courtesy of Jens Lyck

Technical : The complex stuff - exciting but heavy reading, take your time.

Replacing Bosch charging parts

Author: Pete Roper

Replacing Bosch charging system parts with alternatives to the OE items for Moto Guzzi and BMW motorcycles circa 1970-85.


OK, first I'd like to just say that the author takes no responsibility for loss or damage resulting from the use of these instructions. The parts listed all work and I'm afraid if your motorbike bursts into flames when you turn it on it's because you have done something wrong not me :-)

None of this is subject to any real copyright and the author is quite happy for it to be forwarded, used in the print media, put on the bottom of the parrots cage etc. As long as no one seeks to profit financially from the information it contains I couldn't give a fat rat's arse what people do with it.

If you have some knowledge of charging systems, you can go for the details straight away. If not read on for a bit of background first.

"Let's make some electricity."

For starters lets have a brief look at how the system works. The basis of making electricity is that if you pass a conductor, like a wire, through a magnetic field an amount of Electro Magnetic Force (EMF) is produced in the wire. The reverse is also true. If you run an EMF through a wire it produces a magnetic field. Both of these principles are used in the Bosch alternator that flaps about on the end of your Guzzi crank.

Your alternator has two substantial parts. The rotor that sits on the end of the crank and, strange though this may seem, rotates and the stator which is bolted to the front of the crankcase and is therefore static.

If you look at your rotor you will see it's made of several different lumps bonded together in an interlocking sort of way with a coil of insulated wire in the middle.

From this coil of wire two ends come out and connect to the two brass rings on the outer end of the rotor. These are called the 'Slip Rings' and are so called because the 'Brushes', two little carbon blocks, slip round on them, we'll get back to them later.

Each of the interlocking lumps is a soft iron magnet with a North and a South pole on it and as it whizzes round, the magnetic field from these magnets passes through the coils of wire in the Stator affixed to the crankcase and tries to produce an EMF. The problem is that the magnetic fields produced by these magnets are very weak and consequently the amount of EMF produced is very small, not nearly enough to power the requirements of a rattling shitbox like a Guzzi. This is where it all starts getting very cunning!!!!!

If a current is passed through the coil of wire in the middle of the rotor, it produces a much larger magnetic field in the magnets of the rotor. There is a resistance to moving the magnetic field through the windings of the stator, but, because the rotor is being spun by the motor, all that happens is that a small amount of the power the motor produces as mechanical energy is used to overcome this and in turn produces electrical energy. This is why on some cars when you turn on the headlights, spotlights, windscreen wipers, heater fan etc. the engine speed will drop at idle. It is drag on the alternator and the same thing happens with Guzzis but is considerably less noticeable.

The power for the coil of wire in the rotor is provided through the 'Slip Rings' and amount required to maintain a steady output of about 14.5 volts is controlled by your 'Regulator'. As the amount required to produce this is constantly changing due to load requirements, (Are the lights on. Braking light etc. etc.) and engine speed, (The faster the rotor spins the more electricity is produced.) the amount of power needed in the rotor windings needs to be varied constantly. The regulator takes care of all this without you having to worry your head about it !!!! Good Eh!

There is one other teensy-weensy little problem though. As the magnets of the rotor spin the magnetic fields change polarity as first one pole and then the other swing through the coils in the stator. Each time this happens the current goes first one way and then the other. This is called an Alternating current (AC.)and is about as much use to us as tits on a bull. What has to happen is that all the power has to be made to travel in one direction and so become Direct current (DC). This is where the Rectifier comes in. This is what a rectifier does by using some clever little things called diodes that only allow current to pass through them in one direction. They are arranged in such a way that all the power surges are 'channeled' through a series of these diodes until they are all going the same way. If you look closely at a diagram of a rectifier pack, or even in some cases the pack itself you can usually work out the path it has to take. Unless you are dead keen, I wouldn't bother but it's useful to understand how it works.

The stator has three separate coils to generate the AC. These help provide a smooth flow of power rather than a series of pulses and each coil is called a 'Phase'. Therefore your bike has a 3 Phase alternator. This is why when you look at the front of it on the bike you will see three pins close together with a sort of black plastic connector with three yellow wires coming out of it. Each wire is a Phase and is carrying AC to the rectifier. After the AC has been rectified to DC it goes to the battery and the ignition switch. These are the two red wires that come off the big pins at the front of the Bosch rectifier pack.

