Allison 1000 & 2000 Gen 5 Fault Code:P0963 Main Pressure Modulation Solenoid Circuit High

CIRCUIT DESCRIPTION:
Pressure Control Solenoid (MAIN MOD) is a Normally Closed (N/C) solenoid used to modulate the transmission main pressure schedule. The TCM commands the solenoid ON when specific transmission and engine conditions are met. When MAIN MOD is commanded on, pressure is routed to the main regulator valve lowering the main pressure schedule.

The TCM sends control current to MAIN MOD from High Side Driver 1 (HSD1) via wire 111. HSD1 is continuously on unless the TCM detects a fault condition. The TCM regulates the amount of current to MAIN MOD by switching MAIN MOD’s low side driver on and off. Wire 174 completes the circuit between MAIN MOD and its low side driver. DTC P0963 indicates that the TCM processor has detected a short-to-battery condition in the low side of solenoid MAIN MOD’s electrical circuit.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0963 is active, the following conditions occur:
1. The CHECK TRANS light illuminates (non-OBD II strategy).
2. The TCM forces Variable Modulated Main off.
3. The DTC is stored in the TCM history.

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.

4. DTC P0963 indicates a short-to-battery in the electrical circuit for the MAIN MOD solenoid.

5. 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.

Inspect the wiring for poor electrical connections at the TCM and the transmission main connector. 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.

Inspect OEM wiring harness routing. Look for possible contact points where chafing could occur leading to an open circuit condition. Moving parts on the vehicle could be contacting the harness; this includes parking brake drum, suspension components, etc.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.

If the DTC appears to be temperature related, suspect a defective MAIN MOD solenoid. It is possible for a solenoid to be temperature sensitive causing resistance values to fluctuate. This may cause an intermittent DTC to be set.