Allison 3000 & 4000 Gen 4 Fault Code: P0973 Shift Solenoid 1 (SS1) Control Circuit Low

DIAGNOSTIC RESPONSE:
DNS, SOL OFF (hydraulic default)

CIRCUIT DESCRIPTION:
Shift Solenoid 1 (SS1) is normally closed solenoid used to properly position C1 and C2 logic latch valves in forward ranges. The Transmission Control Module (TCM) commands the solenoid ON to supply control main pressure to the C1 and C2 logic latch valves. When SS1 is commanded OFF, control main pressure is released from the C1 and C2 latch valves.

The TCM sends control current to SS1 from High Side Driver 2 (HSD2) via wire 171. HSD2 is continuously ON unless the TCM detects a fault condition. The TCM energizes SS1 by switching the solenoid’s low side driver ON. Wire 151 completes the circuit between SS1 and its low side driver. DTC P0973 indicates that the TCM has detected a failed low condition in the low side of the SS1 electrical circuit.

CONDITIONS FOR RUNNING THE DTC:
TCM initialized normally and engine speed above 200 rpm.

CONDITIONS FOR SETTING THE DTC:
This DTC sets when the TCM detects SS1 low side driver has been detected in a failed low state.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0973 is active.
1. If failure occurs while in a forward range, then shift to another forward range. If failure occurs while in Neutral (N), the transmission remains in Neutral (N). If failure occurs while in Reverse (R), the transmission remains in Reverse (R) or shifts to Neutral (N). Once range is attained, transmission will be locked in range. There is no response to shift selector requests.
2. DTC is stored in the TCM history.
3. The TCM inhibits Torque Converter Clutch (TCC) engagement.
4. The CHECK TRANS light illuminates.
5. The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without failure.

DIAGNOSTIC AIDS:
DTC P0973 can be caused by:
1. SS 1 issues such as:
– W151 high resistance, open, or shorted to another wire in chassis harness or chassis ground.
– W151 (terminal 10 of 20-way) high resistance, open, or shorted to another wire or to the control
valve module in the internal transmission harness.
– SS1 defective.
2. Broken or defective connector assemblies allowing pushed back pins to short together.
3. Unlocked connectors, missing seal plugs allowing moisture in connectors.
4. Unlocked connectors, expanded terminals, poor pin crimps or unlocked pins causing high resistance
or open circuit.
5. Defective TCM.

Review applicable information in Control System and Transmission Specifications to find additional circuit specifications, system and connector diagrams, and troubleshooting tips.