Allison 1000 & 2000 Gen 5 Fault Code:P0751 Shift Solenoid 1 (SS1) Valve Performance – Stuck Off

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 Pressure Switch 1 (PS1) monitors the SS1 valve positioning and relays it to the TCM. When PS1 is in the open state, SS1 valve should be in the destroked position. DTC P0751 sets when the SS1 valve is commanded on and PS1 status remains Exhausted/OFF for a period of time that is temperature dependent. The intent of this DTC is to detect a shift valve timeout condition which normally indicates a stuck shift valve in the destroked state, mechanical solenoid failure or an open circuit condition.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0751 is active, the following conditions occur:
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 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) or R (Reverse) and transmission is compromised by overspeeding or direction change, transmission shifts to neutral.
3. DTC is stored in TCM history.
4. The CHECK TRANS light illuminates (non-OBD II strategy).
5. The TCM forces Variable Modulated Main off.
6. 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 therefore 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 PS1 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 P0843 and P0751 are set in combination, the TCM logic re-attempted to command the shift solenoid on. A circuit or pressure switch concern exists. Refer to DTC P0843 for further instruction.

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 Intermittent Fault 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.

Whenever performing any internal valve body repair be sure that your work area is free of airborne dust and dirt that might contaminate the valve body assembly. Always store any reusable oil in a clean, covered container. It is recommended that a funnel which incorporates a screen filter be used to refill or strain the oil through an appropriate screen.

Use of a shallow sump filter in a deep sump pan can also set this DTC. Inspect for correct filter size.