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LED alternator warning light

Posted: Mon May 25, 2009 8:18 am
by bigdaddykarl
I've read some articles - I think on other websites - about some difficulty with using a LED to replace the stock warning light in the speedo. I don't have a stock speedo, and I have LEDs for all my warning lights. Will I not get a charge from my alternator because of the minimal load from the LED? Also, how would I wire it up correctly, I'm a little confused with the diagram for the stock light, it's almost as though it's backwards... can someone educate me on how exactly that circuit works?

Posted: Mon May 25, 2009 1:28 pm
by Speedy Jim
You are 100% correct; the LED light will probably not allow the alternator to begin charging.

My sketch below shows how to overcome this.
The 100 Ohm resistor will pass enough current to start the alternator.
The additional diode protects the LED against reverse voltage and also prevents engine run-on which can occur, especially with any kind of electronic ignition. An ordinary 1 Amp diode (200V rating) will suffice here.

I'll assume that the LED lights you have are rated for 12V operation and have some kind of limiting resistor built-in. If not, the light will need an additional resistor in series.

Speedy Jim
http://www.nls.net/mp/volks/



Image

Posted: Tue May 26, 2009 8:32 am
by bigdaddykarl
This is beautiful. Thank you. Just so I can wrap my mind around it, I tried to isolate the oil light circuit, do I have this correct?
My LED is from Watsons (if I recall correctly) and was for 12V. There's a white lead and a black lead. Does it matter which direction the current flows?
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Posted: Tue May 26, 2009 10:11 am
by Speedy Jim
Yes, your (nice) diagram is correct.

The LED won't work if connected backward. But I don't know what polarity Blk and Wh is on their LED's. Try it and see which works.

Jim

Posted: Tue May 26, 2009 10:19 am
by bigdaddykarl
Thank you. I'll let you know how it turns out.

Posted: Wed May 27, 2009 8:44 pm
by bigdaddykarl
So far, so good. Well, sorta. Rewired, turned the key, and on popped the light. Fired up the car, struggled to get her to run (that's another story entirely) and the light won't go off. I think it's because I've been cranking and cranking, and not able to keep the car running long enough to recharge the battery. She's on the charger now, so we'll try her again tomorrow.

Fixed my oil light circuit too, thanks to Speedy Jim's oh-so-easy diagram. Thank you!

Now if only I can get her to run right...

Posted: Thu May 28, 2009 8:49 pm
by bigdaddykarl
Circuit works like a charm, and the alternator is charging. Thanks for the help!

Posted: Wed Jun 24, 2009 12:40 pm
by zazothex
Bit of a bump from me: I'm running the stock light on a 75W internal alt. My gen light won't go off until my RPMs hit about 2600, right about the time my main jets kick in from the idles on the Webers. Once the revs get high enough, the light goes off, my volt meter begins to read ~13V. however, from this point forward, the Alt continues charging regardless of RPMs.
Any thoughts?

Posted: Wed Jun 24, 2009 2:19 pm
by Speedy Jim
That happens with some alternators.

The fix is to add a small resistor across (in parallel with) the dash warning light.

Try the 100 Ohm 2 watt resistor (or anything close to that). 100 Ohms will add 120 ma of excitation current initially which should be enough to get the alternator going even at low RPM's.

If you get run-on after adding the resistor, stick the diode shown above in the circuit.

Speedy Jim
http://www.nls.net/mp/volks/

Posted: Sat Jul 04, 2009 1:49 pm
by zazothex
Thanks much, Jim. I'll let you know if it works next time I have a spare minute to try it

Posted: Mon Jul 06, 2009 6:13 am
by MNAirHead
ZAZ..

Thanks for the post.

timmy

Posted: Mon Aug 10, 2009 10:18 am
by zazothex
uh..no problem

Haven't been able to try the fix yet; I'm sure if you do a search, you can find out that I've been trying to get my newly busted transmission sorted out. Priorities.

LED warning lite for Alternator

Posted: Thu Aug 27, 2009 11:33 pm
by chayes
I followed Speedy Jim's instructions and diagram to put an LED as warning lite. It worked well first try....but I see it charging my battery at almost 17 volts ! !
Any ideas of what I should do ?

Re: LED warning lite for Alternator

Posted: Fri Aug 28, 2009 4:32 am
by david58
chayes wrote:I followed Speedy Jim's instructions and diagram to put an LED as warning lite. It worked well first try....but I see it charging my battery at almost 17 volts ! !
Any ideas of what I should do ?
Welcome to the STF chayes. Can you post a pic of you Alf or Gen?
The regulator controls the amount of charge.

Posted: Fri Aug 28, 2009 5:32 am
by Speedy Jim
Can you post a pic of you Alf or Gen?

Alf:

Image



Gen:


Image


:lol: