P0840

ALLISON AUTOMATIC TRANSMISSION

DTC P0840
Circuit Description for Allison Automatic Transmission
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 transmission control module (TCM). When the C pressure switch is in the open state, the C shift valve should be in the unstroked position.

DTC P0840 is a type A DTC.

DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0840 Pressure Switch C Circuit Malfunction

Conditions for Running the DTC
^ The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts.
^ Engine speed is greater than 200 RPM and less than 7,500 RPM for 5 seconds.
^ C shift valve is commanded to the OFF, unstroked, position.

Conditions for Setting the DTC
DTC P0840 sets during steady state operation when pressure switch C indicates ON for 125 milliseconds after C solenoid has been commanded OFF.

Action Taken When the DTC Sets
^ The TCM illuminates the malfunction indicator lamp (MIL).
^ When in a forward range, the transmission will fail to another forward range, unless the transmission is compromised by over speeding or direction change, then the transmission will shift to NEUTRAL while diagnostic response is active.
^ If the shift selector is moved from a forward range to NEUTRAL or from a forward range to REVERSE, the transmission will shift to NEUTRAL.
^ If the shift selector is moved to REVERSE, the transmission will shift to REVERSE if it is not compromised by over speeding or a direction change. If compromised, the transmission will shift to NEUTRAL.
^ DTC P0840 will be stored in TCM history.
^ The TCM freezes shift adapts.
^ The TCM inhibits torque converter clutch (TCC) engagement.

Conditions for Clearing the DTC
A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.

Diagnostic Aids
^ Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions:
- A bent terminal
- A backed-out terminal
- A damaged terminal
- Poor terminal tension
- A chafed wire
- 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.
^ You may have to drive the vehicle in order to experience a condition.
^ This DTC may indicate that C solenoid is mechanically defective or the shift valve is stuck in the stroked state.
^ 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.

Test Description








The numbers below refer to the step numbers on the diagnostic table.
5. This step tests the TCM response function.
6. This step tests the transmission wiring harness integrity.
7. This step tests and evaluates the internal wiring harness.
8. This step tests the PSM for a failed condition.