Allison 1000 & 2000 Gen 4 Fault Codes: P0840 Transmission Pressure Switch Solenoid C Circuit

Allison 1000 & 2000 Gen 4 Fault Codes: P0840 Transmission Pressure Switch Solenoid C Circuit

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 C pressure switch monitors C shift valve positioning and relays it to the TCM. When C pressure switch is in the open state, C shift valve should be in the destroked position.

CONDITIONS FOR RUNNING THE DTC:
1. Components are powered and the ignition voltage is greater than 5V.
2. DTC P0561, P0562, or P0563 is not active.
3. C shift valve is destroked.
4. Transmission temperature is above -25°C (-13°F).
5. Vehicle shutdown is not in process.

CONDITIONS FOR SETTING THE DTC:
DTC P0840 sets during steady state operation when C shift valve is in the destroked state (commanded OFF) and C pressure switch status is detected as stroked (commanded ON). Steady state is defined as attaining a valid range. The intent of this code is to detect shorts to ground in the pressure switch wiring circuit or the pressure switch.

Later software levels incorporate retry logic that allows the TCM to command C solenoid ON and OFF. If the pressure switch state does not change, and remains in the ON state, DTCs P0840 and P0842 will set.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0840 is active, the following conditions will occur:
1. If the failure occurs while in a forward range, the transmission shifts to 1st, 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.

DTC P0840 is stored in TCM history.

The CHECK TRANS light illuminates.

The TCM inhibits TCC engagement.

The TCM freezes shift adapts (DNA).

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 only a P0840 is set, look for an intermittent short to ground at the pressure switch circuit or intermittent hydraulic failure (sticking valve). This code is set when the condition occurs more than 3 times during the current drive cycle. See Intermittent Fault information.

When a P0840 and P0842 are set in combination, this may indicate a short to ground at the pressure switch circuit or the shift valve is stuck in the stroked state. See Troubleshooting Process information.

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