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Thread: Gen light on- WTF

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  1. #1
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    Gen light on- WTF

    77 R75/7 Gen light on full time. Bike has relatively low miles 23000. Gel battery only 1 year old.
    How do I diagnose this? I`m not sure where to start looking and do not want to start replacing parts unnecessarily.
    I`m in Leamington, not far from Michigan and do not know any airhead mechanics in the area. I`m quite willing to trailer it several hours for good service. Any leads or pointers appreciated

  2. #2
    Sir Darby Darryl Cainey's Avatar
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    1st thing to do is get this book from Motorrad Elektrik!


    2nd thing is read book.

    3rd thing, deduce step by step.
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  3. #3
    BeemerBoy terham's Avatar
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    Quote Originally Posted by Darryl Cainey View Post
    1st thing to do is get this book from Motorrad Elektrik!
    +1 on this book. I recently had gen light issues with my /5 and my '83 RS and in both cases was able to track the problem with this book as a guide. I had to replace the rotor on the /5 and the diode board/grounding wires on the RS. I also recommend the solid diode board mounts from Motorad Electrik as IMO1131 says. Both my airheads now have the solid mounts. If you follow this book, I don't think you'll be replacing parts unnecessarily and should be able to find the problem. Good luck. Let us know how it goes.
    Terence
    R75/5 R100RS K100RS R1100S

  4. #4
    Stage Crew beemerPhil's Avatar
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    Nice bike! 75/7s are a rare bird- not many were made before they punched it out to 800cc.

    If your gen light stays on all the time, even with the key off, you're probably looking at a diode board failure.
    If the light is on all the time, only when the key is on, suspect a rotor failure. This is much more likely, esp. on an older airhead.

    If you have an ohmeter, this is an easy test. Disconnect the battery ground and remove the front engine cover. You'll see that the alternator has two contact rings, or slip rings, that are contacted by a pair of spring-loaded carbon brushes. One of the brushes is grounded directly to the stator, or alternator body. The second is connected to a black wire- this comes from the voltage regulator under the tank. Unplug the black wire. Now turn on your ohmeter, zero it, if neccessary, and check the resistance between the two slip rings. You're looking for about 3 ohms; a little more or less is fine, but a lot less is a problem.

    Rick @ motorrad has rewound and epoxy-dipped rotors for around a c-note. I used to get mine rewound at an automotive electric shop in Seattle for cheaper, but that was near 15 years ago now!

    Replacement is simple. Disconnect the remaining wires on the alternator- it doesn't matter which way you put the 3-wire plug, that gets sorted out by the rectifier, so it'll work the same either way. Carefully- I use a small screwdriver under the wires- draw the brushes out against their springs until you can push the springs off to one side and remove the tension on the brushes. Now you can remove the 3 allen screws retaining the alternator stator. The stator is often a somewhat stubborn fit against the timing cover; a few GENTILE taps will help dislodge it. Be sure that you're getting the whole thing, and not just the aluminum front cover- you want the assembly to separate at the timing cover. When it comes free, set it aside on a friendly surface; there will be wire windings exposed at the back- they're pretty sturdy, but you don't need to damage them.

    Now to remove the rotor, which is a snug taper fit on the crankshaft. This takes a 6mm allen wrench and a good yank; if the engine wants to turn, you might want to put it in gear, but I've found that if you give it a sharp pull, the inertia of the rotating parts provides enough resistance to pop the screw free. Back it all the way out into your hand.

    Now to remove the rotor...

    If you're in the business, the rotor removal tool is a good thing to have; if not, you don't need one, it's an easy work-around. I've used a cut-off piece of a 1/4" drill, about 1 1/2" long. In a pinch, roadside, one of the small studs securing the valve covers will work fine. You put one of these into the hole you just took the screw out of, as a spacer, and put the screw back in on top of it. Tighten the screw, while holding onto the rotor with your other hand, and it'll pop right off.

    Reassemble in reverse order....

    Notes:

    If there's any sign of oil leaking past the front crank seal(behind the rotor) now's the time...

    Be sure the taper surfaces on the crank and inside the rotor are clean before they meet.

