That little red light..

[wvcajun]

I am appealing to the collective genius of this great group of riders and mechanics. I have a '77 R100RS with about 80,000 miles. Today, the red "Gen" light on the cluster stays lit when riding. I gets dim while riding and brighter when idling. I notice no adverse symptoms, though. What does it mean and what should I do?

Please help.

MG

[srb]

The Gen light on below 1500 rpm is normal. If it goes out while you are riding, then everything is a-ok. If it stays lit at higher rpms then you have a charging system problem.

[jgr451]

diode board,alternator brushes,battery,poor ground,or other loose,poor or corroded connections....soon all go boom(in a limp sort of way)

[acehosedme]

My '92 PD was doing that a fwe weeks ago, it turned out to be the positive battery cable end had cracked and was barely making contact. The end got bent after sticking a gel battery in the bike, doing that meant I had to bend the terminal 90 degrees, weakening it.
Good luck.

[James.A]

If your diode board has failed, the generator light will get brighter, over time, as the battery discharges. Your battery can not absorb A/C, and dies a slow death. With a bad diode board and a fully charged battery it takes about 15 minutes of running for the little red light to start to glow.

[rocketman]

Deffinately need to resolve/find problem or you will find yourself running on battery power only and then..... it can be long push home! Check out all of the above and one should resolve the problem, those bikes just don't have that much to the electrical system.

RM

[Braddog]

My red gen light only comes on while idling, and only on initial startup during the first few minutes. After that, I normally don't see it, even at stoplights.

Since I was a little skeptical of the bike's voltmeter, I tested using my own voltmeter across the battery leads. Everything seems to be OK.

Mine does NOT stay lit while riding, so I think it would be wise to take some action.

[gambrinus]

I was just watching the debate last night and they said that the little red light meant "Time is up Mr. Kerry.."


RW

ps... How many of the 9 members of the Brady Bunch can you name? Now how many of the 9 Supereme Court justices can you name? --Jon Stewart

[James.A]

Additionally...... on my /5, an open circuit on the rotor should/will cause the anti-restart relay to disable the starter. I know the /6 and later starter relays are a different part number, but serve the same purpose. Therefore, based on my experience, if your starter engages, it is probably not the rotor. It took me a week and 3 cases of beer to figure that one out. That would leave the field winding and the voltage regulator. I've never had a stator field go bad. If you want to send me an e-mail, I'll give you some test proceedures.

ps. I can name all nine members of both groups, what's my prize?

[wvcajun]

Thanks to all who responded. I 'll have fun trying to diagnose this thing since I am a rookie to electrics and BMW's.

MG

[MikeC]

Motorrad has a good book about Boxer electrics.Its a little pricey but contains diagrams and troubleshooting tips.

[djkehoe]

Have an electrician, electronikker or electical engineer test your battery charging voltage. 13.5-14.5 is charging. <13 is not charging. These are DC voltages. AC voltage present at battery means bad diodes. Since you notice a change, something has happened. May be bad brushes, connection, diodes or lost winding. Good luck.

[The_Veg]

OK so what does it mean if my light NEVER comes on? I've already determined that the bulb is good, and that every component and wire in the system is individually good. The battery seems to charge, as several long hard rides running acccesories within pushing distance of home seems to indicate. Everything is apparently right except for the light never coming on.

[Weasel]

Mine came on but I saw where the horn wire had chafed on the front engine cover. Wrapped some electrical tape around the wires and bent the horn forward a bit. Red light is out. I will say the light gives off a dim glow at speed, but it's done that for the last 20 years or so with no problem. It's hard to see it in the day time, but in a tunnel I can see it there. It's charging, though...

[wvcajun]

I think that's what's happening to me--it's charging but shows dim.


MG

[James.A]

Here is the location of a very good post on Airheads.org on this very topic.

http://p220.ezboard.com/fairheadtechforumfrm12.showMessage?topicID=15.topi c

[nhlkats]

If your diode board has failed, the generator light will get brighter, over time, as the battery discharges. Your battery can not absorb A/C, and dies a slow death. With a bad diode board and a fully charged battery it takes about 15 minutes of running for the little red light to start to glow.The GEN lamp on my /7 is dim when idling, and gets brighter and brighter as you increase throttle. what does that mean?

