LED replacement options for instrument bulbs

[nhmaf]

Interesting article someone on the ABC list made about changing over ALL the filament indicator bulbs in the dash panel and in the instrument housings to LED replacements, which are available in packages to plug into existing filament sockets, mostly.   Some of these filament bulbs seem to last almost forever, but the LEDs will certainly last even longer.   One potential pitfall that I see possible in these cooler and damper climates is that the LEDs used for backlighting the gauges may not generate enough heat to help evaporate moisture from the faces of the instruments, which is a common issue.   But for the indicators, they should be fine.

http://hstrial-wniglobalinc.intuitwebsites.com/BMW-LEDs.html

[Bob_Roller]

I tried LED replacement lamps for indicator backlighting a few years ago, unless they have come up with better lamps, I wouldn't waste the money, the indicators were not very well illuminated .

I do quite a bit of night riding, and the LEDs were not up to the task at that time .

I believe they were PIAA lamps .

I tried them in the turn signal indicators on the instrument housing, and they would wash out in bright sunlight .

Also, don't use an LED lamp in the "GEN" position, not enough current passes through it, to properly operate the charging system .

[nhmaf]

IF it is an array of several LEDs with integrated resistors, it might be OK in the GEN position, but otherwise I'd opt for putting a 330 ohm or 470 ohm 1 watt resistor in parallel with it (the ol' filament bulb bypass trick) to ensure sufficient current for the voltage regulator&rotor.   I think the ABC guy in the article had already performed the resistor bypass, which may be why it is indeed working.

[montmil]

Here's Snowbums take on the topic, including the correction he made today in a second ABC post:


There is a reason I do not like 12 volt LED's, even with a resistor in parallel,
in place of the GEN incandescent lamp. The reason is that the incandescent lamp has a
rather low resistance (about 48 ohms) when lit (LESS resistance when not lit, but that is not pertinent here). That low resistance, combined with the voltage regulator transistor being in full-on operation, means a relatively decent current for creating the magnetic field in the rotor is available at ignition key turn-on, and as rpm rises over idle.

It takes a certain rotor magnetic field strength to get TRUE charging to the battery started early. A lesser magnetism will delay charging.

If you used a 470, or 330 ohm resistor, and there is an LED lamp, the current is much less than stock (roughly 10 times). Charging will be delayed.

Those that would like to experiment and find out what I did, decades ago, can do it (even without an LED which typically runs at 10 to 20 milliamperes when lit).

Remove the incandescent lamp.  Substitute various resistor values, and monitor the system with a voltmeter at the battery, which is the easiest method. Determine at what RPM the battery just barely starts showing a voltage increase as rpm's are slowly increased.  Try various resistor values, perhaps start at a thousand or so ohms, and maybe go as low as a common 47 ohm resistor.

Replace the LAMP, and repeat tests without a resistor. Do report back your results. I did these sorts of tests back in the seventies, and repeated them in the eighties, and also again when the last lowest ohms rotors were installed by BMW. When I did these tests I monitored the diode board output with a current meter, and monitored rotor current too, but fancy testing is not needed for anyone to find out what I did.

continued on next reply post...

[montmil]

Snowbum continued:

Again...do report back. I made my recommendations; NO LED LIGHTS in place of the GEN lamp.
No, I'm NOT going to report on this list what I found out. Remember the old saw about teaching someone to fish?

SO....for the best charging characteristics (and ONLY for that), I can NOT recommend using an LED light for replacing the GEN lamp, unless the resistor value is quite a bit lower than even 330 ohms....and if so, it will get hot. If low enough (47 ohms, say) to be essentially equivalent to the 3 watt stock lamp, then the heat produced in the resistor is 3 watts.  That is a fair amount of heat, and if the idling period is long enough, that heat could damage something....especially if the resistor was mounted/soldered at the lamp socket.

NOTE that you CAN get adequate charging with an LED and a 470 or 330 resistor, the rpm needed for beginning output and adequate charging will be a bit higher. How high?...you report back.

Once the alternator is generating enough voltage/current, that is applied through diodes to the lamp side of the voltage regulator, and the lamp goes out, and the alternator self-energizes.  That's how it works.

Snowbum

. . . . . . . . . . .

All that said, I do use an LED with the add-on resistor circuit on my 1981 R65. I have noticed no problems with the installation.   Monte