Allison 1000 & 2000 Gen 4 Fault Codes: P0876 Transmission Reverse Pressure Switch Circuit Stuck Open

Allison 1000 & 2000 Gen 4 Fault Codes: P0876 Transmission Reverse Pressure Switch 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. Fluid pressure is supplied to the reverse pressure switch, holding it open, when the manual selector valve is in any position except reverse. When the manual selector valve is moved to Reverse, pressure to the reverse pressure switch is cut off allowing the switch to close.

CONDITIONS FOR RUNNING THE DTC:
TFT is greater than 0°C (32°F).

DTC P0706, P0708, or P0875 is not active.

There are two cases where this DTC runs.
Case 1: Ignition voltage is above 9V (12V TCM) or 18V (24V TCM). The PRNOL state and the reverse pressure switch state do not agree.
Case 2: Ignition voltage is under 9V (12V TCM) or 18V (24V TCM). All speed sensors are under 50 rpm.

CONDITIONS FOR SETTING THE DTC:
DTC P0876 sets in two cases:
1. When PRNDL and reverse pressure switch do not agree. When reverse range is selected and the reverse pressure switch is detected in the mechanically open/electrically OFF state for more than 1 second.
2. When engine shutdown and reverse pressure switch do not agree. When engine shutdown is in process and the reverse pressure switch is detected in an improper state, mechanically open/electrically OFF, for more than a period of time that is temperature dependent. The time period is 5 seconds at 35°C (95°F) to 30 seconds at -20°C (-4 °F).

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0876 is active, the following conditions will occur:
1. Calibration Dependent – The transmission will either lock-to-neutral or shift to 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 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.
5. If an output speed fault or NSBU fault is present when this test fails, then hydraulic default is implemented.

DTC P0876 is stored in TCM history.

The CHECK TRANS light illuminates on the second 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:
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 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 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. To help reduce multiple pressure switch DTCs from setting due to a plugged control main filter or slow pump prime, later software levels have a time delay up to 30 seconds before a pressure switch test is enabled.