A wiring challenge for Tim or Mike, ..or others if you understand this kind of stuff. Options

[ThePaintedMan]

I've been on a mission to add a second set of brighter, more visible idiot lights since this car is mainly a track car. I bought a set of 2 cheapo 24-led green/red boat lights from Ebay (seen here). I wired the green in line with the existing oil pressure light and it works perfect - very bright and obvious on startup until it shuts off with adequate pressure.

However, I'm still stumped as to how the alternator idiot light is wired and if the same thing can be done with the red one. Someone posted an excellent thread awhile back explaining how alt light is wired to come when the circuit is no longer charging. But I can't figure out how to wire this red light up to work with it in the same way that my oil pressure light works. Obviously when I wire the led light in parallel with the existing alt light, it doesn't work the same way, ostensibly to the added resistance? Wondering if it's even possible to add a second light.

Sorry if this is confusing. I'll take some pics tonight to try and illustrate my point, but as always, thanks for your help in the meantime.

[Rotary'14]

I read somewhere here (forgot which thread,,,) that you cannot use a LED for the alternator warning light. Something about the LED prevents the alternator from charging.

OK,, I found the thread,,
http://www.914world.com/bbs2/index.php?sho...p;#entry1948685



Just tossing in my .02

-Robert

[timothy_nd28]

Yes it could be done, but I think you would need to build a small circuit to make it work right. Usually people have the opposite problem, by replacing the incandescent bulb with a single LED, which doesn't have enough load to excite the windings in the alternator. You are going in the opposite direction, with a monster load of LED's. Do you have the spec's (current draw) on this clustered LED?

Another question, do you have a 6 cylinder in your 914? If not, why is it important to know when your alternator fails? I would think just having a bright oil pressure warning lamp would be enough?

[ThePaintedMan]

Yup, Tim got what I was saying. This isn't replacing the stock alt light, just adding another light in a different location.

Nope, no /6 in this car Tim, so having a light isn't crucial. I think the biggest reason to move it for me is that I can't see the stock light with the smaller diameter steering wheel I'm using. Again, not completely necessary, but I just wasn't sure if it was something that could be done.

Here is a similar bulb from SuperBrightLeds. They list their red 24 LED at 82 mA. Not sure how much that helps.

[Mike Bellis]

Add a relay. Trigger it by the existing idiot light. Ad a diode at this connection, this is you negative trigger. Wire the other side of the relay to 12V switched. run you new idiot light through the COMM/NO connections of the relay.

The relay will turn on when the key is on but the alternator is not charging.

Do not forget the diode!!!

[worn]

But I can't figure out how to wire this red light up to work with it in the same way that my oil pressure light works. Obviously when I wire the led light in parallel with the existing alt light, it doesn't work the same way, ostensibly to the added resistance? Wondering if it's even possible to add a second light.

As I remember it, the problem you face is that the light in the dash uses current running in both directions, depending on the situation. So you would need two LEDs polarized in opposite directions.

Turn the key on and the light filament provides current from the battery to the windings allowing the alternator to bootstrap itself up as the car starts. Then the output of the alternator roughly matches battery voltage and the light goes out. You need this for alternator function! A 1.8-2 watt bulb is needed or the equivalent conductance.

If the main diodes fail, then the alternator will stop producing output. If the secondary diodes are still OK the lamp will see a higher voltage at those diodes than it does at the battery and current flows in the opposite direction, from alternator to battery.

In my TR6, the blinker indicator light on the dash uses the actual blinker lamps opposite the ones signaled as a ground. The current from the indicator is not enough to light up the tail lights, so it works. It saved them a couple feet of wire, but no LED is gonna work easily there!

[worn]

Yup, Tim got what I was saying. This isn't replacing the stock alt light, just adding another light in a different location.

Nope, no /6 in this car Tim, so having a light isn't crucial. I think the biggest reason to move it for me is that I can't see the stock light with the smaller diameter steering wheel I'm using. Again, not completely necessary, but I just wasn't sure if it was something that could be done.

Here is a similar bulb from SuperBrightLeds. They list their red 24 LED at 82 mA. Not sure how much that helps.

That is LESS power than the dash lamp! You should be getting about 0.98 watts with those LEDs. The dash lamp needs to be 2 watts. I know this is an addition, not a replacement but puts it into perspective. LEDs are great!

If you go the relay route you do want to maintain the correct wattage (i.e. current) flowing to the alternator field windings. Too low and it won't want to start charging. Too high and you take over for the regulator.

[timothy_nd28]

(IMG:style_emoticons/default/agree.gif) We would need to know the exact current draw of the clustered LED you have. The original incandescent draws around 0.150 amps, I'm thinking the one you have draws around 0.082 amps?

[r_towle]

use this

[ThePaintedMan]

Okay, I clearly don't have enough electric knowledge to tackle this one. It seemed to me that it would be more complicated than I thought, and this confirms it. I'm not sure I would be able to find out enough about the LEDs to make the relay work either. No worries, but thanks for the help anyway guys!.... except Rich (IMG:style_emoticons/default/mad.gif).

[stugray]

One simple way you could do it:

Hook up the proper resistors to precharge the Alt field winding.
Then put in TWO idiot light LEDs,one pointing in each direction.

The stock alt light illuminates when there is an imbalance between the two sides.
If the dash voltage is higher than the Alt voltage, current flows TO the alt illuminating the light
If the dash voltage is lower than the Alt voltage, current flows FROM the alt illuminating the light.

If you put a LED pointing each direction, ONE of them will illuminate for either condition.
Just make sure the LEDs are predesigned to be excited by 12V (the will have current limit built in)

Whether, or not, you can excite the ALt windings with the an LED is a different subject.

[r_towle]

Okay, I clearly don't have enough electric knowledge to tackle this one. It seemed to me that it would be more complicated than I thought, and this confirms it. I'm not sure I would be able to find out enough about the LEDs to make the relay work either. No worries, but thanks for the help anyway guys!.... except Rich (IMG:style_emoticons/default/mad.gif).

maybe these might work better???

[timothy_nd28]

Actually this should work without building a circuit. The clustered LED should have a 12 volt power wire and a ground wire (not sure what color is which). If you tie the clustered LED's 12volt power wire to 12 volts, the red/white wire and you tied the LED's ground wire to the blue wire, it should work. Leave the idiot light installed, as this would still be needed by the alternator.

[ThePaintedMan]

Thanks Tim. I'll give that a shot tomorrow. Hopefully no smoke will follow. (IMG:style_emoticons/default/confused24.gif)