after I changed the old alternator that doesn’t charge, the battery instead of charging discharges; also the ABS light appears on the board and it still doesn’t show the idle speed
Ask your shop to do their standard battery alternator test. My diy’er version is: The battery voltage should measure about 12.6 volts before the first start in the AM; then 13-15.5 volts after starting the engine. Suggest to address any battery or alternator faults before considering ABS or idle rpm display involvement.