R1150 Alternator Start-Up Fix & AFR
The other day while taking some AFR data on cold start and warm up I noticed that my alternator light was on. I haven't noticed it before but after going back through a year's worth of data I can see that it has happened several times in the past. After a couple minutes the alternator started on its own and the light went out. Or if I blipped the throttle the alternator came on-line and the Batt light went out. So I might have lived with it by blipping the throttle. However:
During the time the light was on, there was no alternator output and the battery voltage was at the Motronic was about 12V. When the alternator came on-line the voltage jumps to 14V.
Then I noticed something very interesting. During warm-up when the voltage was at 12V, the mixture was about 8% leaner than a moment later when the voltage jumped to 14V. Thats too lean and i could feel it in a slightly rough idle. The Motronic sensed the lower voltage and lengthened the injector pulses, trying to compensate but in Open Loop it didn't compensate enough. If the alternator stayed off line until the bike went closed loop, the AFR was at the correct level before and after the step up in voltage. This means that the Motronic closed loop algorithm could fully compensate for the low voltage. That was lesson 1 for me.
So then I tried to understand why the alternator wasn't self starting. I for some feedback that many bikes do this. The starting current for the alternator flows though the Batt bulb on the Instrument Panel. I decided to try a resistor across that bulb and found that with a 220 ohm resistor across the bulb, the alternator aways starts.
The bulb is 1.7W and let's about 140 mA through the alternator rotor to start it. Adding the 220 resistor boosted the start-up current by a mere 50 mA for a total of 190 mA. BTW if the Batt bulb burns out the alternator may not start, it depends how much residual magnetism is in the rotor. Lesson 2.
Looking at the parts list, it originally specified a 1.7W bulb. But that is crossed-out now and a 3W bulb is now called for. So to get at the Batt bulb I first had to pull the High Beam bulb (they're both in a knuckle-busting location). As luck would have it, the High Beam was 3W even though the Batt bulb was only 1.7 W. So all I had to do was swap the bulbs, its hard to tell the difference of brightness. I plan to carry a 3W spare (Toshiba A14V3W).
Running the numbers, the magnetizing power with the 1.7W bulb was about 75 mW. The 3W bulb boosts that to 225 mW--a big jump
So if your alternator doesn't start on its own you might want to check the Batt bulb and see what it's power is.