- Remove Engine Covers
- Engine Wiring Sub-harnesses
- Remove Diode Board
- Remove Alternator
- Remove Ignition Sensor, aka The “Bean Can”
I want to update the charging system (upgrade the alternator and diode board to 400 watts), clean and lube the ignition sensor (aka, electronic points) advance unit and replace the hall sensors. I also want to replace the timing chain, timing chain tensioner and crankshaft sprocket and nose bearing. And I’m going to remove the crankshaft to inspect the main bearings.
To do all that I first need to remove all the engine electrical components inside the front engine cover, the diode board, alternator, ignition sensor (aka, the “bean can”, aka, the “electronic points”).
Although my engine is out of the frame, you can remove these electrical components with the engine in the frame.
I use a special hardened bolt to remove the alternator rotor from the crankshaft. I acquired this one from Euro MotoElectrics
I use an oil filter wrench to keep the rotor from turning as I remove the alternator rotor bolt.
I shot a short video summary of how I do this work that you can see here.
Remove Engine Covers
You need to remove the (-) battery ground cable BEFORE you remove the front engine cover. The cover can touch the diode board frame which has power applied to it. If the cover touches the frame of the board, it will likely short out the board and you get to buy another one.
I remove the front engine cover by removing the two Allan head bolts that secure it to the engine block to expose the electrical components.
I also remove the top engine cover so I can easily access electrical connections on the starter solenoid where some of the wires from the front engine components go.
Engine Wiring Sub-harnesses
There are three wiring sub-harnesses/cables that connect to the components under the front engine cover and to the starter motor solenoid on the top of the engine. They exit the top of the inner timing cover as shown below through rubber grommets.
From left to right in the picture above:
- Voltage Regulator Sub-harness
- Battery (+) Cable
- Ignition Control Unit Sub-harness
Voltage Regulator Sub-harness
This sub-harness has at “T” shaped plug that plugs into the bottom of the voltage regulator on the right side of the frame spine tube.
The “T” plug has two BLUE wires on one terminal, with a BLACK and a BROWN wire on the other terminals. One BLUE wire is from the generator lamp on the instrument cluster that carries a small DC current to energize the alternator rotor coils so they are magnetized before starting the engine. The other BLUE wire carries the DC output voltage from the diode board to the voltage regulator which is used to regulate the charging voltage to the battery. The BLACK wire sends power from the voltage regulator to the alternator brushes via the (DF) brush terminal and the BROWN wire is the ground for the voltage regulator via the (D-) brush terminal.
Battery (+) Terminal Cable
This cable connects to the large screw terminal on the front of the starter motor solenoid. It also includes a RED wire that plugs into the starter relay socket. The starter relay plugs into that socket mounted on the left side of the bike next to the frame spine tube.
Since the starter solenoid sends current to the starter motor, which pulls a lot of current, the starter solenoid is directly connected to the battery. All the current draw of the starter motor flows through the threaded terminal on the starter solenoid.
There is another wire that connects to the starter solenoid threaded terminal that comes from the diode board output terminal. Since that wire and the battery (+) terminal are connected together, the DC power output from the diode board goes directly to the battery to charge it.
Ignition Control Unit Sub-harness
This sub-harness has a special connector plug that attaches to the ignition control unit that mounts on top of the front brake line manifold on top of the frame spine tube.
There is another white connector on this sub-harness the connects to the coils. It has three wires: GREEN–Blue, BLACK and BROWN.
The GREEN–Blue wire sends power to the coils. The BLACK wire is the ground path for the coils but it first goes to the ignition control unit. The ignition control unit opens the ground path when the spark plugs need to fire. The BROWN wire is the ground wire for the ignition control unit and the coils.
The other leg coming from the special connector plug has three wires that go to the ignition sensor inside the front engine cover. This is also called the “bean can” or the “electronic points.” There are two hall sensors inside the ignition senor, each with it’s own wire, and a common ground wire.
Since the ignition sensor is connected to the camshaft which turns at a half-turn for every full turn of the crankshaft, each hall sensor is 180 degrees from the other so one of them fires every half revolution of the camshaft which is every full revolution of the crankshaft. Both of the spark plugs fires every full revolution of the crankshaft. One of them does nothing since the valves are open and it’s piston is not on the compression stroke. Therefore BMW airhead bikes use what is called a wasted spark ignition system.
