Subaru Legacy BN/BS (2015-2019) Service Manual: Dtc p0843 transmission fluid pressure sensor/switch "a" circuit high

Subaru Legacy BN/BS (2015-2019) Service Manual / Continuously variable transmission (diagnostics) / Diagnostic procedure with diagnostic trouble code (dtc) / Dtc p0843 transmission fluid pressure sensor/switch "a" circuit high

DTC DETECTING CONDITION:

Immediately at fault recognition

TROUBLE SYMPTOM:

Shift characteristics malfunction

1. ENGINE TYPE FB

WIRING DIAGRAM:

CVT control system CVT Control System > WIRING DIAGRAM

STEP	CHECK	YES	NO
1.CHECK INPUT SIGNAL FOR TCM. 1) Start the engine. 2) Warm up until the ATF temperature reaches to 40 — 70°C (104 — 158°F). 3) Depress the brake pedal, and shift the select lever to “D” range. 4) Shift the select lever to “P” range. 5) Stabilize the engine speed at idle. 6) Read the data of «Actual secondary pressure» using Subaru Select Monitor.	Is the «Actual secondary pressure» 0.5 — 1.5 MPa-	Check for poor contact of connector.	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0843 TRANSMISSION FLUID PRESSURE SENSOR/SWITCH "A" CIRCUIT HIGH
2.CHECK HARNESS. 1) Turn the ignition switch to OFF. 2) Disconnect the connector of TCM and secondary pressure sensor. 3) Turn the ignition switch to ON. 4) Measure the voltage between TCM connector and chassis ground. Connector & terminal (T10) No. 5 (+) — Chassis ground (−): (T10) No. 33 (+) — Chassis ground (−):	Is the voltage approx. 0 V-	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0843 TRANSMISSION FLUID PRESSURE SENSOR/SWITCH "A" CIRCUIT HIGH	Repair the short circuit of harness.
3.CHECK HARNESS. 1) Turn the ignition switch to OFF. 2) Measure the resistance between TCM connector terminals. Connector & terminal (T10) No. 5 — (T10) No. 33:	Is the resistance less than 1 --	Repair the short circuit of harness.	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0843 TRANSMISSION FLUID PRESSURE SENSOR/SWITCH "A" CIRCUIT HIGH
4.CHECK POWER SUPPLY FOR SECONDARY PRESSURE SENSOR. 1) Connect the TCM connector. 2) Turn the ignition switch to ON. 3) Measure the voltage between secondary pressure sensor connector terminals. Connector & terminal (AT6) No. 1 (+) — (AT6) No. 3 (−):	Is the voltage 4.5 V or more-	Replace the secondary pressure sensor. Secondary Pressure Sensor If the replacement of the sensor do not eliminate the malfunction, replace the TCM. Transmission Control Module (TCM)	Check for poor contact of connector, and if no fault is found, replace the TCM. Transmission Control Module (TCM)

2. ENGINE TYPE EZ

WIRING DIAGRAM:

CVT control system CVT Control System > WIRING DIAGRAM

STEP	CHECK	YES	NO
1.CHECK INPUT SIGNAL FOR TCM. 1) Start the engine. 2) Warm up until the ATF temperature reaches to 40 — 70°C (104 — 158°F). 3) Depress the brake pedal, and shift the select lever to “D” range. 4) Shift the select lever to “P” range. 5) Stabilize the engine speed at idle. 6) Read the data of «Actual secondary pressure» using Subaru Select Monitor.	Is the «Actual secondary pressure» 0.5 — 1.5 MPa-	Check for poor contact of connector.	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0843 TRANSMISSION FLUID PRESSURE SENSOR/SWITCH "A" CIRCUIT HIGH
2.CHECK HARNESS. 1) Turn the ignition switch to OFF. 2) Disconnect the TCM and transmission connectors. 3) Turn the ignition switch to ON. 4) Measure the voltage between TCM connector and chassis ground. Connector & terminal (B54) No. 5 (+) — Chassis ground (−): (B54) No. 33 (+) — Chassis ground (−):	Is the voltage approx. 0 V-	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0843 TRANSMISSION FLUID PRESSURE SENSOR/SWITCH "A" CIRCUIT HIGH	Repair the short circuit of harness.
3.CHECK HARNESS. 1) Turn the ignition switch to OFF. 2) Measure the resistance between TCM connector terminals. Connector & terminal (B54) No. 5 — (B54) No. 33:	Is the resistance less than 1 --	Repair the short circuit of harness.	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0843 TRANSMISSION FLUID PRESSURE SENSOR/SWITCH "A" CIRCUIT HIGH
4.CHECK TRANSMISSION HARNESS. 1) Disconnect the secondary pressure sensor connector. 2) Turn the ignition switch to ON. 3) Measure the voltage between transmission connector and chassis ground. Connector & terminal (T4) No. 14 (+) — Chassis ground (−): (T4) No. 17 (+) — Chassis ground (−):	Is the voltage approx. 0 V-	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0843 TRANSMISSION FLUID PRESSURE SENSOR/SWITCH "A" CIRCUIT HIGH	Repair the short circuit of harness.
5.CHECK TRANSMISSION HARNESS. 1) Turn the ignition switch to OFF. 2) Measure the resistance between transmission connector terminals. Connector & terminal (T4) No. 14 — (T4) No. 17:	Is the resistance less than 1 --	Repair the short circuit of harness.	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0843 TRANSMISSION FLUID PRESSURE SENSOR/SWITCH "A" CIRCUIT HIGH
6.CHECK POWER SUPPLY FOR SECONDARY PRESSURE SENSOR. 1) Connect the connectors of TCM and transmission. 2) Turn the ignition switch to ON. 3) Measure the voltage between secondary pressure sensor connector terminals. Connector & terminal (AT6) No. 1 (+) — (AT6) No. 3 (−):	Is the voltage 4.5 V or more-	Replace the secondary pressure sensor. Secondary Pressure Sensor If the replacement of the sensor do not eliminate the malfunction, replace the TCM. Transmission Control Module (TCM)	Check for poor contact of connector, and if no fault is found, replace the TCM. Transmission Control Module (TCM)

3. OUTLINE OF DIAGNOSIS

• Detect short circuit to power supply or open circuit of the transmission line pressure sensor 5 V system.

• Judge as NG if the voltage detected by the transmission line pressure sensor is higher than the predetermined value.

4. EXECUTION CONDITION

Secondary Parameters	Execution condition
12 V battery system voltage	≥ 9 V

5. DIAGNOSTIC METHOD

If the duration of time while the following conditions are met is longer than the time indicated, judge as NG.

Judgment Value

Malfunction Criteria	Threshold Value
Measured line pressure sensor input voltage	> 4.883 V
(Line pressure)	(> 8200 kPa)

Time Needed for Diagnosis: 1.5 seconds

Malfunction Indicator Light Illumination: Illuminates as soon as a malfunction occurs.

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Inspection
1. INSPECTION OF SWITCH UNIT1. Operate the switches to check the continuity between terminals.Preparation tool:Circuit testerSwitch positionTerminal No.StandardOFFA1 and A3Less than 1 -INTA1 and A3Less than 1 -LOA2 and A3Less than 1 -HIA2 and A4Less than 1 -Washer ONB2 and B3Less than 1 -2. Replace ...