Shortly after I got the bike in 2015, I noticed the battery did not seem to be getting charged when the headlight was turned on, so one thing to check is if I have a bad diode. Also, this year bike originally had rubber diode board mounts which are a solution in search of a problem. So I want to see if I have the original rubber diode board mounts.

It turns out I did have them so I replaced them with metal mounts. Adding the metal mounts improves the grounding and I can clean up the alternator to diode board wiring connectors which will improve charging performance. I also install a new Westco battery and replace the alternator brushes. I cover replacing the brushes in a separate write-up you will find here.

• 12 BMW 1983 R100RS Replace Alternator Brushes

Resources

I found a good deal of information about the alternator and charging system in the following resources written by Bob Fleischer.

• Diode Board and Grounding Wires
• Troubleshooting The Airhead Motorcycle Alternator/Charging Systems

Summary of Diode Board Function

The alternator creates alternating current (AC) with three phases using three independent stator windings. Each phase is converted to DC by diodes in the diode board. There are six large diodes that convert the AC to DC and send the output back to the battery (+) terminal. Each phase has two diodes in what is called a full wave rectifier. This type of rectifier converts the sine wave of AC voltage output from the alternator into two 1/2 waves. As shown in the diagram below, on the left is the AC sine wave where half the time the voltage is (+) and half the time it is (-). The full wave rectifier circuit in the middle converts it into a pulsating DC wave where the voltage is always positive, or zero.

With three phases, there are three AC waves each 120 degrees out of phase so the average DC voltage is higher than for a single phase and the voltage is never zero volts as shown below by the green wave “Resultant DC waveform”.

This is what the diode board does. If a diode is bad, then one of the three AC waves is not being rectified and the output of the alternator is reduced so the battery won’t get a full charge. That’s why I want to check the diodes to see if any are bad.

Removing The Diode Board

I start by removing the battery ground wire from the transmission and covering it with a piece of plastic tubing so it can’t possibly touch the engine while I’m working. This is a “Belt and Suspenders” approach, but it’s better than having to buy a diode board because the ground wire managed to touch the engine or frame while I’m working.

This bike has the oil cooler and it’s right in my way. I remove it by loosening the mounting bracket that goes around the horizontal tube that goes between the two frame down tubes.

I remove the two bolts securing the front cover and manage to wiggle it past the diode board. Here is the board at the top of the timing chest.

I can see that this board still has the original rubber board mounts (see red circle in fuzzy picture below).

I remove the output wire to the battery that is on the right side of diode board as you face it from the front of the bike (picture below) and the black wire that goes to the center tap (Y) of the alternator that is on the left side. I remove the nuts that secure the diode board to the mounts pretty easily with a 1/4 inch 8 mm socket.

Now I can pull the diode board off the mounts. There are two connectors that plug into the back, one is a bundle of three wires from the three phases of the alternator output to the diode board and the other is a blue wire that goes to the voltage regulator.

The BLUE wire tells the voltage regulator when there is enough voltage in the battery to stop charging it. If you look carefully, you will notice that there is a nick in the insulation of the BLUE wire so it could be shorting out intermittently. That’s not good.

Also, there are four BROWN ground wires added, two each to the top rubber mounts as shown in the picture below. This is an upgrade BMW provided to improve grounding of the diode board. The rubber mounts didn’t work well in that department, particularly since the timing chest cover is painted with black paint which is not a good conductor of electricity. So these ground wires ensured there would be good ground path for the diode board when using the rubber mounts.

The earlier metal mounts provided a solid, reliable ground path, so the extra BROWN wires aren’t needed. But I’m keeping them as a “belt and suspenders” method of ensuring a good ground for the diode board.

NOTE:
This picture was taken after I installed the new metal mounts. I kept the ground wires under the top mounts to improve grounding.

I need to remover the nuts that secure the rubber mounts to the engine timing chain cover. I remove the top engine cover so I can get access to the nuts. The cover is easier to remove from the right side.

The left side diode board mount nuts are pretty easy to get to, but the right side nuts are obscured by the starter motor and oil breather pipe that goes to the air box. I remove the wires connected to the solenoid and loosen the hose clamp on the oil breather pipe so I can slide it out of the way. This provides more room to get a wrench on the nuts. After some struggling, I manage to get the nuts loose. The toughest one to access is the lower right nut, but a long handled open end 8 mm wrench just reaches and I can get that nut loose.