[James.A]

If your battery is getting weaker, it's probably the diode board. cjack from Urbana is the true genius on this stuff. I expect he'll have an answer for you later today. Good luck.

[manicmechanic]

I'd be betting on the diode board.

[nhlkats]

yes the battery *seems* to get weaker, it is less than 6mo old. dry cell. westco.

the reason i say *seems* because the bike has trouble starting in the mornings, which could be anything==> knowing the bike has had an "aftermarket ignition", and "creative wiring" done to it before i purchased it last summer also adds more options to debug. even when the bike is on the charger [tender pending purchase] all night long, often the bike would have trouble starting the next morning. sometimes the trouble starting seemed from lack of battery power to "turn over" the starter. sometimes it had sufficient power to turn over the starter, but nothing "caught", no combustion. sometimes it would seem to have enough power, but just barely, to turn the starter very slowly and delayed-like. these symptoms lead me to believe there is more than one culprit here. sometimes the battery will drain with the bike off. perhaps the aforementioned "creative wiring" done by the previous owner parisitcally leaks?

sorry for the piss poor descriptions, i dont know how to properly investigate and report these issues [yet]. what i do know is that i dont take the bike more than a few miles from my house, because it has, on multiple occasions, not started after i leave base, leaving me stranded.

any help is much obliged! :D

[Polarbear]

Well, start with most common failure and consider bike with 80k and no previous work to electrics. Miles considered, brushes are very suspect and very common failure at this mileage. Rotor is number two on list; ever notice all the rotor adds in "ON" magazine and about 95$ gets you one, compared to about twice that or more from your friendly BMW dealer. Shop wisely for rotor. Rotors are common failure at this age bike. No. three is probably a damaged diode board and these don't fail nearly as often, without some mishandling by mechanic. They can be easily shorted out with sloppy workmanship, taking cover off front of motor, etc.. A few other odd things mentioned can come up, as well. The ones above are most common, in order,imo. The brushes are probably the most pain in the neck to change! You'll see...You can test them, before removal. The light on dash is not to come on any above 1200 for most cases. The indicator light detects voltage from rotor to battery, not the other way around. The BMW volt guages are pretty accurate, to about half volt, imo. These systems are not rocket science, but require some very basic electrical knowledge. Happy trails,Randy

[James.A]

The first thing you need to do is restore the battery to full charge. It should read 13.5 + or- volts across the terminals when fully charged. If it achieves this goal and the starter still turns slow, your starter needs help. As an example, my R75/5 is blessed with a 4 year old battery and to get a sufficient roll, I have to park it in the sun for a few hours to get everything warmed up. I know I need a fresh battery AND a starter re-build, but I am living with it for now.

With respect to your gen light, the visible response to RPM (light gets brighter when the motor revs) indicates that your alternator is generating a signal relative to the speed at which it turns. Your alternator is probably O.K. That would leave the diodes or the voltage regulator.

I have never owned anything newer than a /6, but on those bikes it is necessary to lift the tank to get to the VR. It is attached to the right side of the frame backbone. The test for the VR is to jump it out. It has a 3 spade connector that looks like this;

? ?_
? _ _

Put a jumper wire between the 2 recepticles which are side by side. Start the motor. With the tank off, you have about 30 seconds of running time before the carbeuretors run dry, so the motor needs to be warm when you try the test. If the light does not glow, the trouble is with the VR.

There is a bench test for a diode board, but I don't know how to perform it. I always keep a spare that is known-to-be-good on hand. Always disconnect your battery negative (-) terminal before removing the front motor cover to access the alternator and diode board.

Creative wiring is never a good thing.

