How to Best Disengage ABS on a 456M?

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.

 

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.

 

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.

 

Here's the wiring diagram... Good luck

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.

 

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.

 

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.