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ABS algorithms

I don't know how much interest there really is in this, but since I did post
on this earlier I thought I'd get back to the list with some recent
discoveries.  Perhaps the item of the most interest is an answer to a
question asked many times on the list relating to ABS apparent activation
shortly after starting.  This is related to a double redundant safety
circuit built into the system (which explains why I see two identical LSI
chips on the PC board).  Here's an excerpt ...

"The two safety circuits continuously monitor each other as well as all of
the system's components, including the wheel speed sensors, hydraulic
modulator and the wiring harness connections.  The safety circuits also
check the output signals from the logic section to see if they are logically

After the vehicle is started ans a speed of about 3 mph is reached, the
control unit will also initiate a dynamic test.  The safety circuit
simulates a control cycle and checks the reaction of the control unit and
the system's components.  This test cycle may be noticeable to the driver
because of the pressure reduction (return flow) pump will run briefly."

The other interesting thing I learned was that the system really only has
three hydraulic circuits ... both rear wheels have individual sensors, but
one hydraulic line to the rear brake calipers.  

Unfortunately the document doesn't go into much detail about any differences
of ABS dynamics for FWD as opposed to AWD drivetrains ...

Steve Buchholz
San Jose, CA (USA)