Remove Diode Board
The diode board is secured to the inner timing cover with locking nuts and washers on four studs.
BMW originally installed rubber mounts for the diode board. They were chasing diode board failures, but the cause was not vibration. The rubber mounts are a bad idea as they fail in time and then the diode board can fall into the front cover shorting it out. When I first got this bike in 2015, I replaced the original rubber mounts (two were failing) with solid mounts. You can see how I did that work here.
After I remove the nuts and washers, I pull the diode board off the studs. Then I remove the wire on the left side that comes from the alternator “Y” (center tap) terminal, and the wire on the right side that is the DC output from the diode board that goes to the solenoid threaded terminal to charge the battery.
Remove Wires On Back of Diode Board
There are two connections on the back of the diode board. One is a wide plug with three large wires from the alternator stator cover. Each wire carries one of the three phases of AC power produced by the alternator. The other is a female spade terminal with a blue wire that that has DC power and goes to the voltage regulator.
The regulator measures the voltage produced by the diode board using the voltage on this blue wire. If the voltage from the diode board reaches 14.3 volts DC, the voltage regulator shuts off current flow through the alternator’s rotor and when the voltage drops low enough, the regulator again allows current to flow through the rotor. By adjusting the time no current flows through the rotor, the voltage regulator prevents overcharging the battery.
I remove the three wire plug and the blue wire connector from the back of the diode board.
I remove the other end of the “Y” wire from the terminal on the alternator stator cover.
Remove Diode Board DC Output Wire and Battery (+) Cable
I remove the Diode DC output wire from the starter solenoid screw terminal. I remove the nut and washer, then the battery (+) terminal. Notice that the diode board DC output wire spade terminal is bent 90 degrees. This is necessary as there is very little space available for the wires due to the proximity of the crankcase breather valve and hose.
The stator cover of the alternator has a number of wires attached to it. Each of them has a terminal identifier that matches the wiring diagram.
Stator Cover Terminal Markings
Starting at the lower left is the “Y” terminal. This is the center tap of the alternator coils. The wire attached to it goes to the terminal on the left side of the diode board.
The brush terminals are next with one the (D-) and the other the (DF) terminal. These connect to the voltage regulator sub-harness. The (D-) is the ground path for the brushes and the voltage regulator. The stator cover creates a ground path back to the battery (-) terminal via the engine block. The (DF) is the power input to the brushes from the voltage regulator. Consequently, this terminal is insulated from the stator cover with fiber washers and a fiber sleeve that fits around the terminal screw.
At the lower right are three wires going to an insulator with three spade terminals. Each of these wires comes from one of the three alternator phases. Interestingly, a (D-) is shown next to the insulator, but the phase wires are called “U”, “V” and “W” which is cast into the insulator next to the terminals.
I am not sure why the stator cover has the (D-) cast into it at this location. But one thought is it signifies that the stator cover is a ground path back to the battery (-) terminal. That said, I find putting that mark next to where the stator phase wires go to the insulated terminal strip can lead to confusion as the (D-) marking shown on the wiring diagram applies to the brush terminal that is directly connected to the stator cover.
Remove Alternator 3-Phase AC Output Cable
I remove the three stator wires from the terminals on the stator cover and pull out the 3-phase output cable that plugs into the back of the diode board.
Remove Voltage Regulator Sub-harness
I remove the BROWN and BLACK wires that connect to the brush terminals. These wires are included in the voltage regulator sub-harness.
There is also a BLACK wire in the voltage regulator sub-harness that goes behind the inner timing cover to a spade terminal on the starter solenoid. It carries power from the starter relay to the relay inside the starter solenoid which latches the starter solenoid closed to apply battery power to the starter motor through the screw terminal on the other side of the starter solenoid.
Remove Ignition Control Unit Sub-harness
The ignition control unit connects to the ignition sensor, aka, the “bean can”, which is mounted at the bottom of the inner timing cover under the alternator, via the ignition control unit sub-harness. I removed the upper plug of the sub-harness that connects to the ignition control unit and disconnected the white plug that goes to the coils when I removed the electrical system and the brake system. You can see how I did that here:
The sub harness connects to the cable from the ignition sensor using a special connector above the alternator. One half of the connector is black which is part of the ignition control unit sub-harness and the other half of the connector is brownish-yellow going to the ignition sensor.
