Airhead Electrical Systems Documents

Posted on March 6, 2018 by breams

I put together a series of pages about BMW airhead motorcycle electrical systems. It strikes me that electricity in general, and motorcycle electrics in particular, are dark mysteries to many, so I thought I’d shine some light [puns intended 🙂 ].

Lack of understanding hinders confidence when diagnosing and working on electrical projects. As these bikes age, the electrical system is prone to problems as corrosion and neglect makes them behave badly. More owners are having more electrical problems but seem less able to get to the root cause of the problem.

I think one tool that many avoid using is the wiring diagram. The spaghetti of lines, symbols and notations makes the eyes glaze over. “Oh goodness, where do I start?” is the common response to the advice, “Look at the wiring diagram.”

5 Series Wiring Diagram (1970-1973) (Source: Haynes Manual) –> CLICK TO ENLARGE

I’m not an electrical engineer, but I have taken time to learn the basics, have collected comments and input from well respected airhead mechanics and dug into how BMW applied electrical theory when they designed the /5 electrical system. I’ve learned a lot from various reference sources that are scattered about the internet, so my articles include a bibliography of various useful resources. That way both you and I have a nice set of reference materials to consult when problems come up.

I have published three articles (so far), Basics and two about the /5 series: 5 Series Electrical Circuits and, 5 Series Electrical Components. I chose the /5 series to start with because I believe it’s the most popular airhead series for restoration.

I hope to write an Electrical Circuits and Electrical Components document for the /6 and /7 series up to 1984.

Here is the table of contents from the first three documents.

Airhead Electrical Systems-Basics

Airhead Electrical Systems-5 Series Circuits

Airhead Electrical Systems-5 Series Components

1983 BMW R100RS Rebuild: Diagnose Charging Problem

Posted on July 14, 2015 by breams

After I got the bike running, I noticed that the volt meter showed low voltage (under 12 volts) when I was riding the bike with the headlight on. Something in the charging system was not working correctly. I read in Bob Fleischer’s material that a failed diode in the diode board would have this symptom and dirty electrical connections and/or alternator brushes that had gotten worn down could also cause low charging current to the battery. He also talked about BMW using rubber diode board mounts at this time. These mounts can break and allow the diode board to fall enough to short against the front engine cover and also contribute to weak or intermittent grounding.

So, I decided to dig into the diode board and test the diodes, replace the rubber diode board mounts and clean up all the electrical contacts. While I am in there, I also cleaned up all the alternator electrical contacts and replaced the alternator brushes. And, I cleaned up the starter relay and voltage regulator contacts. Last, I installed a new Westco AGM battery so I have a known good battery and date of installation.

Here are the write-ups on this work:

I found that all the diodes were good on the diode board. It maybe worth replacing it due to it’s age and as insurance from an unexpected failure, but I’ll hold off on that until I do the full rebuild. Replacing the rubber mounts is not a fun or easy job. I got very frustrated more than once and had to take a couple coffee breaks to get my attitude adjusted. 🙂 But I did get them installed, so with perseverance and patience, it can be done.

After I replaced the alternator brushes and cleaned up all the electrical contacts, I now get 14.5 volts between the battery (+) terminal cable from the diode board and ground with a momentary rise to 15 volts when the voltage relay does it job. I suspect the brushes were the main reason for the low charging current to the battery.

A couple of pictures follow.

Check Diode Board, Replace Rubber Mounts.

Diode Board-Left Alternator Center Tap Wire, Right Battery (+) Wire

Diode Board Back Side Showing Two Connectors

Left Side Diode Board Mount Nuts

Right Side Diode Board Mount Nuts-Note Solenoid Wires Removed & Oil Breather Hose Clamp Moved

Rubber Mounts-One Broken

Location of Large and Small Diodes

Solid Diode Board Mount Kit From Euro Motoelectrics

Alternator with Wiring

Removing Brush Terminal Lugs

Removing (3) Stator Housing Bolts

Alternator Stator Assembly

New Brush (Top) and Shorter Old Brush

New Brushes Soldered To Metal Tabs of Brush Holder

1983 BMW R100RS Rebuild: Diagnose and Replace Failed Neutral Switch

Posted on July 3, 2015 by breams

When I bought the bike in January 2015, the neutral bulb didn’t light and the starter motor wouldn’t work unless I pulled the clutch. I need to figure out what is going wrong in the starter circuit and the neutral switch. I wrote up the work I did here including how I diagnosed that the neutral switch failed and how to replace it:

61 BMW R100RS Diagnose and Repair Neutral Switch

This write-up makes use of some short videos to show what I do. I’m experimenting with how to use video clips to explain things better than just pictures and text can.

There is a longer video at the beginning in which I explain how I use the Haynes wiring diagram and Bob Fleischer’s material to figure out how the neutral and clutch switches work in the starter switch circuit.

I’ve heard fellow Airheads tell me electricity is confusing and a mystery. So I thought maybe showing how I try and figure out how components work from the wiring diagram when I have a problem may be of some value. Here is a direct link to that video.