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Ron Stygar Carl Buckland Dale Beuning Forums Help
Date: Fri, 8 Apr 2005 16:25:30 -0400 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) design solution. 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 / expandable. 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.
Jeremy
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