Allison WTEC II Fault Code: 13 PID: SPN: 12 FMI: ECU input voltage, low

Main code 13 indicates either a high or low input voltage. Low voltage is less than 8V.
In the event of a power loss, the transmission fails to the ranges indicated in the following, depending upon which latch valve releases ?rst

Possible causes:
1) Bad batteries
2)Faulty vehicle charging system
3) No battery-direct power and ground