Retarder pressure sensor, voltage low
Check the cable harness between the gearbox and control unit, particularly the electrical cables connected to pins 12 and 44 on the control unit. If no fault is found, contact an Allison dealer.
What you'll see
No driver-visible symptom recorded.
System reaction
The Allison fault code number is: P0989.The fault code is set when the control unit has detected 0 V from the retarder pressure sensor for more than 10 seconds.The fault code can only be set if the engine speed is higher than 200 rpm, the components are supplied with voltage and the voltage is higher than 9 V and less than 32 V. Electronic braking must be activated in the control unit.The fault code is cleared automatically, when the vehicle has completed 40 start-up cycles without the fault being detected again.
Diagnostic depth
The control unit has detected 0 V, i.e. too low a voltage, from the retarder pressure sensor.
A possible cause of this fault code is a short circuit to ground or an open circuit in one of the electrical cables to pin 12 or 44 on the control unit.