    IIRC, BMW wants 17 ft/lbs on the rotor screw, but tight enough is tight enough, if you have a good wrist.
    When replacing the stator, carefully guide it into the mounting flanges on the timing cover- it's a close fit- and walk it in with the screws, a bit at a time, to keep it straight. Don't over-tighten these; the stator housing is just aluminum, and the screw bosses CAN break off...don't put the springs back into position behind the brushes until the stator is seated. Position the brushes, and let the springs down onto them gently...


    Interestingly, Moto Guzzi uses the same charging system over these years. Some bikes came with the brush carrier mounted vertically above the rotor, and they found that road vibration was bouncing the brushes off the sliprings, reducing the alternator output. A service bulletin advised us to rotate the stator to an offset position, similar to where your brushes are, to eliminate this problem.



    YMMV- Your bike could be completely different from any other BMW, or I could be thinking of my old Honda 90....
    Phil Keppelman #20331
    MOA Rally Stage Manager
    The shortest distance between two points.............
    ain't how I got here......

  5. #5
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    Good airhead mechanic in Detroit area

    If your self-diagnostic efforts come up short, BMW Motorcycles of Detroit is a first-rate shop. They work on my '77 r100rs. They are in Sterling Heights, about a 35-40 minute drive from the tunnel. http://www.bmwdetroit.com/index.htm Good luck!

  6. #6
    Sir Darby Darryl Cainey's Avatar
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    Good Airhead Mechanic

    There is a good Airhead Mechanic in Stoney Creek Ontario.

    Tomas Roeder tomas.roeder@fkn.ca

    By appointment only!

    You never have responded to any of the advice you were given?
    Ambassador BMW MOA Ontario Canada
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    1977 R100RS, (Retired) 1993 R100GS (just getting started)

  7. #7
    Registered User lmo1131's Avatar
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    You are going to need to do some "wire checking"

    First thing... disconnect the battery.
    Turn your fuel tank petcocks "off" and remove the fuel tank.
    Remove the front engine cover (which is why you disconnect the battery first). Once the cover is off the bike, you can reconnect the battery... BUT disconnect the battery AGAIN before you replace the cover.

    You are looking for the obvious first - loose, broken, or corroded terminals/wire

    Here's a really good article on the subject by Kari Prager > http://www.horizonsunlimited.com/tec...ng-maintenance

    I'm not personally familiar with the /7. But I would be suspicious of the stand-offs (#8) that mount the diode board - if they are rubber isolation mounts you may have have lost "ground" to the board. A quick test would be to make a jumper cable and ground the board to the engine case. If they are rubber you should replace them with solid mounts from Motorrad Electriik.





    You can do this while waiting for the book Darryl suggested.

    Here's the wiring diagram > http://www.omnilex.com/public/bmw78/78r100wire.jpg

    EDIT - some more to read from Snobum > http://bmwmotorcycletech.info/diodebds&grdgwires.htm
    Last edited by lmo1131; 05-18-2013 at 06:50 PM.
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  8. #8
    Sir Darby Darryl Cainey's Avatar
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    There is a BMW Rally close to you this weekend.

    Go pick some Airhead Brains!

    Ontario, Canada

    28th ABC Rally

    Forest City Motorrad Club #159

    Forest City Motorrad Club of Southwestern Ontario will be hosting its 28th rally at the Springwater Conservation Area in southwest Ontario from May 17?20. The rally is located on Springwater Road, approximately three kilometers south of Orwell on Highway 3, and east of St. Thomas, Ontario (N 42.750230 W 081.03331O). Admission is $50, and all motorcyclists are welcome! This includes three nights of camping, showers, continuous coffee, campfire and prizes. Friday has a 5 p.m. setup and biker stew (contribute a can). Saturday has a dice run (win a motorcycle jacket), followed by an evening barbecue and entertainment. Sunday you can tour Elgin County and go on a wine tour. Monday is cleanup and farewells. Contact Rich Stevenson (johnnydundas@gmail.com) for more details.
    Ambassador BMW MOA Ontario Canada
    President Niagara BMW Riders #298
    Knights of the Roundel #333
    1977 R100RS, (Retired) 1993 R100GS (just getting started)

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