The two halves are held together with a wire bale. I use a small screw driver to pull the bail out of the slot it sits in. It’s a bit fiddly to get the second leg of the bale off as it sits under the connector, but with some patience I get it out without mangling it.
Here are some details about the black connector on the ignition control unit sub-harness that the wire bale fits into.
Then I pull out the ignition control unit sub-harness and the battery (+) cable from the top of the inner timing cover. They go through the same grommet so I remove them together.
Remove Stator and Housing
The stator is attached to the inner timing cover with three Allan bolts at 11:00, 3:00 and 7:00 o’clock.
Then I remove the coil springs that push the brushes down onto the rotor slip rings. I use a small screw driver to pull the foot of the spring up and out of the brush holder.
The stator has a steel ring that surrounds the stator coils. It is machined at the back to fit inside three large tabs cast into the inner timing cover. I put my fingers into the holes between the tabs and carefully pull the stator straight out. I don’t want to nick the stator wires as they slide past the rotor.
The stator has a sticker on the side with the Bosch part number. The last three digits “005” indicate this is a 280 watt alternator with the machined part of the steel sleeve sized at 107 mm to fit into an inner timing cover sized for the 107 mm stator sleeve.
The rotor is attached to the crankshaft nose with an Allan bolt. I use a socket to remove the bolt. To prevent the rotor and crankshaft from turning, I clamp the rotor steel fingers with an oil filter wrench as I loosen the bolt.
Once I get the bolt loose, I unscrew it. It will unscrew out of the threaded hole in the crankshaft nose and then become free, but you can’t remove it. There is a second set of threads inside the rotor, so pull it out until it stops and continue to unscrew the bolt until it comes out of the rotor.
Although you would think you can now remove the rotor, you can’t. It has a taper that matches a machined taper on the crankshaft nose. This creates a very tight interference fit that prevents the rotor from spinning on the crankshaft. The rotor bolt applies pressure to the tapers, but the fit is tight enough after you remove the rotor bolt that you need a special hardened extractor bolt to remove the rotor from the crankshaft nose. I got this one from Euro MotoElectrics.
This bolt MUST be hardened. If you use one that isn’t, the end of the bolt can mushroom under the force needed to pop the rotor off the crankshaft nose. It that happens, you have a real problem on your hands as the rotor is now stuck very tightly to the crankshaft nose.
The bolt screws into the threads in the rotor and I screw it in all the way in until it stops. Then I use the oil filter wrench to keep the rotor and crankshaft from turning while I tighten the extractor bolt until it pops off the crankshaft.
The rotor is heavy and you don’t want to drop it when it pops off the crankshaft. Since it is clamped in the oil filter wrench mine is safe. But if you have the engine in the frame and put the transmission in gear to keep the crankshaft from rotating while you remove the rotor, hang on to the rotor with your other hand as you tighten the extractor bolt so it doesn’t fall on the floor.
You can see the taper on the crankshaft and the taper inside the rotor.
Remove Ignition Sensor, aka The “Bean Can”
The ignition sensor, aka, the “Bean Can”, is the bottom component inside the front engine cover. It is secured with two Allan bolts on either side. It has a cable with a special connector that plugs into the ignition control unit sub-harness which I removed earlier. I remove the bolts and pull the bean can out of the hole in the inner timing cover.
The base of the ignition sensor has an o-ring to prevent engine oil from leaking. If you see oil under the ignition sensor, this o-ring has failed.
The ignition sensor has a pinned shaft that engages with a coupling on the end of the camshaft to spin a magnet past the two Hall effect sensors that act as the points. Here are two references about how Hall effect senors work and how they are used.
The Hall effect senors generate a voltage as the magnet goes by which is sensed by the ignition control unit. It turns off the current flowing through one of the two coils of wire inside each ignition coil. Inside each of the two coils, the collapsing magnetic field in one coil of wire creates a large current in the second coil of wire that is connected to the spark plug wire causing the plug to fire.
Here is what the front of the engine looks like now.