Fix P0746: Mechanic’s solutions that WORKED

The P0746: Pressure Control Solenoid “A” Performance or Stuck Off code indicates a problem with the Pressure Control Solenoid “A” in the transmission, specifically that its performance is outside the expected range or it’s stuck in the off position. This solenoid regulates the hydraulic pressure in the transmission, crucial for proper shifting and operation. Issues with this solenoid can lead to noticeable transmission problems.

P0746 quick overview

Meaning	P0746: Pressure Control Solenoid “A” Performance or Stuck Off
Is it serious?	This code can be serious as it may affect the vehicle’s transmission performance, potentially leading to drivability issues.
Possible causes	– Faulty pressure control solenoid “A” – Hydraulic blockages in transmission fluid passages – Low or dirty transmission fluid – Transmission mechanical failure – Faulty Transmission Control Module (TCM) – Damaged or corroded wiring and connectors
How to diagnose?	Inspect Solenoid and Wiring: Check the pressure control solenoid and its wiring for any signs of damage or disconnection. Check Transmission Fluid: Verify the fluid level and quality; replace if dirty or low. Perform Solenoid Tests: Conduct electrical tests (like ohm testing) on the solenoid to evaluate its performance. Hydraulic Pressure Test: Perform a hydraulic pressure test to check for blockages or issues in the transmission fluid pathways. TCM Diagnosis: Diagnose the Transmission Control Module for faults or software updates. Clear and Recheck Codes: After repairs, clear the code with a scanner and test drive to see if it returns.

How to fix P0746 – learn from mechanics

I looked into P0746 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Toyota

Vehicle Information: 2004 Toyota RAV4 2.4L – Hard Incorrect Shifting

Issue + Repair History:

• After transmission rebuild, confusion arose regarding input and output speed sensors due to their identical appearance.
• Testing sensor configurations led to the discovery that one arrangement caused smooth shifting with a P0746 code, while the other caused hard shifting without immediate codes.
• Replaced pressure solenoid A and valve body during the second transmission opening.
• Considering a PCM reflash, but skeptical due to the mechanical nature of the initial failure.
• Persistent hard shifting and P0746 code even after replacing valve body and solenoid.

Suggestions to Fix Issue:

• Check SL1 solenoid signals from ECM.
• Replace ECM due to its potential role in transmission failure.
• Consider turbine or intermediate shaft issues impacting sensor wave signals.
• Evaluate the need for a rebuilt or reprogrammed computer for the transmission.
• Review the possibility of ECU issues causing premature transmission wear.

Final Fix: Replaced the vehicle’s Fujitsu ECU with an updated part number, resolving the issue. The problem was initially difficult to diagnose due to a lack of TSBs or extended warranty coverage from the dealer.

Case #2: Lexus

Vehicle Information: 2003 Lexus ES300 3.0L – Poor/No Shift Quality

Issue + Repair History:

• Initial problem with P0746 shift solenoid A; solenoid replaced after bad ohm reading, but issue persisted.
• Entire transmission replaced, yet the same issues continued.
• Car lacked 1st, 3rd, and 5th gears, potentially operating in limp mode with only 2nd and 4th gears.
• During shifts, the car felt like it was trying to engage two gears simultaneously.
• Same error codes (P0746 and P0796) appeared even after transmission replacement.
• Questioned whether PCM reflash was necessary or if another issue was overlooked.

Suggestions to Fix Issue:

• Replace the Transmission Control Module (TCM) and possibly reflash it.
• Check and clean ground wires on the back of the intake.
• Consider problems with wiring or PCM.
• Explore extended warranties for trans controllers setting false codes.
• Investigate mechanical faults indicated by DTCs and potential debris in valve body.

Final Fix: The vehicle was fixed by replacing the PCM. All engine grounds were checked and cleaned, and the harness was inspected for issues, but none were found. The replacement of the PCM resulted in the car shifting correctly.

Case #3: Chevrolet

Vehicle Information: 2004 Chevrolet Silverado 3500 6.6L – 5th Gear Issue

Issue + Repair History:

• The vehicle, used extensively for towing, experienced sudden revving and redlining when towing. It would slip after stopping but return to normal after turning off the ignition, though the check engine light remained on.
• Another shop had previously replaced the pressure control solenoid after encountering a P0746 code, but the issue persisted.
• The vehicle did not replicate the problem during tests without a trailer, and all sensor readings were normal.
• The transfer case fluid appeared fine, raising questions about the actual cause of the issue.

