ABS/TCS systems have valves and pumps. Our beloved E36M3 has Teves mkIV
systems. The regular E36 switch to the Teves mk20 system for (I'm assuming)
cost/packaging reasons. The advantage to the mkIV is in the ability for
wheel locking pressure to be dropped to near atmospheric (near 1 bar) in an
ABS event. This helps with ABS events on extremely low-mu (low-friction)
surfaces such as glare-ice. The main disadvantage to the mkIV is the
complexity and size. The mk20 units use a spring piston accumulator to
capture the fluid that is removed from the wheel cylinders during ABS
events. This spring and piston seals can combine for as much as 5-6 times
atmospheric pressure and thus can slow the recovery of a wheel on icy
surfaces. Generally though, this is an acceptable (and quite capable)
The 95 has 3-channel ABS only. The 96 (not sure about some bastard M3s out
there, "Hi Neil") and newer have 4-channel ABS/TCS. Our cars are atypical
for passenger cars in that the front suspension/steering geometry dictates
that we use a front/rear split to the brake hydraulic circuit. Typical
passenger cars use a diagonal split to the circuits (1 front & the opposite
rear on a circuit). The system is split so that there is some redundancy to
the brake system in the event of a partial failure.
The fill and bleed procedures may vary for every combination of OEM and ABS
supplier; however, the empty units are typically vacuum filled from empty.
The air is evacuated from the brake system and then fluid is allowed to fill
the space. Vacuum filling essentially cannot be done once fluid has been
introduced into the system as fluids are essentially not compressible /
To answer Don's question about moving fluid through the ABS unit: It has a
pump for each circuit and therefore can move fluid if the valves are opened
properly and the pump is energized. So, the MODIC tool is programmed to
open the set of normally closed valves AND run the pump to move the fluid
through the system. I don't have the specific timings at hand, but this is
the general idea. The service technician would use a pressurized fluid
system and the MODIC tool to properly complete a full bleed in the shop.
Now to complete a full bleed for a DIYer, one should manually bleed the
brakes in the conventional fashion. Start with the wheel that is farthest
from the master cylinder and work back to the closest. Then drive the car
and enter a 4-wheel ABS event to ensure that fluid is cycled in all parts of
the ABS module. This will move the old fluid through the unit and replace
it with new fluid. Then one could complete another full manual bleed.
This may be a little overkill as if you drive the car aggressively you will
be having some ABS/TCS interventions naturally and the fluid will be cycled
at this time. In my opinion, a full bleed / cycle valves / full bleed is
unnecessary as you are reaching a point of diminishing returns (especially
for those of you that change the fluid every few months). Obviously if you
wait years to change the brake fluid, the full aforementioned cycle should
be completed to get all the old/bad fluid out. YMMV.
If I missed anything or anybody wants more detail, feel free to ask and I'll
try to answer.
// The Modic is the computer they use to activate the ABS pistons
// (which are also the traction control pistons).. but how do they
// move fluid through the system? I can't imagine they have one
// tech in the car pumping the brakes.
// I'd have to hazard the guess that they use pressure or vacuum..