Allison 1000 & 2000 Gen 4 Fault Codes: U2104 CAN Bus Reset Counter Overrun

CIRCUIT DESCRIPTION:
J1939 is the protocol currently used in medium duty applications to allow communication between an electronically-controlled engine and the OEM transmission. A signal is sent over a two-wire network harness that incorporates two 120 Ohm resistors in parallel. There are two versions of J1939 currently in use in the medium duty market. Version J1939-11, the most widely used, incorporates a third shield wire. There is also J1939-15, which uses a two-wire connection without shielding.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC U0073 is set when the TCM detects no communication on the Controller Area Network (CAN) backbone harness for 3 seconds or more. This may be due to a shorted wire to ground or power in the CAN backbone harness.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC U0073 is stored in the TCM history.
2. The CHECK TRANS light does not illuminate.
3. The TCM defaults to the last-used adaptive shift values and engine throttle percentage is calculated from torque clip speed.
4. The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to clear the code 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:
1. If communication can be established at the vehicle nine-pin connector and U0073 is listed in failure records, this indicates an intermittent short condition was present.
2. When diagnosing for intermittent shorts-to-ground, massage the wiring harness while watching the test equipment for a change. Refer to Wire Check information.
3. If an active U0073 is present, Scan Tool communication will not be possible. The cause of the active DTC must be discovered and repaired before communication can be established with the TCM.
4. A short between wires 129 and 132 within the CAN harness can cause this DTC.
5. A short between wires 129 and 131 within the CAN harness can cause this DTC.
6. A short to a vehicle ground and wire 129 can cause this DTC.
7. A short to vehicle power and wire 132 can cause this DTC.

Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.

For proper data communications it is necessary to have two 120 Ohm resistors installed in parallel at the J1939 CAN backbone harness.

If this DTC is present in a new vehicle, harsh shifting may occur due to adaptive function inhibit (DNA).