Allison 1000 & 2000 Gen 5 Fault Code:P0873 Transmission Pressure Switch 3 (PS3) 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. PS3 monitors SV3 positioning and relays it to the TCM. DTC P0873 sets during steady state (all shift valves are commanded to their proper state and a valid range is attained) operation when SV3 is in the stroked state (commanded ON) and PS3 status is detected as destroked (Exhausted/OFF).

The intent of this DTC is to detect an open condition in the pressure switch wiring circuit or the pressure switch. This diagnostic test is designed to compare the commanded shift valve position to the pressure switch feedback. Pending failure occurs when the switch indicates stroked for a calibrated time (if main pressure dropout is detected this time increases). In response to pending failure, SS3 is commanded to stroke and unstroke (retry logic). If the PS3 still indicates stroked, a DTC is set.

There is one of three possible failures that could be present:
1. Electrical malfunction – sets P0873
2. Mechanical malfunction – also sets P0761
3. Intermittent malfunction – retry logic attempts to command SS3 up to 2 times

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0873 is active, the following conditions occurs:
1. If the failure occurs while in a forward range a shift to 1st, 3rd, or 5th range is made.
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.
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 only P0873 sets look for an intermittent open condition at the pressure switch circuit. This DTC sets when the condition is present for more than 3 occurrences during the current drive cycle (refer to DIAGNOSING INTERMITTENT DTCs information).

When P0873 and P0761 are set in combination, the TCM logic re-attempted to command the shift solenoid on. A circuit or pressure switch concern is still present or continuous and not intermittent in nature.

When P0761 and P0873 are set in combination this may indicate a continuous open circuit condition at the pressure switch circuit or the shift valve is stuck in the destroked state (refer to 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 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 failures mode where DTC was set.

Multiple inactive pressure switch DTCs may be due to a plugged control main filter.