At 51,000 miles, I noticed the alternator light on my ‘92 ‘GS glowing at idle. Time for a new diode
board (again), I thought. When I removed the board, I found that both of the upper rubber mounting posts were broken–the stud end bolted to the board had come unbonded from the rubber. It was clear from the discoloration of the metal that those studs had gotten very hot. This occurrence is not uncommon as many Airhead owners know, but others who have written on the subject seem not to have noticed what I have.
The problem has been discussed extensively, but my observation suggests that solid mounts and a
bunch of ground wires with soldered lugs are not really going to fix it.
In the world of electricity, current times resistance equals power. If the resistance is in a bad
connection, power is wasted there as heat. Power from the alternator is supposed to replace the
power supplied to the bike and therefore keep the battery charged. But the bad connection I refer
to is at the point where the diode board is bolted to the studs.The voltage drop there causes the
the regulator to flog the alternator more to maintain the desired voltage at the battery. And that makes even more heat at the bad connection.
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Diode boards generally fail by having one or more diodes ‘open’ internally, “open’ means they do not exist electrically. Rarely do they short-circuit. There are small diodes on the diode board with two types of functions. Early boards, such as on the /5, did not have as many diodes as later boards, the later boards had a stator windings center-tap, which used the extra diodes to improve output wattage. Failure of any of the small diodes will cause strange output problems. Usually, a diode board failure is at the solder joint of one or more of the six large power diodes; or, that diode opens. It is quite rare to see shorted large diodes on the board. If a large diode solder joint becomes lousy or opens or the diode itself opens, the alternator will still produce current, but it will be quite considerably lower in total wattage output. The typical result of one large diode opening (or, solder joint failure) is that the battery will show near proper voltage during riding, but when the headlight is turned-on, the voltage will sag rather considerably.
Diode board testing and information about repairing the faulty Wehrle-manufactured ones (I’ve seen a few Bosch boards with the same problems with non-bent-over large diode leads and soldering failures) is covered in depth in the June 1999 issue of AIRMAIL, an article by the late great BMW Guru Oak Okleshen. Airhead owners may want to read the article. The Oak-recommended & difficult to do board modification could be done, with considerable effort, for reasons of solder joint failures, more likely if you have a R100 engine, which develops more heat (worse if an RS or RT). I’ve had good luck with a much simpler method, which is the removing of the ‘paint’ with gel type paint remover (often it has no effect though and abrasion is needed). I always enlarge the solder pad area first, whether the gel remover works or not, by careful scraping with a sharp Xacto thin blade hobbyist knife (don’t remove copper material), & then re-soldering using a high temperature solder (50-50 plumber’s solder and rosin flux) and a extra hot soldering iron. No re-painting or re-coating is needed, but certainly could be done. My repair method is vastly easier to do because drilling the PC board & adding wires & soldering per Oak’s method is quite difficult & best done by total disassembly, which Oak did not get into and is a huge PIA! Yes, it can be done with long tweezers or forceps, but that is quite tricky. I cannot recommend Oak’s method, and I won’t even do it myself. I’ve also seen several Bosch branded boards with this problem, perhaps Wehrle made them? There have been some other Bosch and Wehrle labeled products that made me think either could have manufactured them.
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Many are intimidated by electricity, and the amount of electrical problems with Airheads can be a sore point. Most often problems are due to lack of regular maintenance, particularly cleaning and tightening various connections, checking alternator brushes, changing the battery before a catastrophic battery failure; and general old age and mileage. This is really not much different from other motorcycles. If properly maintained, the Airheads electrical system works fine and is reliable. If, however, the bike is ridden mostly in stop and go city traffic, the charging system may not keep up, and the battery might require nightly re-charging. The Smart type of chargers are especially convenient for that purpose.
The stock charging system starts producing usable amounts of electricity at ~2000 rpm, but upwards of 4000 rpm may be required for enough output to take care of lights (including extra lighting), ignition, heated clothing, accessories, and have electricity left-over for re-charging the battery. Aftermarket alternators of higher output are available that fit inside the timing chest area.
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