will need to do it as shown below:

The 470 ohm resistor limits the current to the LED to around 20 milliamps. Standard red LEDs only require about 2.2 volts across them, connect them directly to a 12 volt supply and they will burn out immediately. You can buy "12V LEDs", these simply contain the 470 ohm resistor or something similar internally. If you get an LED that is designed to work directly on 12 volts then just miss out the 470 ohm resistor. If you get a standard red LED then you will need it. This resistor will be dissipating around 200 milliwatts when the LED is lit so it doesn't need to be anything particularly highly rated. Something like this (470 ohms, 500 milliwatts) will do the job: https://www.mouser.co.uk/ProductDetail/Vishay-BC-Components/SFR25H0004700FR500?qs=sGAEpiMZZMtlubZbdhIBIDRM2xq4Qtv%252B9Fj8WK950Ss%3D

The 40 ohm resistor feeds excitation current to the alternator to get it started reliably. The warning light circuit actually performs a dual function, the lamp lights to indicate that the alternator isn't charging the battery correctly but the alternator relies on the current feed through the bulb to provide the initial excitation of the magnetic field to get it to start reliably. An LED passes a much lower current than an incandescent bulb so without the 40 ohm resistor, the alternator is likely to become unreliable. WARNING: If you leave the ignition switched off without the engine running (so the warning LED on) this resistor will get hot (it will be dissipating 3-4 watts of heat) in the same way as a light bulb gets hot. You need a resistor with a power rating of at least 4 watts. Best idea is a resistor with an aluminium casing which can be screwed to the bulkhead behind the dash to dissipate the heat. I would suggest something like this (40 ohms, 5 watts): https://www.mouser.co.uk/ProductDetail/Vishay-Dale/RE60G40R0C02?qs=sGAEpiMZZMtbXrIkmrvidIpgiW9xqmWBX7%252B6ZL1Tbq4%3D

The 1N4001 diode is sometimes necessary; you may get away without it. It is there to prevent the alternator feeding current back out through the warning 40 ohm resistor when the ignition is turned off. Without this, sometimes people find they can't turn their engines off, they just keep running. There's enough current fed back through the warning system to raise the voltage at the ECU's "ignition sense" terminal above the threshold where it detects whether the ignition is on or off. Note that the diode should be black with a white or silver band around one end. This end is the cathode (NEGATIVE terminal, marked K in the diagram) and must be connected towards the alternator rather than towards the supply. Something like this would do the job: https://www.mouser.co.uk/ProductDetail/Taiwan-Semiconductor/1N4001G-K-R0G?qs=sGAEpiMZZMve4%2FbfQkoj%252BOpeCiqr8ESJlhwZD%2FsxJwA%3D

All of the above parts are from the same supplier (Mouser Electronics) and available to order in multiples on 1.

Hope that helps,

Andrew

SV VVC 170 - 170.4 bhp @ 7100 rpm - 142.4 ft.lb. @ 4900 rpm

randybugger
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Thanks Andrew, brilliant, 

 

Tony

randybugger
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Thanks Andrew, brilliant, 

 

Tony