ABS sensors are on all 4 wheels. The 2 wire multi plugs are situated just behind the front suspension turrets. The wires for the rear sensors are inside the car (from memory). I'm sure the wire oasses through an integrated grommet near the rear wheel arch.
If it is sensor, the resistance values can be measured BUT
be sure to measure the sensor resistance and not the ECU as trying to measure ECU resistance WILL damage it.
Trace the wires to be absolutely sure. Compare the values; the sensor with the obviously highest resistance will be faulty, if it is a sensor fault.
Sometimes an ABS reluctor (built into the wheel bearings) can be faulty. Diagnostic equipment usually gives a fault code and description of a "signal" fault if this is the case, rather than a "circuit" fault (which points to sensor).
Occasionally it can be a brake light switch or a low supply voltage. Generally, if there is a fault from start up, check and clean battery connections (and distribution board) and wires to maxi fuses etc first as that can be the cause.
Have a look here;
https://www.alfaowner.com/Forum/alfa-147-156-andamp-gt/1176693-strange-electrical-faults-fixed.html
Sometimes it can be difficult to know for sure, even with diagnostic equipment. Many people get MES downloaded to a laptop and get the USB/ELM327 connector cable as it gives access to all electronic systems. EOBD only does engine diagnosis and not climate control, body computer, airbags or ABS systems.