Suggestions to Fix Issue:

• Check for wear in the transfer case, specifically the shift fork, which could cause a fifth gear ratio error.
• Replace the complete transfer case if worn shift fork and high/low range sleeve are found, as they might cause the vehicle to fall out of gear under load.
• Utilize GM TSB#2010102 for guidance on snapshotting and analyzing speed sensor readings, especially if the Output Speed Sensor is erratic.
• Consider the possibility of a faulty ignition switch affecting the TCM voltage.

Final Fix:

• Resolved the issue by replacing the rear half of the transfer case due to significant play where the output shaft bearing sits, and also replaced the stretched chain.
• A bad driveshaft support bearing was also replaced.
• The customer reported no further issues after these repairs.

Case #4: Volkswagen

Vehicle Information: 2006 Volkswagen Jetta TDI 1.9L – Transmission Shifting Issues

Issue + Repair History:

• After replacing the DSG clutch unit and valve body, the vehicle exhibited the same problems: no reverse, not shifting out of first gear, PRNDL light flashing, and failure to do the basic setting or enter default mode.
• Despite various diagnostic efforts, including using Rosstech VCDS, the only code present was P0746 (pressure control solenoid performance or stuck off).
• Attempts to induce limp mode by disconnecting the electrical transmission connector were unsuccessful, as the car would not shift out of park or take any gear.

Suggestions to Fix Issue:

• Check if the transmission shifts in manual mode and ensure the correct DSG fluid and filter cartridge were used.
• Monitor ABS wheel sensors and shifter output with a scan tool.
• Consider internal mechanical problems, such as major internal pressure leaks or valve body issues, if reverse is not achieved in limp mode.
• Test line pressure and reverse circuit pressure, and perform air tests on various element circuits.
• Check transmission and body grounds, specifically under the battery, and ensure no debris entered the valve body during the clutch change.

Final Fix:

• Discovered a loose lead in the valve body circuit board, which was repaired using a hot air station.
• After the repair, the transmission functioned correctly, resolving the issues.

Case #5: GMC

Vehicle Information: 2003 GMC Sierra 2500 HD 6.6L – Intermittent MIL Lamp On

Issue + Repair History:

• Persistent, intermittent issue present for nearly a year.
• Initially, the vehicle speed sensor (VSS) in the transfer case was replaced, and the transfer case was rebuilt, but the problem persisted.
• A second-hand transmission computer was tried without resolving the issue.
• Wiring from the VSS to the transmission computer was checked and found to be faultless.
• Some wires near the transmission computer connector were rubbed through and fixed, but this did not resolve the issue.

Computer Codes & Descriptions:

• P0500: VSS Circuit
• P0700: Transmission Control System
• U1152: Loss of ACM/HVAC Communication
• P0721: Output Speed Sensor
• P0746: Pressure Control Solenoid Performance

Suggestions to Fix Issue:

• Inspect the pigtail to the OSS or VSS at the back of the transmission for wire breakage where the pigtail enters the harness.
• Conduct an ohm test on the wires to the speed sensor while wiggling the harness to detect any erratic behavior that might indicate a wiring issue.

Additional Notes:

• The exact nature of the problem the truck is experiencing wasn’t explicitly described in the original post.
• Another truck with similar symptoms came in, and any solution found for that vehicle will be shared.

Advice	Brief Description of Diagnostic Step
Inspect Wiring to Pressure Control Solenoid	Check the wiring and connectors leading to the pressure control solenoid for any damage, disconnection, or wear.
Ohm Test Pressure Control Solenoid	Perform an ohm test on the pressure control solenoid to verify its functionality and to check if it’s stuck or failing.
Check Solenoid’s Power Supply and Ground Connections	Ensure the solenoid is receiving proper power and ground connections, as issues here can lead to malfunction.
Scan for Additional Transmission-Related DTCs	Use a scanner to check for other transmission-related codes that might be associated with or contributing to the P0746 code.
Perform Hydraulic Pressure Tests	Conduct hydraulic pressure tests to determine if there are issues in the hydraulic circuit affecting the solenoid’s performance.
Inspect Valve Body for Blockages or Damage	Examine the transmission valve body for any blockages, damage, or sticking valves that could impact the solenoid’s operation.
Reset and Relearn Transmission Adaptations	After repairs, reset the transmission adaptations and allow the vehicle to relearn shifting patterns.
Check Transmission Fluid Level and Quality	Low or contaminated transmission fluid can cause solenoid issues; ensure the fluid is at the correct level and in good condition.
Consider Mechanical Issues within Transmission	Look for mechanical issues in the transmission that could affect solenoid performance, such as worn components or internal damage.
Update TCM Software if Necessary	Check for any available software updates for the Transmission Control Module that could resolve electronic-related solenoid issues.