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Diagnosing Issues with Factory ABS

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We have an E46 M3 dedicated HPDE car. This car has the MK60 ABS and DSC. Experiencing an issue with ABS going into "ICE MODE" in a downhill braking zone. Other than throwing parts at it, wondering how people end up diagnosing factory ABS systems like the MK60? I have a data logger and am able to log wheel speeds and brake pressures, but nothing in the data seems to explain why ABS is going into ice mode.

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I would guess it's because your rear wheel speeds are about 5% less than the front speeds. In normal dry-braking, you expect to see the front wheelspeeds less than the rear wheelspeeds. BTW, I would overlay all 4 wheelspeeds on the same graph (different colors per channel) to highlight the differences.

Are the tire sizes fitted the same as the spec'ed sizes for the car? There may be settings that can be modified for tire sizes that could help the ABS system do a better job. Check with someone with the proper BMW factory tools to see what's available.

The factory workshop manual might also be useful for a procedure to diagnose ABS issues, and program the system to correct them.

Ah, if I read it correctly, this is a mode in ADDITION to the normal ABS. While the comments below are more for the traditional ABS, it seems the "Ice Mode" is a particularly insidious thing because the ABS' ECU apparently 'decides' that conditions are very slipper, AKA icy, and pre-emptively limits the amount of braking force that can be applied, regardless of the pedal effort. In some cases it seems that this mode is still triggered if the brake pedal is released and re-applied, unlike ABS.

However, I may be reading it wrong - and I would have assumed the outside temperature sender would need to be close to, or below freezing and it seems to also occur at warmer temperatures. There are some suggestions by Lotus owners, with the same problem, that overheating senders can cause issues, but...

Anyway, for the ABS side, it may also be triggered if you have significantly stickier tyres that may be allowing greater load transfer to the front tyres and unloading the rears beyond the point where they start slowing, or locking, up - especially on down-shifts and/or with some lateral loading. Ripple strips, bumps, or other things may also be causing a wheel to 'skip' and trigger.

If you're using modified brakes and/or aftermarket brake pads, the brake pad moments and/or co-efficient of frictions will also affect the brake balance.

I'd suggest paying careful attention to when this is happening, and what you and the car are doing. There does seem to be a means to log the wheel speeds - there does seem to be a way to log them and line pressure, but I don't know it. However, if you have someone filming the vehicle it may pick up pick up a stationary wheel, painting or taping a strip to them will make this more obvious, or puffs of tyre smoke when it's being triggered.

Peter,

MK60 can be reprogrammed with competition software suited to track use that has more aggressive thresholds and behavior. For HPDE you don't have a rules banning this, so it's just a matter of time and money.

You can see the impact of ABS in the outlet pressures if you're able to log them, i.e. Bosch Motorsport ABS, but I believe MK60 only has the 2 sensors which are on the front and rear inlet circuits to the ABS block, before the ABS system determines how much flow/pressure reaches each brake.

The brake pressure you displayed is likely an inlet pressure from the master front or rear circuit to the ABS unit which is why the pressure appears to be a "normal" trace, not indicating ice mode or any ABS intervention, so whether you're getting into ice mode or simply requiring a lot from ABS and having minimal grip and deceleration due to unweighting the car and the anti deceleration nature of going downhill, I don't know.

If you wanted to investigate what's happening further before potentially proceeding with upgraded software, it can be a bit tricky to compare decel performance on flat vs. downhill, especially given the dynamic nature of a downhill transition, pavement changes, track camber and lateral grip demand, etc. for the reasons the others mentioned. If you're certain you're hitting ice mode and tire sizes match front/rear, then reprogramming is a next step people tend to take with mk60.

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