Allison 1000 & 2000 Gen 4 Fault Codes: P0846 Transmission Pressure Switch Solenoid D Circuit Stuck Open

CIRCUIT DESCRIPTION:
The Pressure Switch Manifold (PSM) is a multiple-switch assembly made up of three normally open (N/O) and one normally closed (N/C) pressure switches. The D pressure switch monitors D shift valve positioning and relays it to the TCM. When D pressure switch is in the open state, D shift valve should be in the destroked position.

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. D shift valve is commanded to the ON (stroked) position.

CONDITIONS FOR SETTING THE DTC:
DTC P0846 sets when D solenoid is commanded ON and D pressure switch status remains OFF for a period of time. The time period is 5 seconds at 0°C (32°F) to -40°C (-40°F). The intent of this DTC is to detect a stuck shift valve, in the destroked state, a mechanical solenoid failure, or an open circuit.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0846 is active, the following conditions will occur:
1. If the failure occurs while in a forward range, the transmission shifts to another forward 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 reverse.
4. If the shift selector is moved to a forward range or reverse and the transmission is compromised by overspeeding or direction change, the transmission shifts to neutral.

DTC P0846 is stored in TCM history.

The CHECK TRANS light illuminates.

The TCM inhibits TCC engagement.

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:
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.

When a P0846 and P0848 are set in combination, this may indicate an open circuit condition is present at the pressure switch circuit or the shift valve is stuck in the destroked state. See Troubleshooting Process information.

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 an open condition 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 when the DTC was set.

Multiple inactive pressure switch DTCs may be due to a plugged Control Main Filter. Ensure that the initial 8000 km (5,000 mile) filter change was performed.