-> BMW’s CAN System
by Ron Schmidt
Controlled Area Network
or: Can’t Add Nothin’!
In the early 1990’s the automobile industry needed a manner of reducing the number of wires and complexity of the wiring looms, particularly in the luxury cars that had so many accessories installed. The early CAN systems addressed these issues with limited success due to reliability issues. As the technology improved the systems finally became quite reliable. Beginning this year, all automobiles sold in the USA will require a CAN system for engine management that can be diagnosed by dealership and aftermarket diagnostic equipment.
BMW began using the CAN system in their motorcycles with the R1200GS model in 2005.
BMW states the following advantages for the use of the system:
• Lighter and simpler wiring harnesses
• No fuses
• No mechanical relays (except for the starter relay)
• Extensive diagnostic ability using “Guided Diagnostics” with BMW’s GT1 tester
Basics of the CAN system
BMW uses a “single wire system” (SWS) that is actually made of two wires in a twisted pair. The two wires carry digital messages; the electrical wave form signals carried by each wire are mirror images of each other. This helps resist electrical interference caused by other high voltage or noisy electronics on the motorcycle. This “single wire” carries all the digital data for the multiple computers, controllers, and sensors.
Let’s clear up some confusion about the SWS. The digital signals that it carries are only used to transport input from sensors (although the wiring from the sensors themselves to the CAN are normal wires) to computers, the information cluster, the IABS unit or controllers. A bulb filament still needs two wires (a positive and a ground) to work, a starter motor needs two heavy wires, etc… So, there is not just one little wire that connects everything on the bike together. The motorcycles still have a LOT of wiring to make them function.
The digital messages are sent to every controller in the network, but only the one that recognizes it will be controlled. So, for instance, a digital signal that says “Turn the low beam headlight on” will be ignored by the controller for the left turn signal. The digital messages sent via the SWS are fast enough that there would be no discernable delay in operation should two items be called up simultaneously. When the rider turns the handlebar switch to “Low Beam” a signal is sent to the computer which will then turn a transistor on to supply power to the low beam bulb.
The computers can sense important information such as current. Should, for instance, there be a direct short to ground in the above example of the low beam headlight, the computer would simply shut the power off to that circuit. Rather than using a fuse as in earlier days, the transistor that controls the low beam would be turned off. Additionally, the digital information that would turn off the light would also be sent to the instrument cluster to turn on a warning light display to tell the rider that the motorcycle has no low beam.
So what does it really do for (or to?) you as a rider?
Weight: Light weight motorcycles are a good thing to be sure. It is becoming more difficult to make bikes lighter, so anywhere weight can be saved is a good thing. However, in the real world, the weight savings by this system is probably less than the weight of a quart of gasoline. Very few of us can actually feel the difference in the way our bikes work when the tank is full or ¾ full, so the real world gain here is negligible at best. Additionally, time has shown that wiring failures on motorcycles are much more common on small wires than on the large wires.
Simplicity of wiring harnesses: The harnesses are simpler. No one wants a complex harness. Has the complexity of the old harnesses ever bothered you?
No Fuses: Do you even remember the last time a fuse on your pre-2005 BMW blew? If your answer is “yes” you are certainly in a minority of riders. But if you are, you will undoubtedly also remember why it blew. It likely was from a short, which had to be repaired to keep the fuse from blowing again. The same diagnostics would apply to a CAN system. The CAN cannot repair a problem. It would be nice not to have to carry a spare fuse, but the circuit would still have to be repaired. Also, if the motorcycle is out of warranty and one of the transistors “fuses” fails, the repair cost would be astronomical in comparison to the price of a box of fuses.
Furthermore, the CAN “fusing” system has proved to be finicky. For instance, the auxiliary outlet is rated for 10 Amps. It will reliably allow the use of low current consumers (such as cell phone chargers) but can turn off a heated jacket liner which only uses a bit over 7 Amps. Note that I say “can” turn off. We have seen some bikes run them fine, others run them some of the time and then turn off, and still others turn off right away. A 10 Amp fuse will run 7 Amps very reliably. Also, the outlet turns off a few moments after shutting the ignition switch off, so you can no longer use it to charge the battery as you could on the older bikes. BMW sells a charger that they claim will keep the circuit open to charge the battery; our testing has shown it does not work. Other shops (even franchised BMW dealerships) have come to the same conclusion.
No mechanical relays: As earlier stated, the CAN system uses transistors rather than mechanical relays. A mechanical relay has moving parts, electrical contact points, and springs; all of which can have diminished reliability with repeated usage. Corrosion can cause failures as well, so the relay must be weather-proof to some extent. The contact points must be physically large enough for whatever current the relay is rated for, therefore the size of the relay is relatively large. Most of the relays BMW has used over the years are roughly a 20mm cube. A K1200LT has enough relays to fill a box close to the size of its glove box.
