P0868-LINE PRESSURE LOW





For a complete wiring diagram Refer to Section 8W

Theory of Operation


Line pressure is measured by the Line Pressure Sensor (LPS) and regulation is achieved by changing the duty cycle of the Pressure Control Solenoid (PCS) controlled by the Transmission Control System. (5% duty cycle = solenoid off = Max line pressure, 62% duty cycle = solenoid on = Min line pressure). The Transmission Control System calculates the desired line pressure based on inputs from both the engine and transmission.

The Transmission Control System calculates torque input to the transmission and uses it as the primary input to the desired line pressure calculation. This is called Torque Based Line Pressure. In addition, the line pressure is set to a preset level 827 or 931 kPa (120 or 135 psi) during shifts and in Park and Neutral to ensure consistent shift quality. The desired line pressure is continuously being compared to the actual line pressure. If the actual line pressure is consistently lower than the target while driving, the line pressure low DTC P0868 will set.


  • When Monitored:

    Continuously while driving in a forward gear.

  • Set Condition:

    The PCM continuously monitors Actual Line Pressure and compares it to Desired Line Pressure. If the Actual Line Pressure is more than 10 psi below Desired Line Pressure, this DTC will set.


Possible Causes
  
CHECK FOR RELATED DTCS
  
LOW FLUID LEVEL
  
(F856) 5-VOLT SUPPLY CIRCUIT OPEN
  
(F856) 5-VOLT SUPPLY CIRCUIT SHORT TO GROUND
  
(F856) 5-VOLT SUPPLY CIRCUIT SHORT TO VOLTAGE
  
(T118) PRESSURE CONTROL SOLENOID CONTROL CIRCUIT SHORT TO ANOTHER CIRCUIT
  
INTERNAL TRANSMISSION
  
LINE PRESSURE SENSOR
  
CRACKED, PLUGGED, OR MIS-INSTALLED PRIMARY OIL FILTER
  
STUCK OR STICKING MAIN REGULATOR VALVE
  
POWERTRAIN CONTROL MODULE
  

Always perform the 42RLE Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 42RLE - STANDARD PROCEDURE)



Diagnostic Test

1.CHECK FOR RELATED DTCS



Is the DTC P0932 present also?

Yes
  • Refer to the Transmission category and perform the appropriate symptom.


No
  • Go To 2

2.CHECK IF THE DTC IS CURRENT


NOTE: This counter only applies to the last DTC set.



Is the STARTS SINCE SET COUNTER 2 or less?

Yes
  • Go To 3


No

3.CHECK THE PCM AND WIRING



CAUTION: Removal of the Starter Relay will prevent the vehicle from being started in gear.


WARNING: The Starter Relay must be removed from the PDC. Failure to do so can result in personal injury or death.


NOTE: Removal of the Starter Relay is to prevent a Transmission, NO RESPONSE, condition and disable the starter.







NOTE: All three scan tool Line Pressure readings should be steady and ± 14 kPa or 2.0 psi of the reading specified on the Transmission Simulator.



Did the Line Pressure read within ± 14 kPa or 2.0 psi in all three positions?

Yes
  • Go To 4


No
  • Go To 6

4.CHECK THE LINE PRESSURE SENSOR







Is the line pressure gauge reading within 34 kPa or 5 psi of the scan tool reading?

Yes
  • Go To 5


No

5.CHECK FOR A PLUGGED TRANSMISSION OIL FILTER





Does the Oil Pan contain excessive debris and/or is the Transmission Oil Filter plugged?

Yes


No

6.CHECK THE (F856) 5-VOLT SUPPLY CIRCUIT FOR A SHORT TO VOLTAGE








Is the voltage above 5.5 volts?

Yes


No
  • Go To 7

7.CHECK THE (F856) 5-VOLT SUPPLY CIRCUIT FOR AN OPEN





CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller tool #8815 to perform diagnosis.




Is the resistance above 5.0 ohms?

Yes


No
  • Go To 8

8.CHECK THE (F856) 5-VOLT SUPPLY CIRCUIT FOR A SHORT TO GROUND





Is the resistance below 5.0 ohms?

Yes


No
  • Go To 9

9.CHECK THE (T118) PRESSURE CONTROL SOLENOID CONTROL CIRCUIT FOR A SHORT TO ANOTHER CIRCUIT








Is the resistance below 5.0 ohms between the (T118) Pressure Control Solenoid Control circuit and any other circuit(s) in the Line Pressure Sensor/Variable Force Solenoid Assembly harness connector?

Yes


No
  • Using the schematics as a guide, check the Powertrain Control Module (PCM) terminals for corrosion, damage, or terminal push out. Pay particular attention to all power and ground circuits. If no problems are found, replace and program the PCM per the Service Information. With the scan tool, perform QUICK LEARN.
  • Perform 42RLE TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 42RLE - STANDARD PROCEDURE)

10.INTERMITTENT WIRING AND CONNECTORS






Where there any problems found?

Yes


No
  • Test Complete.