UPDATED: Diode for alternator inline from ignition switch

Crosley.In.AZ · November 03, 2010, 08:47:05 AM

What is the diode needed to wire in from the ignition switch to GM alternator excite wire to prevent engine run - on ?

I want to use a LED light for alternator feed wire and it seems a diode is needed to prevent run -on with the LED lite

model a vette · November 03, 2010, 09:07:50 AM

An LED is a "Light Emitting Diode" so I think you would not need another diode. A diode is a "one way" electrical device so the current will only flow in one direction so it also matters which way you hook up an LED for it to light properly.

rooster · November 03, 2010, 10:53:47 AM

the short wire is the - . If you look into the LED the short wire has what looks a martine glass on that short leg,. Not sure about the way you are using it.

enjenjo · November 03, 2010, 10:58:29 AM

I would try it without a diode. As Ed said, it should work without it. Just make sure it passes enough current to excite the alternator.

tom36 · November 03, 2010, 02:53:57 PM

I used these
http://www.radioshack.com/product/index.jsp?productId=2062591
when an electric fan was powering up the alternator keeping the engine running after shut down. Tom...

wayne petty · November 03, 2010, 04:18:11 PM

http://store.alternatorparts.com/461873-anti-feed-back-harness-wdiode.aspx

this prevents the ignition from being powered by the reverse flow of current from the alternator lamp driver circuit...

the alternator light in the dash is connected to the alternator on one side.. and the switched ign 1 on the other.. this strange feedback can be enough to power the ignition system on some cars. preventing the engine from shutting off...

Crosley.In.AZ · November 03, 2010, 09:27:31 PM

I had read online (I know it may wrong) that an LED lite would not stop the flow of electrons enough and engine run on was possible. I have not hooked it up yet...

This little LED lite I have, may not be setup for alternator use ? I don't know..... I am not even sure where I found this LED lite.

I want to add the excite wire to my 1 wire alternator on my Dodge Lancer and have an idiot... er, I mean Informed driver lite for indicator too.

thanks guys

model a vette · November 03, 2010, 10:19:49 PM

You can test your led with a meter set to test continuity. If the led test ok for continuity one way and "open" with the meter leads reversed, it should be ok for your application.
If it tests good for continuity in both directions it is not really an led and won't work for you.

enjenjo · November 03, 2010, 10:38:17 PM

If you do need a diode. I use 2 watt diodes from Radio Shack, about $1 a pair.

Crosley.In.AZ · November 04, 2010, 09:44:33 PM

OK .... some testing this evening.

I hooked this LED I have up to a 'keyed" 12 volt source under the dash. The red lite comes on... yet the alternator is not eggcited to produce voltage with the LED leads hooked up either direction.

If I blip the throttle and the alternator comes on line, the LED lite goes out.

II hooked up a 12 volt lite side marker single filiment bulb up to the same 12 volt source and the alt.. the lite goes off soon as the engine is started and the alternator is online per my voltage gauge.

in either case the engine did not continue to run when the key was turned off.

enjenjo · November 04, 2010, 09:59:43 PM

It appears that your diode does not pass enough current to excite the alternator.

Crosley.In.AZ · November 04, 2010, 10:47:06 PM

Quote from: "enjenjo"It appears that your diode does not pass enough current to excite the alternator.

yep , looks that way.

story of my life

PeterR · November 05, 2010, 10:01:42 AM

Quote from: "Crosley"
Quote from: "enjenjo"It appears that your diode does not pass enough current to excite the alternator.

yep , looks that way.

story of my life

Try this. The 560 ohm resistor limits the LED curren to a safe level and the 100ohm resistor will pass additional current to start the charge.

LEDs sold specifically for 12V come with the 560ohm resistor already installed in the mounting bezel (represented by the dotted line).

Crosley.In.AZ · November 05, 2010, 09:33:27 PM

Peter... thanks for the diagram and thoughts..

It seems to me I bought a bunch of LED lites in raw form. I was gonna add some LED lites to my Goldwing trim on the saddle bags.. I need to see where I put those LED lites

Crosley.In.AZ · November 09, 2010, 02:36:07 PM

I thought I would add a qucik update....

WZJUNK John offered to send me a diode for my alternator set via snail mail.

I must admit I had wondered where the letter was, cause of the time that had passed.

See the photo of the letter I received today. It was in a clear bag with USPS label that basically apologizes for the problem ... LOL

Looks like a fork lift tire got ahold of the letter. The shiney stuff is tape holding the paper together

I will wire up a test and see what I got going here