The transistors used in place of the relays in the CAN system have no moving parts and are not as prone to failure due to corrosion or the vibrations common to motorcycles. Generally, a transistor will fail almost immediately when put into use; or last for decades if it lives through the first few moments of its life.
We are just starting to see common relay failures in the mid 1970’s BMWs. The only other common relay failure we have seen is with the K1200LT starter relay, but that was due to poor engineering, not the relay itself. So, along about 2035 we will really know if the transistors are going to prove to be a real world improvement over the relays. The immediate fact, though, is that the transistors are far cheaper to produce, take up less room, and should prove reliable far longer than most of the bikes will still be on the road.
Diagnostic ability using “Guided Diagnostics” with BMW’s GT1 tester: Our technical world is becoming ever more reliant on machines testing other machines, rather than having humans trained to think their way through solving problems. Many BMW dealerships do not have personnel who can perform complex diagnostics without the aid of a diagnostic computer. So, in a BMW dealership world, having a machine that can guide a technician through diagnostics is a very good thing. Most of you, however, do not have a BMW GT1 tester in your garage, nor would you know how to operate it if you did.
We have heard story after story from owners who have had failures on the CAN system and have returned to one or more BMW dealers only to have the same problems re-occur after they left the dealership. Normally, these failures are on very new motorcycles with very few miles on them. Often, the failure was found to be in an area that the diagnostic equipment cannot locate anyway. The repair required a technician who could actually do diagnostics. Other times, the symptom was finally repaired by the complete replacement of the CAN part that was related to the failure. Once repaired, though, the system should prove to be very reliable for a long time.
Common areas of concern and what we can do about them
The greatest percentage of new BMWs will not have any problem with the CAN system. The few that do will certainly exhibit those problems early in their life. The problems should be repaired for free during their warranty period. So, for most riders, the CAN system should not cause issues; in fact the existence of the system will not even be noticed.
Auxiliary sockets: For riders who live in Utah or in other areas of the world where it gets cold, the CAN equipped motorcycle can cause aggravation with the auxiliary outlet. If you ride when it is cold, you will be using a heated jacket liner (unless you are just ridiculously tough or have not yet been introduced to this most important piece of safety equipment!). Generally the heated equipment is plugged into the auxiliary socket, which is controlled by the CAN system, which most likely will turn it off.
Another important use for the auxiliary socket is to charge the battery when the bike is not being used for extended periods. As stated before, even the BMW tender advertised for the CAN system will not keep the auxiliary socket turned on, so you cannot reliably charge or maintain a charge in the battery through it either.
The solution for this is easy. We simply re-wire the existing outlet with a standard fuse directly to the battery, or add an additional fused auxiliary socket wired directly to the battery. With this easy modification you will be able to have your BMW reliably supply needed power for the heated jacket, and you will be able to charge the bike battery when it is parked for the stormy part of winter.
Non-factory accessories: In so many ways motorcycles are an expression of the owner’s personality. It is the nature of motorcyclists to modify their bikes to meet their needs. The CAN system makes electrical modifications a bit more difficult, but not impossible.
Additional loads on the CAN system can cause the part of the CAN affected to simply turn off. However, the CAN has to recognize reasonable variances that would be typically found in real world operation. For instance, as bulbs get older they require more current to run. Normal corrosion in wiring also requires more current to overcome the additional resistance. We can use these “allowed variances” to our advantage when adding non-factory accessories.
Standard relays are simply electrically controlled switches that require very little current to operate. Often we can simply install a relay and “fool” the CAN system into not recognizing the additional load. In areas of the CAN that are hyper-sensitive, we have micro relays that operate with such low current needs that they are not noticed by the CAN. The only problem with the micro relays is that they can only support minute amounts of current, insufficient to operate most accessories. However, they will support enough current to operate another standard relay. In those cases, we piggy back the micro relay to the standard relay and are thusly able to “fool” the CAN.
As a result, we can add any electrical accessory that you might want to your CAN equipped BMW. Obviously, we have to stay within the alternator’s ability to supply enough current to keep the overall system positive.
Synopsis of the CAN system
I never get finished with a ride on my 1989 R100RS and think “This is truly a better motorcycle than my 2004 R1150GS”. Technology tends to improve the breed of motorcycles, which can add to our enjoyment of the sport of motorcycling. While the CAN system could be viewed as “technology for the sake of technology” it probably will become the norm rather than just new technology in our sport. Time will likely find it to be cost effective, light weight, efficient and most importantly, reliable. It will become, like most of the other systems on modern motorcycles, unnoticed as it just does its job without fanfare. Our attention will be focused on the love of the ride, not on the magic happening between us and the road. Technology—resistance is futile.
