Allison 3000 & 4000 Gen 4 Fault Code: P2793 Gear Shift Direction Circuit

DIAGNOSTIC RESPONSE:
Ignores PWM input from shift selector

CIRCUIT DESCRIPTION:
The OEM 4th Generation Controls shift selectors communicates with the Transmission Control Module (TCM) by exchanging standardized digital messages over the SAE J1939 Controller Area Network (CAN). The shift selectors are also equipped with a single wire backup to the SAE J1939 CAN datalink. The OEM 4th Generation shift selectors transmit directional information (Forward, Neutral, and Reverse) in form of an analog Pulse-Width Modulated (PWM) signal via wire 134 to the TCM. The shift selector switches an internal driver ON and OFF to vary the duty cycle of the voltage on wire 134. When the driver in the shift selector is ON, the voltage on wire 134 is pulled to ground. When the driver is OFF, the driver’s output is open and the voltage on wire 134 is high. Since duty cycle is measured when voltage is high, the driver’s OFF-time determines the duty cycle. For example, if wire 134 duty cycle is 15 percent, the shift selector driver is ON (pulled low) 85 percent of the time and OFF (open) 15 percent of the time.

CONDITONS FOR RUNNING THE DTC
After TCM initializes normally and the engine speed is above 200 rpm for more than 5 seconds.

CONDITIONS FOR SETTING THE DTC:
When the duty cycle is invalid for more than 1 second.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2793 is active, the following conditions occur:
1. The DTC is stored in the TCM history.
2. The TCM illuminates the CHECK TRANS light.
3. The TCM ignores PWM signal from shift selector.
4. If CAN signal is also lost, the TCM locks in last valid direction.

CONDITIONS FOR CLEANING THE DTC/CHECK TRANS LIGHT
The diagnostic tool may be used to clear from DTC from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
This DTC may be caused by:
1. Short to power or short to another wire in W134.
2. High resistance or open in W134.
3. Poor vehicle grounds.
4. Unlocked connectors, expanded terminals, poor pin crimps or unlocked pins causing high resistance or open circuit.
5. Unlocked connectors, unlocked pins, causing shorts in connectors.
6. TCM ground is at different voltage potential than the ground of the OEM voltage source responsible for shift selector.
7. Defective TCM.
8. Defective shift selector.
9. The PWM signal characteristics are listed below:

Primary Shift Selector 977 Hz 10 Hz (When Active):
1. Unknown: 15% ± 2% Duty Cycle
2. Park: 30% ± 2% Duty Cycle
3. Reverse: 45% ± 2% Duty Cycle
4. Neutral: 60% ± 2% Duty Cycle
5. Forward: 75% ± 2% Duty Cycle
6. Error: 90% ± 2% Duty Cycle

Secondary Shift Selector 871 Hz 10 Hz (When Active):
1. Unknown: 15% ± 2% Duty Cycle
2. Park: 30% ± 2% Duty Cycle
3. Reverse: 45% ± 2% Duty Cycle
4. Neutral: 60% ± 2% Duty Cycle
5. Forward: 75% ± 2% Duty Cycle
6. Error: 90% ± 2% Duty Cycle