Do you have a scan tool that can cycle the transmission controller and transmission pressure gauge? Here is the diagnostic process. One major item not in this tree is that the line pressure solenoid that controls the first gear clutch, controls the line pressure throughout the transmission. If it was failing the transmission would slip in all gears. Not just first.
During normal driving modules the PCM/TC adapts line pressure to prevent clutch slip and harsh shifts. If the first clutch is slipping after the pressure is adjusted to the high line, it is determined that there is no first gear. First gear clutch is fed directly from the manual valve and is always applied in all forward gear selections.
Important: First gear is a mechanical hydraulic apply. Other than line pressure, there is no electrical circuit that affects this gear.
Conditions for Setting the DTC
DTC P0731 will set when the PCM has detected the following twice in an ignition cycle:
Vehicle speed is greater than 5 km/h (3 mph).
Input shaft speed is greater than 224 RPM.
No input shaft or vehicle speed signal noise.
High line pressure has been commanded.
Ratio failure counter equals 225 for an in-gear failure, or the shift failure counter is greater than 100 on a downshift to first during acceleration. 128 equals 0 or normal operation. Or second gear must be commanded ON for the vehicle to move.
Numbered steps below refer to numbered steps on the chart.
Check for DTCs P0745, P0716, and P0717. They must be eliminated first as a possible cause for DTC P0731.
Pressure test at the same temperature that the fault occurred to assure the fault is not a pressure problem that only occurs in a particular temperature range. Low line pressure, which would be indicated by DTC P0745 line pressure low, must be eliminated first as a possible cause for DTC P0731.
Refer to Test Section Diagnosis in Automatic Transaxle for pressure apply points. Remove the valve body and air check the first clutch apply through the case passage.
This step targets the specific mechanical failure.
Remove the end cover and check for damaged first gear hub seal rings and check for a blocked passage in the cover or case.
If the seal rings are okay, remove the first clutch and replace the leaking piston.
Servo leak causing loss of 1st oil pressure -- Remove the valve body and air check and check the first oil pressure passage to servo. Also, check the one, three, and four passages to servo.
No Blockage In Passages
The first gear sprag failed.
The first clutch plates failed as a result of low line pressure or inadequate lube.
The output shaft is broken.
The incorrect gear ratio is installed.
Monitor the DTC menu Code Status for Pass Since Code Clear and Pass This Ignition Cycle. When both have passed, the repair is verified.
Criteria for pass:
Steady in first gear with no slip for 0.4 seconds.
Engine torque is greater than thirty counts for 0.1 seconds.
To air check the first clutch, use the first clutch pressure port in the test section. Apply compressed air with the rubber nozzle on an air gun.
DTC P0731 may be set under certain conditions if an intermittent loss of the vehicle speed signal occurs. If the customer is also experiencing speedometer problems, check the VSS circuit as a possible cause of DTC P0731.
A short-to-ground or open in the turbine speed sensor wiring will set DTC P0731. Verify using the scan tool.
Sunday, January 22nd, 2017 AT 8:52 AM