Idiot Lights - LED and Standard Bulbs, Working Together

[CBDZ]

I’m looking for a little help on how to wire a second “idiot” light for both the Oil Pressure, and Alternator Charge lights. I have a stock speedometer, with the stock Oil & Alt bulbs installed. I also have installed in the dash two Large LED lights which would be more visible than the two speedometer mounted gauges.

Prior to mounting the two LED lights, both the oil and alt worked perfectly.

Now that I have mounted the two LED lights, I have two issues.

1. This is the more important of the two, but I don’t have a functioning charge system.
2. The oil pressure LED will light, but the standard speedometer incandescent bulb will not.

This is how I have them wired. What do I need to change in order to get proper function?

[Marc]

Diagram's not showing.

[CBDZ]

-----------------Fixed Photo Link-------------

[Marc]

Yes, and as I suspected you've wired them in series rather than parallel. Try this:


Note that sometimes enough voltage manages to backfeed from the alternator, through the LED, to the ignition switch (and thence to the coil) that the engine won't shut off when you turn off the key ....depends upon the characteristics of the LED and the coil. If you have this problem, get back to us - you'll have to go to Radio Shack (or open up some junk old transistor radio/whatever) to get a diode to wire in between the LED and the alternator...meanwhile, disconnect the - lead from the LED.

Unlike incandescent bulbs, LEDs are polarity sensitive - so if an LED doesn't work, reverse the leads going to it. If it's one that has its polarity marked, you want the + lead going to the ignition switch.

[CBDZ]

Thanks a ton. After you said that it needed to be parallel, I know exactly what I needed to do.

Will I need a resister at the LED for the Alt.? Will the current flow along the path of least resistance, and therefore bypass the Speedo light, causing a "not charging" condition?

I already have the diode at the Alternator.

[Marc]

Current will ALWAYS follow the path of least resistance, but in a parallel circuit there'll still be some current going through the high-resistance side (since the same voltage differential exists on both paths)...if the incandescent bulb doesn't light, I suppose you might have to add resistance in series with the LED but I don't think it should be needed in this application, pretty sure that's only a concern to make the flasher work when using LEDs as turn signals. I've never tried your setup before, please be sure to tell us how it works out for you.

The resistance of the stock warning lamp should be in the 100Ω ballpark. (R=V^2/W) ....so if it's necessary to add a resistor to balance the branch load, subtract the forward resistance of the LED from that to get a value to try. http://www.instructables.com/id/Determi ... s-of-LEDs/

[CBDZ]

I rewired in parallel WITHOUT the resister. The lights in the speedometer DO NOT light, and I am not getting a charging Alternator. Looks like I need to go hunt down some resisters.

It looks like I need to do something like this, but what is the diode doing? Is that the "after-run" diode?

[Dale M.]

Diode is "anti run on"... You are pretty close with "after run"....



Dale