Allison 3000 & 4000 Gen 4 Fault Code: P278A Kickdown Input Failed ON

DIAGNOSTIC RESPONSE:
Inhibit kickdown operation

CIRCUIT DESCRIPTION:
The Transmission Control Module (TCM) can be calibrated to receive an acceleration pedal kickdown input from either an analog input wire or a digital datalink message. When the operator activates the kickdown feature in the Economy Mode, the TCM uses Performance shift points. A momentary, normally open switch attached to the throttle pedal typically generates the kickdown input signal. The switch usually provides a detente feel when full-throttle is achieved. Some OEMs may opt to send a kickdown datalink message when a threshold for accelerator pedal percentage has been reached (e.g., 95%), and not implement the momentary switch in the circuit. However, on vehicles with the switch, the kickdown function is activated when the operator steps through the detente.

For implementation of the kickdown feature via datalink with a momentary switch, the switch input is sent to the controller responsible to send a kickdown active message over the datalink to the 4th Generation TCM.

The SAE J1939 Controller Area Network (CAN) is the preferred digital datalink. The TCM communicates with the Engine Control Module (ECM) and other controllers by exchanging standardized digital messages over SAE J1939. The Physical network consists of a two-wire twisted pair, two 120 Ω termination resistors, and in most cases a third shielded wire. A 120 Ω termination resistor is located at each end of the network Vehicle OEMs may choose to install external termination resistors or use internal termination resistors built into many SAE J1939 electronic modules, including the TCM and shift selector. Refer to the OEM Technician’s Library for additional information.

CONDITIONS FOR RUNNING THE DTC:
1. TCM initialized normally and engine speed is above 200 rpm.
2. For vehicles requiring analog kickdown input, the proper kickdown input function must be enabled.
3. For vehicles requiring a datalink kickdown message, an acceptable OEM source address must be autodetected or enabled in calibration.

CONDITIONS FOR SETTING THE DTC:
DTC P278A sets if the TCM is calibrated to receive the kickdown input signal and both of the following conditions are met:
1. Throttle percentage is less than 20 percent.
2. The kickdown input signal is ON for more than 5 seconds.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P278A is active, the following conditions occur:
1. The CHECK TRANS light does not illuminate.
2. DTC is stored in TCM history.
3. TCM inhibits kickdown operation.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
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:
When analog input wires are used, the kickdown function is active when a switch is closed to complete the circuit between wire 122 and signal return wire 103. If a datalink is used, the TCM receives accelerator pedal kickdown input as part of a SAE J1939 message parameters PGN 61443, Electronic Engine Controller 2 (EEC2).

DTC P278A indicates the TCM has detected a kickdown input signal for more than 5 seconds with less than full throttle conditions. The code can be caused by:
1. Chassis wiring shorted to ground, high resistance or open.
2. Faulty connections to the accelerator pedal kickdown switch.
3. A faulty accelerator pedal kickdown switch.
4. Another controller improperly broadcasting kickdown signal on the data link when throttle conditions are not met.
5. A faulty TCM.
6. SAE J1939 Kickdown status can be read on the diagnostic tool. Monitor data link communications using Data Bus Viewer.
7. A faulty accelerator pedal.

If DTC is still active, review applicable information in the OEM Technician’s Library, Control System and Transmission Specifications, and other related information.