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How to Best Disengage ABS on a 456M?

Discussion in '456/550/575' started by Tifoso, Apr 8, 2020.

  1. Tifoso

    Tifoso Karting
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    Mar 8, 2005
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    Concord, NH
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    Carmine
    I have a 2000 456M and have noticed that the ABS engages prematurely, particularly when braking from higher speeds. I am braking decisively, but nowhere near threshold braking where the ABS should get involved.

    I have aftermarket 19" Novitec NF2 wheels and tires, and due to very limited tire availability for the 12.5" wide Novitecs, my rear Michelins are 345/30 19s, more than 1" larger in diameter than the OEM rear tires (285/40 17). Fronts are the same diameter as stock. Not sure if the larger, much heaver wheels and tires, or the larger diameter rear wheel/tire, might be contributing to the ABS issue, but no one else running Novitecs has reported any issues with ABS.

    I typically run all 4 at 36 psi, and after doing some searching here, those pressures may be a little high so I may try to lower my tire pressures to 32 psi in the event that is an issue.

    In any case, if there isn't an easy solution, I really don't feel that I have any need for ABS and would like to "disconnect" it. Can anyone tell me how best to do this? Is it as simple as pulling a fuse?

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  3. Laserguru

    Laserguru Formula Junior
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    Aug 7, 2016
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    Eric
    Carmine, do you have a consistent fault indication or just early intervention by the system? You definitely have a different wheel rotation rate between front and rear with the diameter difference and that may well be confusing the heck out of the system under deceleration.
     
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  4. Qavion

    Qavion F1 Rookie
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    #3 Qavion, Apr 8, 2020
    Last edited: Apr 8, 2020
    Just some considerations:

    The ABS fuse also powers the traction control (ASR).
    More than likely you will get warning lights if you pull a fuse.
    Offering somoene advice to disable a safety feature is somewhat risky for legal reasons. Some Euro Ferraris do offer ABS cutout switches (not sure about the 456M), but the regulations are different there.

    The workshop manual describes ABS operation, but the text is a little confusing. I'm not sure if ABS action is based on relative wheel speeds rather than absolute wheel speeds. Relative speeds might help take tyre diameter out of the equation. Tyre diameter would be affected by tyre size and by tyre pressures and centrifugal forces. The manual text mentions "car speed" which is derived from "diagonally opposed wheels". As Eric says, perhaps having different (non-standard) wheel sizes (front/back) might be confusing the system?

    Of course, braking performance is based on a number of factors including tyre contact area and friction. How old are the tyres?

    P.S. By the way, nice colour combination... I had a Lotus Esprit in these colours.
     
  5. tazandjan

    tazandjan Three Time F1 World Champ
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    Jul 19, 2008
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    Terry H Phillips
    On older Ferraris, the brake fluid in the ABS system itself is often not changed when the fluid is changed and the brakes are bled. There may be contamination, water, or air in your ABS system. See if your tech can bleed the ABS system to eliminate that as an issue.

    That wheel tire combination is being used by at least a dozen 456/456M owners, including my old set of NF2s. If ABS was having a problem with front to rear diameter differential, you would be getting an ABS lamp just driving at a steady speed.

    Disconnecting something that affects so many other systems is not a great idea.
     
  6. Tifoso

    Tifoso Karting
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    Mar 8, 2005
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    Concord, NH
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    Carmine
    Thanks for the thoughts, guys.

    Eric, to answer your question -- I do not have any other indication (such as a warning light) that there are any issues with the ABS system.

    Also, the tires are quite new, replaced in the last year. I had to replace the original Pirellis (which were the correct diameter at 355/25 19). The reason why I wondered whether the diameter mismatch may be contributing is that I only noticed this issue with my new tires. Of course, it could just be a coincidence. As I stated earlier (and as Taz confirmed), I have not heard of anyone else running NF2s and these Michelin tires having any ABS issues.

    While I don't care about ABS and personally wouldn't hesitate disabling it, I would not want to disable the traction control as well, so I will try to isolate the problem.
     
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  8. Qavion

    Qavion F1 Rookie
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    Here's the wiring diagram... Good luck :D

    Ferrari_456_ABS_&_ASR_Wiring_Diagram

    The description of ASR isn't so clear. It's probably translated from german to italian to english.

    "The ASR device, integrated in the ABS system, avoid (sic) driving wheel skidding in acceleration through check of the hydraulic pressure on the brake calipers of the same wheels, and dialogue with the Motronic to reduce the engine torque".

    Sounds hopelessly integrated with the ABS.
     
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  9. tazandjan

    tazandjan Three Time F1 World Champ
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    Ian- Affirmative, the system reduces torque by cutting back the throttle (pretty abruptly with no digital throttle) and tries to regain control by braking the rear wheels.
     
  10. Laserguru

    Laserguru Formula Junior
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    Eric
    Yeah, that's a software rewrite, not a wiring change. ASR has to work off the individual wheel speed sensors to determine differential rotation rates between free wheels (front) and individual driven wheels (rear) and then do something about it. And you and Taz are both correct, it reduces engine output torque and differentially brakes the over-rotating wheel (the one spinning faster than the others).

    As for ABS, it has to look at the individual wheel rotation rates. If it is, as you posted, working off the diagonal pairs calculation as its expected norm, the difference in wheel diameters (what started this conversation and thread) will be normalized out in the diagonal pairs averaging. This is likely why lots of folks have run this NF2 setup without issue. The system would still be able to discern any particular wheel locking up and dropping its rotation drastically in the pairs calculation and thus in the pairs comparison. Again, not so easy to rewrite this software.

    But ASR and ABS tied this intimately together AND interacting with the engine control Motronic would be a major rewrite.
     
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