Allison 3000 & 4000 Gen 4 Fault Code: P0842 Transmission Pressure Switch 1 (PS1) Circuit Low

DIAGNOSTIC RESPONSE:
DNS, Lock in current range

CIRCUIT DESCRIPTION:
C1 and C2 latch valve logic states (stroked/destroked) control clutch pressure routing based on latch valve location. The four possible combinations for C1/C2 latch location are the following:

C1 Latch Valve C2 Latch Valve
Destroked Destroked
Stroked Destroked
Destroked Stroked
Stroked Stroked

The TCM knows which solenoids should be commanded to attain a specific range and will use the PS1 switch state (open= PRESSURIZED and closed = EXHAUSTED) to confirm if the actual transmission range attained matches the TCMs known commanded range.

PS1 is used by the TCM to sense the PCS2 regulator (trim) valve position (stroked or destroked) and the C3 clutch state (pressurized or exhausted) when the C5 clutch is filled. C5 is filled in Reverse, Neutral, and 1st range. Also, PS1 is used to sense the C1 and C2 latch valve positions (stroked/destroked) when the C5 clutch is exhausted. C5 is exhausted in Low (7-speed) and 2nd through 6th ranges.

During normal operation the TCM expects PS1 to indicate PRESSURIZED (open) in Reverse and 2nd through 5th ranges. The TCM expects PS1 to indicate EXHAUSTED (closed) in Neutral (N), Low (for 7 speeds), 1st and 6th range. DTC P0842 becomes active if the PS1 switch is detected as EXHAUSTED (closed) when it should be detected PRESSURIZED (open).

With key ON and engine OFF, normal PS1 state is EXHAUSTED because it is a Normally Closed (NC) switch.

When PS1 is closed, W177 (PS1 input) is switched to W158 (analog return) and the diagnostic tool shows the switch EXHAUSTED. When PS1 is open, W177 will pull up the voltage internally on terminal 77 and the diagnostic tool shows the switch PRESSURIZED. PS1 opens at approximately 175-200 kPa (25-30 psi).

CONDITIONS FOR RUNNING THE DTC:
1. TCM initialization is complete.
2. Hydraulic system pressurized.
3. Transmission fluid temperature greater than -10°C (14°F).
4. When C5 clutch command is not in transition.
5. When C5 clutch is filled and PCS2 regulator (trim) valve is commanded stroked, or C5 is commanded to exhaust and both latch valves are commanded stroked.

CONDITIONS FOR SETTING THE DTC:
DTC P0842 sets when PS1 indicates EXHAUSTED (closed) when the TCM expects PS1 switch state to be PRESSURIZED (open). DTC may set while commanding Reverse and 2nd through 5th ranges.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0842 is active:
1. If failure occurs while in a forward range, then shift to another forward range. If failure occurs while in Reverse (R), the transmission remains in Reverse (R) or shifts to Neutral (N). Once range is attained, the transmission will be locked in range. There is no response to shift selector requests.
2. The TCM inhibits TCC engagement.
3. DTC is stored in the TCM history.
4. The CHECK TRANS light illuminates.
5. The 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 failure.

DIAGNOSTIC AIDS:
DTC P0842 can be caused by:
1. Unapproved fluid, contaminated fluid, e.g., glycol, water, etc.
2. W177 shorted to ground in chassis harness.
3. W177 (terminal 3 of 20-way) shorted to ground in internal transmission harness.
4. PS 1 issues such as:
– PS1 switch stuck mechanically closed.
– PS1 intermittently will not open.
– PS1 shorted to ground.
5. PCS1, PCS2, PCS3 solenoid or trim valve issues such as:
– Broken trim spring.
– Debris in trim valve bores causing sticking valves.
– Solenoids mechanically defective.
6. C1/C2 latch valve issues such as:
– Debris in valve body sticking C1 or C2 latch valve.
7. Defective solenoids SS1 or SS2 (7-speed models).
8. Broken or defective connector assemblies allowing pushed back pins to short together.
9. Unlocked connectors, missing seal plugs allowing moisture in connectors.
10. Incorrect service parts installed, e.g., non-prognostics channel plate gasket used on prognostics control module, etc.

NOTE: Channel plates, solenoid and main valve bodies, separator plates, and channel plates are incompatible between former non-prognostics control valve modules and current prognostics control valve modules. Do not intermix parts because DTCs will result.

11. Low or loss of C5 pressure.
12. Low main pressure.
13. Defective TCM.

Review applicable information in Control System and Transmission Specifications to find additional circuit specifications, system and connector diagrams, and troubleshooting tips.