The 446 LED headlamp bulb is a technological marvel, and considering its modest draw from the electrical system, it is a “must have” for almost all owners of vintage British or American motorcycles (who can forget Harley’s generator engines, they could really use this headlight). The chip is designed to allow either polarity grounding (+ or -) and at almost any voltage 6-volt, 12-volt, or even 24-volt. While this tech is hard to beat, some customers have had some problems getting their 446 LED headlamp bulb to operate with the integrated circuit chip (IC) as designed.
For those who have had issues with the 446 LED on their bike, every one tested in the shop has proven to be 100% working as designed. For our customers however, on some bikes the 446 LED headlamp bulbs did not work at all. These possible issues turned out to be:
handlebar switches,
grounding, or
corroded earth connections
On some bikes they went from full to dip beam, or from dip to off at high RPMs. This also turned out to be grounding issues regarding the spring clip holding the 446 into the light itself. The changing resistance of the vibrating ground caused the IC to constantly reset between 6v and 12v. It appeared to be a full/dip issue, but it turned out to be a changing resistance issue. However, the most common and problematic observation by some customers has been that they only run on full (high) beam and will not go to dip (low) beam.
A few days ago, a client dropped off a 1972 A65 BSA (Lightning model) due to it having charging problems. Also, in the shop, there is a 1966 TR6SR for a total engine overhaul (somebody did not set up their Morgo kit correctly). I took advantage of this situation to investigate the whole 446 LED kerfuffle.
First, focusing on the TR6, we removed the headlamp and installed the 446 LED bulb. I saw no problems at all. High and Low beams worked perfectly and were a lot brighter than the incandescent bulb that was in it. With a full battery charge, she was bright enough to blind someone.
Next, focusing on the 72 Lightning, we fully charged the battery and installed the 446 bulb. I discovered that, sure enough, only the high beam would work. Regardless of the switch position, the 446 bulb was always on full beam. I then removed the LED bulb from the motorbike and bench tested it. The 446 bulb worked like a charm. This did not make any sense.
The voltage from the two contacts on the bottom and top of the plug is completely different – even with different circuits on the harness but with a common ground (metal mounting ring). How can a bulb be that smart yet make such a complete mistake? Then I realized they were working correctly. They are so dang smart that they are too smart for their own good.
“They are so dang smart that they are too smart for their own good.”
No matter what the programming, no matter how the schematic is laid out, the fact is that electricity goes to the path of least resistance. This is a fact that will never change. And what is the difference between the 66 and the 72? All you Lucas fans (both of you), know the answer…the main beam warning light. Eureka!
Going off this hunch I unplugged the 281 green light (main beam warning) and sure enough the 446 bulb lit correctly. The main and dip switch worked as designed. Since the warning light is in constant contact (the filament) THAT is the path of least resistance. The IC chip in the bulb does not know that – only we do. The 446 LED bulb is too smart for its own good. It is taking voltage from either input and then sending it only to the main beam because the warning lights ground easily.
So, what is to be done?
The problem is the incandescent bulb allows a back-feed to the LEDs in the 446 LED. There are three ways to stop this back-feed. The most effective is to install a LED warning light bulb. Diodes and ‘Light Emitting Diodes’ work in a manner that is best described as a one-way valve for electrical flow. The illustration below explains how a LED warning light bulb will stop the back-feed (power-flow is highlighted).
The problem is the incandescent bulb allows a back-feed to the LEDs in the 446 LED. There are three ways to stop this back-feed. The most effective is to install a LED warning light bulb. Diodes and ‘Light Emitting Diodes’ work in a manner that is best described as a one-way valve for electrical flow. The illustration below explains how a LED warning light bulb will stop the back-feed (power-flow is highlighted).
The ones we offer here at The Bonneville Shop are negative ground. To use them with positive ground Lucas system you just switch the + and – wires at spade connectors on the 99-1209 warning light bulb socket for the correct polarity.
Once the back-feed is stopped in this manner the high beam, low beam, and warning light will work normally. The dip switch will be the only component completing the high beam circuit. The LED basically plugs a leak in the high beam circuit.
Another solution is to use a single diode to prevent this back-feeding. On a positive ground/earth system the Lucas 83225A/Triumph 60-7082 can be attached to the high beam warning light’s ground/earth spade connector, then the ground/earth wire attaches to the diode’s male spade connector. Personally, I’m not a big fan of this solution. It works but it’s a pain to find a decent diode and wire it up. Here’s a diagram with highlighted power-flow to show how this solution works.
The third solution is leaving the high beam warning light bulb out completely. The empty socket will also prevent the back-feed. The wires can be safely left connected to an empty socket.