There are also a pair of wires that emerge from the rectifier connected to spades adjacent to the three phase spades in the slot in the middle of the rectifier. One of these goes to the idiot light the other one to the D+ on the regulator.

One final feed is connected to the spade at the rear of the rectifier. This goes to one of the other spades on the Stator, either the 'other' brush that feeds the rotor or the central pole of the stator windings. Why the confusion? I don't know, but on my bike and an identical one we both use different pins. I have never been able to work out why this is so but it is and both systems work well so experiment for yourselves.

Substituting the alternative bits

OK. First thing to do is to connect the three phase wires which are connected to the alternator via a triple spade connector to the rectifier. These are shown on the pic below as 3 Phases. On Guzzis these wires are yellow and can be found appearing at the rectifier end of the circuit in the frame triangle on the right of the bike in most post T series bikes. Here there is another identical triple spade connector that if you are going to use Ingrams rectifier type 9692 will have to either be cut off or joining wires soldered on to the rectifier to enable them to be connected.

These three wires must be connected to the three main inputs on the back of the rectifier unit.

See the above pics showing the three "tails" of the main AC inputs.

The other main component in the charging system is the regulator. To replace this is easy as it has a direct solid state replacement for the original moving contact type in the Bosch RE55, (2 pin.) or RE57,(3 pin) these are dirt cheap, common as muck and even mount on the same holes as the original item so there should be no need to explain this any further.

What we do have to do now is make sure that all the assorted wires go to the right places. I think perhaps it would be best to just indicate first which wires go where without explaining why. For those of you who don't care about the how but just want a cheap replacement for a buggered part this would seem to be the way to go. For others who want a bit more of an insight I'll rattle on a bit more later.

If you are using the RE 57 regulator there will be three pins on the underside of it. One will be marked DF, one will be a G or ground and the third will say D+. The G or ground pin is an earth, (the RE 55 does away with this by earthing out through the body of the unit.) so just connect it to a convenient bit of the bike.

The DF pin is the one that supplies power to the field windings of the alternator and controls the output of AC power. This should be connected to "Spade 1"on the alternator brush holder. You will note that if you look carefully this spade and the brush it is connected to are insulated from the main body of the alternator. This is very important. The third wire, the D+ is connected to the pin or plate closest to the mounting point of the rectifier unit. See pic below.

Also from this 'inner' plate, either from the same spade or one of the others on the plate, (They are seen in the photo as being 'above' and 'below' the indicated D+ spade.) a wire is taken to the "Idiot" light on the dash. This is VERY important as without the power that comes from this tiny circuit the whole system will fail to charge. The other side of the "idiot" light MUST be connected to a +'ive feed that becomes live when the ignition is turned on.

The rectifier unit only has the one mounting point and on most Guzzis I have found it will fit quite conveniently on one of the holes where the previous much bulkier Bosch item sat.

Adjacent to the spade that the D+ wire goes to are two large spades coming from the middle of the three 'plates' of the rectifier pack. These are the main DC feeds one of which goes to the battery +'ive post and one of which goes to the ignition switch.

The above picture shows the final feed. This goes to one of two places. The reason for my confusion is that on my bike and my mates one we both have identical charging systems but for some reason I've never bothered working out the feed from this point on the rectifier goes to different spades on his and my alternators! You go figure it out. I can't be bothered. The thing is they both work equally well on the respective bikes!

On my machine this feed goes to 'Spade ' which is essentially an earth as it's the other brush of the rotor feed. On Dave's bike it goes to 'Spade' which is the central pole of the stator windings, (I think!). Either way it's weird but one or other of these combinations *will* work.

The biggest advantage to using these parts, the Ingrams 9692 and the Bosch RE55/57 or Ingrams equivalent is that they are ridiculously cheap when compared to the Guzzi/BMW original equipment prices. Both the rectifier unit and the regulator retail for about $AU 25 to 30 or about $US 18 to 22. Compare this to the OE price and there's a lot of money left over for beer.

That will do for now.