warning light-part 2- trying to understand

[Dany]

hi chaps

 

i'am still trying to resolve my warning light issue. i dropped a email to caige to get the tacho wiring diagrame, and they advised me to put a resistor because the probleme may be that my alternator is not build to be connected to a LED warning light

here is the answer from caige:

 

Hello,

Some alternators need a bigger load to switch, ie they have been designed to

switch a lamp on and off, rather than a low current led.

Please obtain a 68 ohm, 4 watt resistor and connect it between the warning

light input to the speedometer, and switched ignition.

This should allow the alternator to correctly turn off your led.

Best regards,

Ian

 

 

i have bought the resistor, but i really don't understand where it should goes.

can anyone point me toward the right direction?

 

cheers

[revilla]

Basically you are trying to make the low-current LED look more like a high-current bulb to the alternator. The LED will have one end connected to a wire from the alternator (Brown with Yellow Stripe) and the other end connected (probably inside the dash unit you have) to a battery supply which is switched through the ignition switch and passed through a fuse (Green Wire). You need to connector the resistor in the same way, so one end to the alternator warning wire (Brown with Yellow Stripe, probably easiest to connect it somewhere near where the wire goes into the dash unit but anywhere will do) and one end to an ignition switched fused power supply (probably ANY of the Green wires at the back of the dash instruments will do, but check against a wiring diagram for your car to be sure). I've drawn a quick sketch diagram to show what you are trying to achieve - basically you are adding the red bit to the existing black bits:

 

http://www.revilla.plus.com/Alternator%20Warning%20Light.png

 

Hope that helps!

 

PS: The write colours above are correct on a 2003 K-Series; whilst fairly standardised they may be different on your car, you need to look at the appropriate wiring diagram to be sure.

 

PS: If the ignition is left on without the alternator running for any length of time, the resistor may get quite hot. Make sure it isn't touching anything sensitive or plastic. If it is the type that has a metal case with a bolt hole, bolt it to the aluminium bulkhead behind the dash to keep it cool.

 

PS: DON'T connect it directly to the battery, missing out the ignition switch or it will just get hot and drain your battery even when switched off, and DON'T connect it to an unfused (White) wire for safety reasons.

[elie boone]

I could be wrong but would the alternator go the way of least resistance ? if you would like the alt. to think there is a bulb wouldn't you need to put the resistor in between the led and the alt ?

[revilla]

No the resistor will be in parallel with the LED as I drew it; for example if the alternator then pulls it's output terminal to ground to turn on the "bulb", there will be 12.6V across the resistor which will then draw 12.6V/68R = 185mA current. The LED will then have 12V across it and will draw about 20mA in parallel, so 205mA in total. 185mA at 12V dissipates 12V*0.185A = 2.22W in the resistor, but the alternator will see the total current of 205mA which is equivalent to 12V*0.205A = 2.46W. So the alternator will see this as approximately a 12V 2.5W bulb load.

 

Actually I simplified the diagram a bit, the LED as shown is a "12V Type" that has built in series resistor. This series resistor will have a value of around 520 Ohms, so with 12.6V across the combined series resistor and LED and an LED forward voltage of 2.2V there will be 12.6V-2.2V = 10.4V across the series resistor giving a current of 10.4V/520R = 20mA through the LED.

 

PS: I've updated the diagram now to show the series resistor. Elie, your arguments of "least resistance" is partially correct. MOST of the current will flow through the path of least resistance, however A LITTLE will flow through the higher resistance path - the current will be split between the paths in inverse proportion to the resistance as the voltage across each path will be the same. This is exactly what is needed - A LITTLE current to feed the LED but a LARGE current to keep the alternator drive happy.

 

Edited by - revilla on 22 Jul 2014 17:35:27

[Dany]

thank you both for your advices. will try this asap

 

cheers

 

[Dany]

hi revilla

you were right on everything! that works a threat!

thanks very much for your help

cheers

[revilla]

Glad to hear its all sorted 😬

[BBL]

Thank you very much, revilla! My 2006 7 has had the alternator warning light (a.k.a. alternator light) on whenever the engine is running since new. I thought I had read on here that this is a known issue with a fairly simple and straightforward solution, but could not easily locate anything with a search. edit -- Until now.

Thanks again!

 

Edited by - BBL on 25 Aug 2014 17:23:55

[revilla]

No problem, just happy that I was able to post something of use to somebody

[Jonathan Kay]

...as if it were the only time!

 

;-)

 

Jonathan