355 Teves brake system error

That's ok, I have the manual. So what function does pin 10 on the ECU have? Maybe it is used in a way for faultfinding. i.e. it helps by isolating faults. e.g. if all four brakes seem not to be working, it doesn't report the brakes as being faulty if, say, the fluid level is low.

 

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One thing to keep in mind when considering using the ABS pressure switch from another car is that there were two versions having different states of one of the switches (the switch marked as "ABS Fault s/w" on Ian's diagram in post #25): Normally Closed and Normally Open. Diagrams of the two versions (examples from Jaguar XJS with Teves ABS):

Normally Closed (appears to be same as the Ferrari's):
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Normally Open (opposite action):
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In the case of Jaguar XJS, to be able to use the NO switch on models which originally had NC switch, the owners added a relay to the switch line to reverse its signal (otherwise the warning light would be constantly "on").