Allison 1000 & 2000 Gen 4 Fault Codes: P0768 Shift Solenoid D Electrical

Allison 1000 & 2000 Gen 4 Fault Codes: P0768 Shift Solenoid D Electrical

CIRCUIT DESCRIPTION:
Shift solenoid D is a normally closed (N/C) solenoid that provides control main pressure to stroke the D shift valve. The TCM determines the proper solenoid command logic to move the D shift valve to attain a particular range requested. A pressure switch, located at the end of the shift valve, sends shift valve position feedback to the TCM.

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. TCM initialization is in process or engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.

CONDITIONS FOR SETTING THE DTC:
DTC P0768 is set when the TCM detects one of the following conditions for more than 100 milliseconds:
1. Open circuit or short to ground detected when D solenoid is commanded OFF.
2. Short to power is detected when D solenoid is commanded ON.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0768 is active, the following conditions will occur:
1. If the failure occurs while in a forward range, the transmission shifts to 1st, 3rd, or 5th range.
2. If the shift selector is moved to N (Neutral), the transmission will shift to neutral.
3. If the shift selector is moved to R (Reverse), the transmission shifts to neutral or reverse.
4. If the shift selector is moved to a forward range or reverse and transmission is compromised by overspeeding or direction change, transmission shifts to neutral.
5. If this failure is present at the solenoid electrical feed wire 231, the response is hydraulic default.

DTC P0768 is stored in TCM history.

The CHECK TRANS light illuminates on first occurrence.

The TCM freezes shift adapts (DNA).

The TCM inhibits TCC engagement.

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:
The diagnostic test performed to detect this DTC is very sensitive. Therefore, there is a high probability that an intermittent circuit condition may be causing this DTC to set. Make sure you check for the following conditions at the OEM harness first and then at the transmission internal harness.

Inspect the wiring for poor electrical connections at the TCM connector and the transmission main connector. 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.
7. Inspect OEM wiring harness routing, looking for possible contact points where chafing could occur. Moving parts on the vehicle could be contacting the harness. Check for contact at the parking brake drum, suspension components, transmission shift linkage etc.
8. Inspect the internal transmission wiring harness for possible contact areas where chafing may occur.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. It may be necessary to check for shorting to ground at individual wires within a harness to isolate an intermittent condition. Refer to Wire Check information.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time, etc. This data can be useful in reproducing the failure mode where DTC was set.

If the DTC appears to be temperature related, suspect a defective shift solenoid. It is possible for a shift solenoid to be temperature sensitive causing resistance values to fluctuate. This may cause an intermittent DTC to be set.

If the circuit problem (open or short) is present at the 231 solenoid feed wire, other shift solenoid electrical DTCs may be present (C, D, E electrical codes).