Using LED as Alternator Light?

My car is a 1973 MGB, using a GM C130 type alternator. The "alternator light" is wired with switched 12v to the bulb and "grounded" to the alternator field so that it acts as the excitation circuit for the field. This is a pretty normal lashup from what I understand...although with all electronic dashes these days there's probably a different way to do it now.<G> The bulb is a 2W 12v BA7 bayonet incandescent. Changing the wattage of this bulb will affect the operation of the alternator. For my question I am assuming this to be accurate. Several of the indicator lights are corroded enough to make the bulb un-removable (especially since they are sunken) or they are flakey. New bulbholders of the original design are no longer available. Many of the possible replacement items are "throwaway" units that include the light and the holder in one sealed piece. It blows out, you toss it and buy new for $5 or so. That's not a problem. What is a problem is that most of these units appear to have LED "bulbs" and those that are incandescent don't seem to have any information available regarding the wattage. Now with one of them in hand I can measure the resistance and figure wattage at 12v. But I'm gonna bet they will not be 2 watt and... therefor the question (sorry for the leadup but I wanted to make clear what I am trying to do). 1) If I use one of the incandescent lamps and it is measured & calculated to be 3w, can I go to Radio Shack and get a 24 ohm resistor and put it in series with the lamp (to change the 3w resistance of 48 ohm to that of a 2w's 72 ohms? 2) any LED lamp would, I assume, already have the appropriate resistance to be plugged in to 12v wiring, but it is not going to have the resistance to make the alternator work. ...so can I simply put a 72 ohm resistor in parallel with the LED (such as between the + and - leads on the lamp)? or?

If memory serves me the light is only on when there is not output from the alt. so any 12v rated buld or led should work.

No - the charge-light also provides a trickle or tickle current to excite the rotor in older alternators. The rotors may have enough residual magnetism to begin charging without the tickle, but one day they won't. (I too once thought it was only an indicator. Unfortunately I found out the hard way - an irate Kerrigans customer a few days after my work...)      In other newer alternators/regulators - typically single wire D+ and 2-wire S&L types - the charge light may also be required as a signal to turn on the alternator's regulator. However LEDs might provide enough current for this, but it's not guaranteed. Though the safest bet is to use a resistor equivalent (eg, 47R 5W or 82R 2W), it may work reliably with a 150R 1W or 330R 1/2W resistor. Keep in mind that the resistor can be anywhere - even at the alternator (regulator), it merely does between the L or D+ terminal (charge lamp) and switched IGN +12V. Not that it matters, but I presume the alternator is a Delco CS-130 with 4 terminals? (SFLP, even though in practice it is effectively a 2-wire S&L type. F (Field) is unused, and P (tacho) is irrelevant). I recall one Pommie conversion thread where they were described as being "self exciting". It is - provided you have the charge light or equivalent (ie, it is not self exciting!). At least the older style external-regulator type was upgraded. The newer all-in-one alternator-regulators are the only way to go, not only to overcome certain destructive earth/ground fault situations. And alternators with the "remote Sense" S terminal are the best option (not D+ or Ig & L with no S). PS - and since clever motorists have a voltmeter (as per the 3-digit unit I added to the dash in my 1965 vehicle) it's easy to tell if the resistor value is too high, or broken, or some other charging problem, or even if the battery is at its end of life....

Thanks. Since I have 3-4 of them, I think I'll try using a 2w bulb as the resistor in parallel with the LED light and just let it hang in back of the dash. I'm curious to see if the LED unit "works" like the bulb (i.e. goes out when the applied voltage is equal at each leg and goes on when it is uneven). I have several people telling me it won't accept current coming from the alt so the light won't see equal voltage applied to each leg and thus won't go out. Easy enough to see, I guess.

Isn't it cute when car enthusiasts think they are so correct? I suppose next they'll tell you the oil pump determines the oil pressure? Whilst it's true that the LED won't "reverse conduct" and hence "accept current" from the alternator, that's not the issue. The LED/bulb lights when its IGN +12V side is +12V and the alternator/regulator side is 0V (gnd). When the D+ or L alternator side goes to +12V there is 0V across the LED/bulb so it extinguishes. It's a case of the voltage drop across the device, not "accepting reverse current". FYI only... Note that in some vehicles, charge lights may be dimply lit even when charging. That depends on various factors, but it was not uncommon to have ~1V - 2V across the charge lamp (enough to be seen at night). The problem is that that low voltage might be enough to significantly light LED - especially if red. But a white or blue LED with its knee voltage somewhere above 2V should not be a problem.

Well, thanks a bunch! I was wondering along those same lines (if it looses ground it goes out or...?) this afternoon. Quite pleased so hear that this is how it should work. I will be ordering a number of the LED things from Rock Auto (closeouts for a buck or two each, from Dorman) tomorrow. As far as incorrect opinions....well, I've suppose I've disseminated a few (too many, I think) in my time as well--all based on my particular (or peculiar) experience or on what I thought I saw or knew or what I had painted myself into an idea corner on. I thought I was right. I wasn't. And in the few cases, I've been prodded gently to the correct understanding. In most cases, it was less gentle but no less educational. Thank you again!

Oh yeah, I've been wrong too, and learned from mistakes. LOL - to again quote Charlie Brown after Lucy said "You learn more from mistakes..." - "That makes me the smartest person in the world". Yep - I am reasonably smart . I find however with my local fellow car (club) members and other auto-associated people that they seem intent on pushing their so called expertise.    I on the other hand am a bit gentler - I merely believe that everyone has a right to MY opinion! And though I may be diplomatic in stating what IMO is right or wrong - and knowing that often there are simply other methods that have their advantages etc - ie, nothing is impossible, and the unbelievable may actually be correct - I am occasionally amazed at how I have misunderstood or been on the wrong track for decades. (Though my oil-pressure comment earlier was a bit of a tease, my recent ~6 year stance on oil-pressure being a totally wrong method to control fuel pumps (for engine protection, and to meet Regulatory & Safety requirements) is more to the point.) That's my ramble anyway. Maybe a few car-related future considerations, but more so to support your behavior. (Welcome to the club!) But heed your feelings and thoughts, and discuss them when suitable.     To quote someone else on this forum "There is no such thing as a stupid question. Only stupid answers." (I think that's in their signature.) As to discussing "stupid" ideas or misunderstandings, I think more of what can come out of it (apart from clarifying the specific issue). But people that know or have experienced "brainstorming" should know what I mean (unless brainstorming sessions have been subject to ridicule {by idiots)). Back to the OP - a lamp or equivalent load "in parallel" somewhere is all that's required. Resistors don't blow like bulbs, but bulbs are easier. Just make sure the resistors or bulbs do not overheat anything - though that's more of an issue when people require dummy loads for bulb-type flasher-cans when using LEDs instead.    And BTW - I am really glad you have upgraded to a "new" style of alternator. I can never go back to the old styles. And the extra 70A capability is quite nice too!

If you want to elimnate the bulb you can swap out the alt regulator for a 1 wire one, if thats what you want to do.

AFAIK, D+ alternators always require a bulb. Besides, they are not as ideal as types with an S Sense wire which accurately sense the battery's voltage no matter where it is (and charging systems are geared to battery requirements). The single-wire D+ type can only monitor its own output voltage - and hence to a max of 14.4V etc - even if there is a significant voltage drop to the battery.