If the unit has a 6v battery (3 cell) or 8v battery (4 cell) but also uses a 9v or has the option of a 9v battery in the back of the console:
Using a known good (test if necessary) 9v battery in the back of the console, see if the console will light up and operate. If the console does light up and the unit will operate, then it is likely the 6v battery (3 cell) or 8v battery (4 cell) did not have sufficient voltage to power the console on its own. The 6v battery (3 cell) or 8v battery (4 cell) may recharge with use of the unit. If the battery does not recharge or hold a charge, it should be replaced.
CAUTION: PINCH HAZZARD - Battery/Alternator test performed while pedaling on the unit. Make sure all parts and persons are clear of all moving parts. Use a voltmeter and connect to the 6v (3 cell) or 8v (4 cell) battery (located beneath the plastic shrouds). Stride on the unit. Does the battery voltage climb?
If yes, then the alternator is good. You have confirmed the alternator is putting out voltage which goes through the ACB (Altenator Control Board) and the ACB (Alternator Control Board) is sending the voltage to the battery.
Using an ohmmeter, check for continuity in the data cable that connects the ACB (Alternator Control Board) to the console.
You test this by locating each end of each wire in the data cable and probe into each end with a lead from your ohmmeter. Set the ohmmeter on the lowest ohm setting. A reading of any value near zero confirms continuity. If continuity cannot be confirmed at any tested point, the cable should be replaced.
Swap the console with a working machine.
If the problem follows the console, it needs to be replaced.
Swap the ACB (Alternator Control Board) with a working machine.
If the problem follows the ACB, it needs to be replaced.