[pmdave]

On the /7 models, the alternator requires a 12v current to "excite" the coils so they can charge. The "exciting" supply to the alternator coils is through the red "generator" light. When the alternator is producing a voltage higher than the battery, the red light goes out (because there's 12v approximately on both sides of the light). If the red light illuminates at road speed, it indicates the alternator is not generating voltage as high as the battery, or the voltage regulator has failed and the alternator is overcharging (but an overcharging /7 alternator is an oxymoron).

If the red light illuminates dimly, it hints that the alternator is charging somewhat but there is insufficient voltage. Likely causes are a diode board with a bad diode, a bad ground between diode board and engine case, worn alternator brushes, loose connections, a bad voltage regulator, or corroded voltage regulator contacts. Red light on with engine at road speed = bad.

The red light should come on bright when power is switched on but before the engine is started. If the red light does not come on, the bulb could be burned out, or the pins to the cluster corroded, or the flexible circuit board damaged. There won't be voltage to the exciter coils. However, if the alternator, diode board, and regulator are still good, it may be capable of producing a charge with an inop red light.

With the engine above about 2,500 rpm, the alternator should be charging, the diode board turning AC into DC, the voltage regulator keeping voltage within limits, and and the battery being charged. It would be nice to see voltage above 13.5v after being generated, rectified, regulated, and blessed by Matt Parkhouse and Paul Glaves. Note that the stock BMW voltmeters are not especially accurate, which is why it is wise to use a good voltmeter at the battery terminals to see what the actual charging voltage is.

The /7 airhead charging system is uhhh, "weak" even if in pristine condition. It needs both tender loving care and some modifications to work reliably. Once a year or so, clean all contacts and lubricate. (Vaseline is acceptable) Clean the battery terminals, top up with distilled water, and apply acid-proof grease to the terminals. Make sure the ground strap and all grounding points are clean and snug. Once every 5 - 8 years, disassemble the instrument cluster, clean the circuit board, replace any damaged bulb sockets, and change the bulbs. Matt Parkhouse keeps reminding you of these things in "Keep 'em Flying" in ON.

Replace the alternator brushes before they get too short, and replace the three-wire bundle between alternator and diode board if the plugs appear to be getting burned or corroded. Consider using an electronic voltage regulator. Ensure that the (brown) ground wires to the diode board are clean and snug, and think about adding additional ground wires attached to the screw hole in the engine case behind the diode board.

The /5 airhead charging system is similar, but the lights in the dash have less complex connections, and the diode board is mounted straight to the engine case without rubber isolators, so it gets a good ground and even a bit of heat transfer to the case. For whatever reasons, the /5 system seems to work better than the /7 system, even with a lower (theoretical) output and a smaller battery.

etc. etc. Or ignore all of the above and get a good manual on the charging system that provides both maintenance techniques and troubleshooting tests.

pmdave

[OscarMayer]

what if the red light doesn't come on at all???

Unless I touch the little blue wire from the front of the aternator to the aluminum housing - not until then do I get a red light on the dash.

What then???

:|

[bmwmick]

Could be a failing regulator OR field winding. Look at the circuit here:
www.buchanan1.net/charge.shtml

[kbasa]

what if the red light doesn't come on at all???

Unless I touch the little blue wire from the front of the aternator to the aluminum housing - not until then do I get a red light on the dash.

What then???

:|

Check the rotor for an open. It has two rings. Slide a matchbook cover under the brushes to isolate them from the rotor. With your multimeter check for continuity between the two rings. If you don't have continuity, your rotor is toast.

You can get a replacement for $99 from Motorrad Elektrik. You'll need to tool to pop the old rotor off. The tool is cheap.

[20774]

You'll need to tool to pop the old rotor off. The tool is cheap.
And important!! Otherwise you'll ruin your day. :nono

Kurt in S.A.

[kbasa]

And important!! Otherwise you'll ruin your day. :nono

Kurt in S.A.

Indeed. It's just a little piece of welding rod, cut to the appropriate length. I'm sure there's a pennytech tool out there somewhere, but it's almost easier to just order it with the rotor. :ha

[bmwmick]

Airhead Rotor removal tool on the cheap:
http://www.airheads.org/content/view/194/98/

Do NOT use a welding rod. :nono