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Bosch alternator repair

Discussion in 'Electronic Repair' started by jcpearce, Feb 3, 2006.

  1. jcpearce

    jcpearce Guest

    I am not a big electronics chap so this may be an obvious answer but..

    I drive a BMW R1100RS motorcycle and the alternator has hit the dirt,
    removing the voltage regulator/brush unit
    ) and testing with an ohm meter I can the see the To-3 regulator has
    failed (high continuity between input and ground). A new unit cost
    $120 but I'd like to repair if possible.

    The battery is the usual 12v and the running voltage is 14v. The
    regulator regulates the voltage on the slip rings which excites the
    rotor, but I assume the voltage for the rotor varies with the load, so
    if the bus voltage is <14v more voltage is sent to the rotor. So why
    kind of regulator is this and can I find one in Mouser?

    Thanks and cheers,
  2. default

    default Guest

    I have a '72 R75/5. It had an old mechanical reg (still excited
    field type - same as used in most cars and your RS)

    I replaced it with a solid state one I found/adapted from an auto
    manual. Two transistors, 5 fixed resistors, one variable (so I can
    set the limit) one zener diode and a couple of rectifier diodes.

    I'm not aware of anyone marketing a TO3 style regulator for lead acid
    batteries but I seem to remember GM alternators had one that fit on
    the brush housing (so it would be three connections, ala TO-3)

    My homemade one uses four wires - one to ignition (but it works with
    that tied to the alternator output - used it on a Toyota Land Cruiser
    in that mode), one to ground, one to the alternator diodes (diode
    board in the R75 with a set of 3 diodes that only fed the regulator),
    and an wire for the slip rings.

    My bike has a "charge" indicating lamp on it. When the alternator is
    just starting - the current to excite the field comes through the
    idiot light - when the alternator is spinning, and generating voltage,
    the power for the slip rings comes from the diodes, turning the idiot
    light off.

    I can supply the schematic of what I used.. You'd need a modicum of
    electronics knowledge (ability to observe polarity of the diodes,
    connect the semiconductors, solder and package it (no heat sink
    necessary if you use a TO-3 or TO-220 or TO-66 or similar transistor
    for the output. I used a pair of 2N3054 transistors and heat-sinked
    the pass transistor - but it never got mildly warm.

    Mine has worked since 1973 - the transistors are exposed and I've been
    varnishing them to slow the rust. I have a 81 Honda 750 and it uses
    the same type alternator - I rewound the rotor when it shorted -
    that's a lot harder than building a simple regulator.

    I can scan and send the schematic if you want to try building one,
    remove capital Xs and treat like an email ady.

  3. Warren Weber

    Warren Weber Guest

    I also would like a scan of schematic if you would. Can send direct to
    [email protected] Of course leave out the no spam TIA Warren
  4. Mark

    Mark Guest

    I suggest you ask this question on

    They will design a new one for you...


  5. Are you in England? If you were in the states, I'd suggest calling
    National Automotive Lines in Shelbyville, Indiana. They do not have a
    website. They sell car parts, but the link you provided suggests that
    the brush-regulator assembly is used in a variety of automotive
    alternators, not just bike regulators.

    The toll-free number is 1-800-428-4300. The toll-free fax number is
    1-888-442-9222. It's a great outfit to do business with.
  6. James Sweet

    James Sweet Guest

    Have you looked online? Volvo and Saab both use a very similar Bosch
    alternator with the onboard regulator, the last time I needed one it was
    only about $30 online though the dealer charges around $170.
  7. none

    none Guest

    If it's a modular unit it's a simple matter of buying a new one.
    Usually around 20-30 bucks US.(unless you purchase direct from the
    dealer then of course it may cost more.)
  8. Ken Weitzel

    Ken Weitzel Guest


    Your use of the word "chap" leads me to believe you're
    probably somewhere in the UK, so don't know how helpful
    this might be.... but...

    Here we can buy rebuild kits for virtually any alternator
    for about 20 dollars cdn. They come with everything,
    the regulator of course, as well as new brushes, springs,
    bearings, etc.

    Our most "famous" source is Canadian Tire, perhaps you have
    something equivalent over there?

    Take care.

  9. jcpearce

    jcpearce Guest

    I have english background, but I live in the States.
    I have checked Kragen's and the part retails for $110. Amazing as I
    bought a whole rebuit alternator for my experimental plane for $20
    (with core). Probably some generic american brand would be quite
    cheaper. I can get the actual BMW part for $85 so I will probably just
    do that vs run around on an experiment I soldered together. A new BMW
    alternator costs $575, talk about bending over.
  10. James Sweet

    James Sweet Guest

    Check out, it crosses to the same site I get
    Volvo and Saab parts from. I don't know what model your BMW is but they
    list the same Bosch regulator/brushpack that many other European cars
    use and it's $26 aftermarket or $44 OEM. That place has great service
    too, I've ordered items several times and had them show up within 48
    hours with standard shipping.
  11. Not that it's relevant, but I used to own a BMW, a 1970 1600. I
    eventually installed a Bosch alternator from an Opel in the BMW. The
    only difference was in the mounting ears on the exterior of the
    alternator. The internals were identical.

