123quattro

Hi Greg,

About the ABS. The wheel speed sensors are not CAN signals. Depending on the vintage they will be analog or digital, but not CAN. Are you talking about just straight ABS, or ESC (stability control)? If it's just ABS you will have the 4 wheel speed signals and an analog brake light switch, BLS, that is on the brake pedal. If it's ESC, you will have 4 digital wheels speed sensor, an analog BLS, a digital brake pressure sensor in the hydraulic unit, CAN steering angle sensor, SAS, engine torque via CAN, a CAN combination sensor that goes under the center console that reports lateral acceleration and yaw, and a fancy 4 channel hydraulic unit.

You can retrofit ABS from a later model 928 onto an earlier version. You will need to make sure you have the correct tooth count tone wheels on all four wheels for the version of ABS you are installing. You should also use the calipers that came on that year of ABS you want to use. You can, but SHOULD NOT, attempt to install ESC on a car that never had it.

Both are a lot of work and probably not worth it.

svpmx83

CAN is relatively simple to implement - the hard part is documenting the messages

The simplest example i can give is from AUDI/VW

h271 ignition, 1 byte, 100ms
That is a message with ID of 271 with 1 byte of information every tenth of a second
271 1 11 key in
271 1 01 keys in pos 1, igniton off
271 1 05 keys in pos 2, ignition off
271 1 07 keys in pos 3, ignition on
271 1 0B key in pos 4, igition on, starter turn

So every 10th of a second a message with the ignition key status is broadcast across the network for other modules to pick up if they need it. The radio will want to know if the acessories are on, the dash will want to know if the key is in the ignition and the headlights are on etc.

dr bob

Rich in Phoenix spent a bit of time in Germany with Intel, and was involved with development of CAN bus there, I'm guessing with Bosch. He shared a few hints and a paper on CAN a couple years ago.

I'm guessing that 100ms packet scans are not fast enough for CAN-distributed ACS or VSC sensor or control messaging. The 100ms layer would be more supervisory.

Vilhuer

I believe BMW 850 was first car to use CAN to control windows and sunroof, not first ones but some later MY cars. Without CAN number of wires needed for all features in most complex modern cars would be so great that wires would basically take entire trunk space and large part of the passenger area. Using CAN was simply mandatory.

karl ruiter

I'm getting the idea that Greg's original question was more basic. The CAN bus is a digital serial protocol. So when information needs to go from one point to another it is sent as a code number telling what is going on and, if necessary, further data numbers giving details. The numbers are sent in the natural binary form one bit at a time. At every tap point for the bus there is a microcontroller of some sort that is able to communicate digital data onto and off of the bus. The microcontroller is typically doing things like reading analog temperature values, and turning them into digital data to send over the bus, or receiving messages to run windows up and down or turn lights on and off, or reading digital values like keyin/keyout and coding them to go over the bus.

I have never played with an actual CAN implementation, but I would think the ultimate goal would be to wire most of the car with basically two wires (signal, and power). In real life I suspect it is a lot more than that: multiple fused power, signal ground, clock (if it a sync protocol), digital power, etc. But two wire would be cool.

I have thought about developing a CAN system for the 928, and offering it for sale. It would allow me to get a toybox (garage space) and perhaps even some more toys and write it off as a business expense. I mean, how could I test my CAN implementation for the GTS if I don't have a GTS? Just a daydream, really. But a nice one.

slate blue

Guys all good and helpful responses, thank you! Karl interesting stuff. All looks pretty complex, too complex for me at the moment. The reason for the interest was some updated electronics and many aftermarket systems use can bus too. I was chatting a to a Porsche racer, an early 997 and he said that the ABS system on those cars wasn't very good. Another racer had replaced his ABS with the new Bosch Motorsport system. It seems lots of things these days revolves around Can Bus.

Greg