I have a 2007 Thomas Saf-T-liner. The ABS light is on. I have SID code 13 FMI 5. DBR retarder output is open. I have checked the relay box under the dash and above the front axle. There is no relays in the DBR socket. I am not sure what the dbr even is.
If your bus does not have a retarder as part of the transmission and your ABS sees a resistance on the retarder circut wire it expects to see it every time during start up. to make the ABS ECU unlearn a component you must use Wabco tool box to "reset memorized". Reasons for seeing resitance may include retarder circut wire shorted to grnd, touching wire with old style test light or moveing one ucu to another unit. Pin #7 in the black x2 connector is the wire in question.