LED alternator warning light

bigdaddykarl · Post by **bigdaddykarl** » Mon May 25, 2009 8:18 am

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?

Speedy Jim · Post by **Speedy Jim** » Mon May 25, 2009 1:28 pm

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/

bigdaddykarl · Post by **bigdaddykarl** » Tue May 26, 2009 8:32 am

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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Speedy Jim · Post by **Speedy Jim** » Tue May 26, 2009 10:11 am

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

bigdaddykarl · Post by **bigdaddykarl** » Tue May 26, 2009 10:19 am

Thank you. I'll let you know how it turns out.

bigdaddykarl · Post by **bigdaddykarl** » Wed May 27, 2009 8:44 pm

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...

bigdaddykarl · Post by **bigdaddykarl** » Thu May 28, 2009 8:49 pm

Circuit works like a charm, and the alternator is charging. Thanks for the help!

zazothex · Post by **zazothex** » Wed Jun 24, 2009 12:40 pm

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?

Speedy Jim · Post by **Speedy Jim** » Wed Jun 24, 2009 2:19 pm

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/

zazothex · Post by **zazothex** » Sat Jul 04, 2009 1:49 pm

Thanks much, Jim. I'll let you know if it works next time I have a spare minute to try it

MNAirHead · Post by **MNAirHead** » Mon Jul 06, 2009 6:13 am

ZAZ..

Thanks for the post.

timmy

zazothex · Post by **zazothex** » Mon Aug 10, 2009 10:18 am

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.

chayes · Post by **chayes** » Thu Aug 27, 2009 11:33 pm

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 ?

Post by **david58** » Fri Aug 28, 2009 4:32 am

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.

Speedy Jim · Post by **Speedy Jim** » Fri Aug 28, 2009 5:32 am

Can you post a pic of you Alf or Gen?

Alf:

Gen:

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