    I suspect that interchangeability is still the rule, and that if you
    order the Bosch alternator as a Volkswagen part or an Opel part, for
    example, you will pay less for it than you would had you ordered from
    the BMW dealer.
  12. Or get an exchange unit from a Bosch dealer. Or indeed an exchange unit
    from any reputable source.
  13. none

    none Guest

    Try any import parts dealer for an aftermarket. Beck/Arnley
    specializes in high quality aftermarket electronic/electrics for
    In the US there is also Standard electrics who manufactures all sorts
    of crossable aftermarket part for auto electrics. You can usually get
    Standard brand at NAPA or any good speed shop.
    Bosch also should have that component, I've used Bosch parts in alot
    of the competitive racers I helped build.(I especially liked their
    "blue coils" and the adjustable brain boxes they offered. very easy to
    adjust top performance and super tough.)

    If you can find an old part man who really knows his stuff he'll most
    likely have a cross-parts manual, something that's very hard to come
    by. I had a fellow who had a cross manual for all things
    British/Italian to Most American electrics.( Used him alot to clean
    out all the bad electrics that came on British cars. When I'd finished
    that MG or Jaguar would be running on a Ford/autolite
    altenator/regulator and a good marine grade all copper wiring
  14. none

    none Guest

    NEVER buy from the dealer. Go to any decent import parts store and get
    the beck/Arnley aftermarket part. Or Bosch.
  15. ;-) What problems did you have with the actual wire?

    Lucas got the blame for the poor quality components but in reality it was
    the penny pinching car makers demanding the lowest prices. The original
    Jaguar company being notorious for this. If Lucas didn't make them down to
    their price they'd have bought elsewhere. But Lucas also made very high
    quality stuff for Rolls Royce - and of course aerospace work.
    Funnily the vehicle which sticks in my mind with the very worst electrics
    I've owned was a small Bedford van - made by GM in the UK. With mainly
    Delco electrics. It was a miracle to have all the basic stuff working at
    any one time...
  16. Re: the Lucas to Autolite conversion.

    If the OP can find a way to adapt a GM three-wire alternator to his BMW
    bike, he will never have to worry about finding Bosch alternator parts
    again. That's a bit of a modification though, I suspect.
  17. Three wire alternators are usually just a battery sensing device so not
    difficult to modify the wiring for - unless I've got it wrong?
  18. James Sweet

    James Sweet Guest

    He's referring to the old style with a 3 wire cable going to an external
    regulator. Bosch made those too, but when I encounter them I usually
    swap to the later style alternator with an onboard regulator, one less
    cable stringing around the engine bay.
  19. none

    none Guest

    Well... GM electronics have allways been notoriously bad.
    Had a brother in law who was a designer for Delco at one time and
    later Packard electric. He was always complaining about GM and how
    crappy they could be in regards to design standards.

    As for the Lucas wiring harnesses, aluminum wire and incredibly cheap
    grade vinyl insulation. Or have you never seen a TR4 that'd gone up in
    smoke from harness fire?
    In the very last years of production at British Leyland that damn
    aluminum wire would find it's way into just about everything they
    rolled off the line.
    And what with the poorly designed electronics in the late models any
    resistance load variances over the harness could make for some very
    wonky functions.
    Never have liked anything turned out by Lucas. ( I'd have a constant
    stream of customers who'd need new rectifiers for their Lucas
    altenators. Those things tended to burn out every few months or so and
    many customers insisted on keeping their British import authentic.
    Even when I tried to tell then that not all MG's had Lucas under the
    hood and that it'd be ok to replace with more reliable electrics. Take
    for example the v8 mgb's with the 3.2 litre v8 from buick. Very rare
    state side but very popular back across the pond. All quality Delco,
    at least compared to what Lucas stuck in 'em.)
  20. I've had countless cars with Lucas electrics dating back to the early '50s
    and never ever come across aluminium cable. I still own an '85 SD1 Rover.

    Why they should have used this stuff for export defeats me.

    Aluminium household wire was around for a very short time when there was
    the copper crisis in the UK caused by UDI in Rhoadesia -the UK main

    IMHO electrical fires in older UK cars was caused by inadequate individual
    fusing of circuits - indeed those most likely to be accident damaged like
    the lighting ones had often no fusing at all. But the cable was pretty
    well common to all cars - in the UK at least. I'll do some more research.
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