Allison 1000 & 2000 Gen 5 Fault Code:P0878 Transmission Fluid Pressure Switch 4 (PS4) Circuit – High

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.

DTC P0878 sets when:
1. IMS PRNDL and PS4 do not agree. When R (Reverse) range is selected and the PS4 is detected in the Pressurized/OFF state for more than a calibrated time and more than a calibrated number of occurrences.
2. Engine shutdown and PS4 do not agree. When engine shutdown is in process and PS4 is detected in an improper state, Pressurized/OFF state, for more than a period of time that is temperature dependent.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0878 is active, the following conditions occurs:
1. Calibration dependent – Either lock to neutral or transmission shifts to 3rd or 5th range.
2. While diagnostic response is active, if the shift selector is moved to N (Neutral), transmission shifts to neutral; if the shift selector is moved to R (Reverse), transmission shifts to reverse. If the shift selector is moved to D (Drive) range or R (Reverse) and transmission is compromised by overspeeding or direction change, transmission shifts to neutral. If output speed fault or IMS fault is present when this test fails, then a hydraulic default is implemented.
3. The CHECK TRANS light illuminates (non-OBD II strategy).
4. DTC is stored in TCM history.
5. The TCM forces Variable Modulated Main off.
6. The TCM inhibits TCC engagement.
7. TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC:
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 the DTC recurring.

DIAGNOSTIC AIDS:
The following procedures are not documented in any OEM procedures or in any OEM Troubleshooting or Service information and should not be used as a method to diagnose any transmission DTCs, function, or shift quality concern:

1. Back-probing any connections used for transmission features or functions may damage and/or unlock terminals from the back-probed connector creating permanent or intermittent shorts and/or open circuits. If possible, use the J-39700 Breakout Box, the appropriate harness adapters, and appropriate magnetic overlays to troubleshoot the vehicle.

2. Load-testing any transmission-related circuits with any other electrical devices such as vehicle lamps or relays, especially with the TCM connected to the harness. Use J-39700 Breakout Box and appropriate harnesses with the diagnostic tool to monitor the circuit performance in question unless otherwise specified in the various Troubleshooting information.

3. Piercing a wire to check for voltages, shorts-to-grounds or other wires anywhere in the circuit but especially at the TCM. This creates a leak path for moisture and damages the wire and insulation.

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 and DIAGNOSING INTERMITTENT DTCs 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 failure modes where the DTC was set.