The top left mount as you face the front of the diode board is broken, so this was a disaster waiting to happen. The rubber gets brittle and eventually multiple mounts break and allow the board to sag down and short out to the cover. Then you don’t have a charging system and your battery runs down.

Diode Board Inspection

Here are pictures of the diode board front, back and sides. Some of the large diodes (there are six) and the small diodes (there are five) are shown; blue boxes around the large and red ovals around the small ones.

NOTE:
On the backside of the diode board you can see that the bottom mounts are insulated while the top mounts are not. That’s why the four grounding wires go to the top mounts.

Testing Diodes with an Ohm Meter

I use an ohm meter to test each diode. The six large diodes are arranged in two rows. All the diodes in one row let electricity flow in one direction and all the ones on the bottom row allow electricity to flow in the opposite direction. I put the black lead (-) on the round case and the red lead (+) on the diode lead inside the diode board. All the diodes in a row should show the same reading; either approximately zero ohms (conducting) or approximately infinite ohms (blocking). Then I check the other row for the opposite flow through the diodes.

For example, for the top row diodes, if the black probe is on the case and red probe is on the diode lead inside the board and the ohm meter shows the diode is blocking current flow (infinite ohms), then for the bottom row of diodes I put the red probe on the case and the black probe on the diode lead and all the bottom diodes should show they are blocking current flow.

As another check, I reverse the ohm meter leads and I should  get the opposite behavior for the diode: if it was blocking, when I reverse the ohm meter leads, it should be conducting (approximately 0 ohms).

The readings for all the top row diodes had the same blocking reading and conducting reading on the ohm meter needle and so did the bottom row diodes in the blocking and conduction directions.

I checked the small diodes as well. These have a black line that indicates the lead that should block current that flows from that lead across the diode. They all blocked in the blocking direction and conducted in the opposite direction. I did not find any that were bad.

I used some 600 grit wet/dry paper and cleaned all the leads on the board and then sprayed them with electrical contact cleaner. They were all dull with oxidation so a good cleaning was in order to improve charging system output.

Install New Metal Diode Board Mounts

I got new metal mounts for the diode board from Euro Motorelectrics. They come with extra washers and nuts.

But, unfortunately the hardware is stainless steel so you can’t hold or pick them up the nuts and washers with a magnetic pickup, which is a problem because you will be dropping them inside the timing chest cavity. The good news is there is a shelf in the cavity that usually catches the nuts and washers when you drop them, but I manage to lose a nut inside the starter motor housing where it lodged tightly against the front mount bracket.  There it shall stay until I pull the starter motor for some reason.

I decided to use the original steel hardware. I have a magnetic finger but I couldn’t get it where I wanted it to position the washer and nut.

However, the magnetic finger is too strong and pulled the washer off the threads plus there is too little tactile feedback to help me position the washer or nut well.

I decide to use a tiny dab of silicone seal on one face of the washer to hold the steel nut on the washer.

I use a pair of extra long needle nose pliers to position the combined washer/nut on the backside of the timing chest cover and then hold them there with a finger. Then I screw the metal mount against the nut until it catches. This is an exercise in stretching my arms around the lower fairing to insert the mount while not dropping the nut/washer. After the sixth try on the top right mount (viewed from sitting on the bike) I got it threaded. To get the lower right nut/washer positioned, I had to remove the oil breather pipe from the bracket so I could get the pliers and a finger in the proper position. This too took about six or seven tries before I get the mount to thread on the nut. The mounts on the other side are much easier. I don’t have to attach the washer and nut with silicone seal so I use the stainless self-locking nut and washer on that side.

Repairing Cracked Insulation on Blue Wire

I use some RTV Silicone on the cracked insulation of the blue wire and let it harden.

Install Diode Board

I  attach the two cables to the back of the diode board and attach the left side black wire and right side battery (+) wire (as seen from the front of the bike) to the board.

Then I attach the board to the new metal mounts using the stainless locking nuts and washers and tighten them down with a 1/4 inch 8 mm socket.

Revisions

2019-11-22  Updates, edits and typos.