Solenoid PCS3 jammed in the ON position
Contact an Allison dealer.
What you'll see
No driver-visible symptom recorded.
System reaction
The Allison fault code number is: P0797.The fault code is set when the control unit detects a predefined number of times that next gear ratios are incorrect.For the fault code to be set, the following conditions must be met: The hydraulic system is pressurised.Output shaft speed is higher than 200 rpm.Turbine rotational speed is higher than 200 rpm.Cold mode is not activated.The lock-up cannot be locked. The control unit shuts off gear changing adaptation. The control unit attempts to engage the previous gear.While the fault code is active, the control unit ignores all information from the drive mode selector. The fault code is cleared automatically, when the vehicle has completed 40 start-up cycles without the fault being detected again.
Diagnostic depth
PCS3 still supplies a clutch despite that the clutch should be shut off. The fault is detected when the control unit detects that a previous gear ratio remained engaged for a long time period despite the clutch having received a signal to be shut off.
The cause can be: Irregular sensor signal from turbine or output shaft speed.Obstruction in a discharge circuit for clutches C3 or C4.Defective PCS3 solenoid.A governor valve for PCS3 has jammed.