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 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.
-
This step tests the TCM response function.
-
This step tests the transmission wiring harness integrity.
-
This step tests and evaluates the internal wiring harness.
-
This step tests the PSM for a failed condition.
Step | Action | Value | Yes | No |
|---|---|---|---|---|
Schematic Reference: Automatic Transmission Controls Schematics | ||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
2 | Measure the transmission fluid level. Refer to Transmission Fluid Check . Is the transmission fluid level correct? | -- | Go to Step 3 | Go to Transmission Fluid Check |
3 | Measure line pressure. Refer to Line Pressure Check . Is the pressure within the specified value? | -- | Go to Step 4 | |
4 |
Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records for reference. The Clear Info function will erase the data. Does the scan tool indicate TFP SW. C status is ON? | -- | Go to Diagnostic Aids | Go to Step 5 |
Does the scan tool indicate TFP SW. C status is ON when grounded and OFF when open? | -- | Go to Step 6 | Go to Step 12 | |
Does the scan tool indicate TFP SW. C status is ON when grounded and OFF when open? | -- | Go to Step 7 | Go to Step 11 | |
Does the scan tool indicate TFP SW. C status is ON when grounded and OFF when open? | -- | Go to Step 8 | Go to Step 10 | |
Does the scan tool indicate TFP SW. C status is ON when grounded and OFF when open? | -- | Go to Diagnostic Aids | Go to Step 9 | |
9 | Replace the PSM. Refer to Pressure Switch Manifold Replacement . Did you complete the replacement? | -- | Go to Step 13 | -- |
10 | Replace the AT internal wiring harness. Refer to Wiring Harness Replacement . Did you complete the replacement? | -- | Go to Step 13 | -- |
11 | Repair engine wiring harness circuit for an open or short. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 13 | -- |
12 |
Important: In most cases, the TCM is not at fault. Investigate thoroughly before replacing the TCM. Replace the TCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 13 | -- |
13 | Perform the following procedure in order to verify the repair:
Has the test run and passed? | -- | System OK | Go to Step 1 |
