failure signal c 3 is at the high level and therefore the output of the inverter 22 at the low level is supplied through the or gate 21 to the and gate 24 via the inverter 25 thus the control signal a 2 drives the driver 19 through the and gate 24 the contacts 17a and 17b of the relay 17 are also operated by the failure c 3 so that the exciting coil 18 is driven by the output of the driver 19 the sub microcomputer 20 samples as the diagnostic signal b 2b the potential at a point between the driver 19 and the contact 17b if the diagnostic signal b 2b indicates a normal operation with respect to the control signal a 2 the sub microcomputer 20 generates a low level signal as the failure signal c 2 during the period t 7 t 8 as illustrated in fig6 g if the diagnostic signal b 2b indicates an abnormal operation the sub microcomputer 20 provides the failure signal c 2 at the high level which set to the low level momentarily during the period t 7 t 8 as illustrated in fig6 i in the fourth mode at the time t 8 the.