The alternator feeds the diode board through a three-wire connector. The wires in this harness are quite thick (2.5mm or 16 gauge black) and carry AC current to the diode board (bridge rectifier). Check the wire ends for signs of overheating, replace if in doubt of the condition of wires. Bench measurement of resistance with an ohmmeter may not reflect performance in operation. Remove and examine the diode board itself. Look for signs of melted solder or measure the diodes with an ohmmeter to determine that they pass electricity only in one direction. The diode check function in a multimeter or test light is a better test, as an ohmmeter may not sufficiently load the diode to reveal a problem. Visually check the soldered connections between the diodes and the diode board. Does the surface of the diode carrier have a crinkly appearance from overheating? Replace the board if anything looks questionable or face the likelihood of later failure on a long ride. If your diode board is rubber isolated, always replace the four rubber mounts and two ground wires when the diode board is removed for inspection or replaced. If your diode board is solidly mounted, carefully clean all paint or corrosion from the aluminum timing chain housing where the diode board grounds to the housing before reinstallation. Note which mounting holes are grounded and which are insulated. You should also add supplementary ground wires to the crankcase from the grounded mounts as described below (A new supplementary ground wire set will soon be available from BMW under part number #12 31 1 468 013.) If your diode board is rubber mounted, it will have two grounding wires connecting the diode board to the timing chain housing. First, remove all paint and corrosion from the timing chain housing where the wires make contact. Then clean and resolder the crimped connections on the factory ground wires and large red B+ wire. (We re-tin even new wires.)
It is best to run a third wire from the diode board mount back to one of the ground-ended mounting bolts at the starter to get the best possible connection from the diode board to the crankcase itself (We use a factory grounding wire and open up one terminal connector hole a little for the starter bolt.) These wires are specified l.25mm or 16 gauge brown. This third wire is important to insure a good connection between the timing chain housing and the crankcase itself, especially on motors with painted timing chain housings, as on most later motors. The new supplementary ground wire set will make this easier. The main positive B+ wire (4mm/6 gauge red) from the diode board to the starter also needs inspection. This wire carries the output from the alternator and diode board to the starter and hence to the battery. If the ends look cooked or the wire seems old and brittle, replace this wire! While you arc replacing it, you should clean and inspect the main positive lead (16mm red) from the starter back to the bat7tery, checking the ends for corrosion or oxidation, especially at the battery connection. Check also that the large connection nut at the starter is properly dean and tight. We use a multimeter test to determine the condition of these wires. Voltage drop from the B+ wire to the battery may be measured either with an accurate multimeter/voltmeter in parallel from the diode board to the positive terminal of the battery. A good wire will have a voltage differential of l volts; not so good wires will show a greater difference, perhaps .4 volts or more. This test may reveal faulty wires that seem okay when checked with ohmmeter. The short negative lead from the battery to the transmission breather/ speedometer cable bolt needs to be dean, properly tightened and free of corrosion. If in doubt, replace it. If you over tighten the transmission breather bolt (easy to do) you will strip your transmission cover!
The wires to the regulator can be quite oxidized without having much effect on charging efficiency since they serve a switching function only, but the regulator itself must turn the charging system off reliably at the desired cut-off to replace the harness connectors, or in rare occasions the entire harness. The alternator itself must be in good condition, with clean slip rings, good solder connections between the brushes and the brush-holder, and brush springs in good condition. New alternator brushes measure about 16.5mm in length. The rotor should measure about 4 ohms resistance measured across the slip rings. A failed rotor will have a broken wire and usually show infinite resistance across the slip rings. 'Bad' or open rotors are the second most common part failure, after diode boards. Occasionally a rotor will test okay on the bench and fail under operating conditions. If the slip rings are dirty, they may be cleaned with very fine emery or crocus cloth.
Did you know that if your red alternator light does not come on when you switch on the bike, your charging system is probably not working? The alternator exciter circuit passes battery voltage through the alternator bulb and the rotor to ground. No alternator light means no current to excite the alternator. If the light does not come on, it usually means either a bad bulb (cheap) or a bad rotor (expensive). Less often, it can mean a bad bulb socket, broken harness/connector or corroded instrument circuit board. The importance of good test equipment and the necessity of a logical approach cannot be overemphasized. A good Fluke Multimeter or other accurate and properly understood electronic tester is absolutely necessary if you are going to diagnose a problem and trace it to a defective component or circuit. Remember: if you have not tested a component and proved that it is working, you cannot assume that it is okay. just because it is the part you most recently replaced or because it looks fine does not mean that it works. Read your manuals and do not do anything if you do not have a logical reason for what you are doing. If your dealer is understandably reluctant to accept easily damaged electronic components back for refund over the counter if you discover that swapping parts randomly does not fix the problem.
Finally, all your good efforts will go for naught if your battery is not maintained properly. This means using only distilled water for filling, not filling more than the correct amounts (overfilling a battery has the effect of discharging it), keeping terminals clean and tight, and above all charging the battery regularly with an external charger capable of at least 2 amps charging current.
Occasional charging is very important to keep the rest of your charging system healthy., Many charging system problems originate when owners ride around with an old, sulfated, nearly discharged battery, forcing the charging system to work at maximum output all the time. Your charging system requires a battery in good condition with a proper charge level in order to operate reliably. Failure to maintain your battery in a fully charged state or unwillingness to replace it before it fails completely put a tremendous strain on your charging system. Riding your bike frequently no substitute for regular charging with an external charger of suitable type. A new service bulletin, No. 12 019 93, from BMW of North America, validates the points made in this article, and describes a new four-wire harness specifically designed to improve the ground circuits as described above. The part number for this new harness is 12 31 1468 013. Good luck with your charging system. When properly serviced and maintained, the BMW charging system is as reliable as any on the market. If you have any questions, do not hesitate to call our Service Department at California BMW, 415-966-1183.