|Trouble Code||Fault Location||Probable Cause|
|P0746||Transmission fluid pressure (TFP) solenoid -performance or stuck off||Wiring, TFP solenoid|
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What Does Code P0746 Mean?
SPECIAL NOTES: Non-professional mechanics should note that if code P0746 occurs on an application with a “filled-for-life” transmission, the wiser option is to refer the vehicle to the dealer, or other competent repair shop for professional diagnosis and repair.
“Filled-for-life” transmissions are not fitted with dipsticks with which to measure the fluid level, and determining the actual level of the fluid in these transmissions is all but impossible without equipment that only the dealers possess. Thus, removing the oil pan on these transmissions to check the condition of the fluid, or to check for the presence of large wear particles (evidence of mechanical failure(s)) is NOT recommended, since it is nearly impossible to refill these transmissions with the correct amount of transmission fluid.
Bear in mind that over filling a transmission is as bad as under filling it and either condition can cause catastrophic transmission failure. END OF SPECIAL NOTES.
OBD II fault code P0746 is a generic code that is defined as “Transmission fluid pressure (TFP) solenoid – Performance or Stuck Off”, and is set when the PCM (Powertrain Control Module) detects a difference between the desired transmission fluid pressure, and the actual transmission fluid pressure at any given moment. While most causes of this code involve malfunctions in the control circuit of the pressure control solenoid, be aware that there are many possible non-electrical causes as well.
NOTE: In this definition, the words “stuck off”, refers to a condition in which the solenoid that controls the transmission fluid pressure is stuck in the “OFF”, or closed position. In this condition, the pressure of the transmission fluid cannot be controlled, directed, or regulated at all.
While the setting parameters for this code vary greatly between manufacturers, it should be noted that all automatic transmissions depend on pressurized transmission fluid to function properly. The pressure is delivered /developed by a pressure pump that is driven by the engine via the torque converter, and effective control of the pressurized fluid is essential for efficient gear selection and torque converter lock-up clutch operation.
In terms of operating principles, pressurized transmission fluid needs to be circulated through the transmission casing to provide proper lubrication to moving parts, as well as to provide pressure to move, and maintain pressure on clutch packs and planetary gear sets to effect gear changes, and to maintain a particular gear ratio selection.
Thus, to ensure that all parts of the transmission is supplied with both an adequate flow of transmission fluid, and transmission fluid that is pressurized sufficiently to act effectively on moving parts, the fluid is passed through a dedicated pressure control solenoid. Depending on operating conditions such as throttle opening, engine speed, transmission turbine speed, the currently selected gear, and the vehicle’s road speed, the PCM calculates the pressure required for the transmission to react appropriately to changing conditions.
For instance, based on current operating conditions such as hard acceleration, the PCM might instruct the TCM (Transmission Control Module) to initiate a measure of torque converter clutch slippage (to protect transmission components), before selecting a lower gear as an appropriate response to a large, and sudden throttle input. To perform these actions effectively, the transmission fluid needs to be at a certain pressure, which the PCM will provide by adjusting the flow rate through the transmission fluid pressure-control solenoid by manipulating the solenoid’s opening.
In a fully functional system, these actions are virtually seamless and the driver is not aware of the process. However, in a system that can control neither the flow rate, nor the pressure of the transmission fluid, the transmission cannot react appropriately to inputs such as the large throttle input in our example above. When this happens, the transmission may slip, gearshifts may become harsh, erratic, or unpredictable, and damage to various transmission components could occur as a result of the torque converter lock-up clutch not functioning as it should.
Note that on almost all applications, the PCM will set code P0746 and illuminate a warning light on the first failure cycle. Also, be aware that on some applications, the PCM will initiate a failsafe, or limp mode to protect the transmission against further damage.
The image below shows a typical transmission pressure control solenoid. Note though that while all such solenoids are located inside the transmission, the actual appearance, and exact location of these solenoids within the transmission varies greatly between applications and manufacturers.
What are the common causes of code P0746 ?
Some common causes of code P0746 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and /or connectors, both inside and outside of the transmission
- Defective pressure control solenoid
- Mechanical failure of some internal transmission components that can cause blockages of hydraulic circuits in the valve body and elsewhere in the transmission
- Failure of the pressure pump
- Dirty, contaminated, or degraded transmission fluid
- Low transmission fluid levels
- Use of unsuitable, or incorrect types / grades of transmission fluid
- Failed or failing PCM / TCM. Note that controller failure is rare, and the fault must be sought elsewhere before any controller is replaced
What are the symptoms of code P0746 ?
Common symptoms of code P0746 could include the following, but note that the severity of one or more of the symptoms listed here can vary between applications-
- Stored trouble code, and an illuminated warning light. Note that code P0746 can be accompanied by many other transmission related codes, some of which may be manufacturer specific
- Gearshifts may be harsh, erratic, unpredictable
- Depending on the nature of the problem, the transmission may not shift at all, or remain stuck in one gear
- Transmission may slip, or there may be varying degrees of hesitation before any gear is selected
- In some cases, the engine may stall when the vehicle is slowed prior to stopping because the torque converter lock-up clutch may be fully engaged
- Some applications may enter a failsafe, or limp mode that will persist until the problem is resolved
- Depending on the nature of the problem, some transmissions may emit mechanical noises such as whining, grinding, rumbling, or even knocking/thudding sounds in some cases
- On some applications, the transmission fluid may contain large or excessive amounts of metal wear particles. Note that this is can be either a cause or a symptom of code P0746.
How do you troubleshoot code P0746 ?
SPECIAL NOTES: Properly diagnosing this code requires that the functioning of the pressure pump in the transmission be checked. Doing this requires the use of a dedicated pressure gauge, but it should also be noted that all transmissions have multiple pressure-testing points. Different functions can be tested /checked at different points, which makes important to obtain a diagram of the hydraulic circuits in the transmission, as well as a diagram that CLEARLY indicates the location and function of ALL pressure testing points on the transmission.
Since failure of the pressure pump can set code P0746 on many applications, it is important to eliminate or confirm pump failure as the cause of the code on these applications, and the only way to do that is to test the pressure at the CORRECT pressure-testing point. Testing the pressure at any other point(s) will result in a misdiagnosis, wasted time, and the almost certain unnecessary replacement of expensive parts and components, if not the entire transmission. END OF SPECIAL NOTES.
NOTE: As noted above, a dedicated pressure gauge with which to verify transmission fluid pressure, as well as a diagram marking the location of all pressure testing points will be most helpful in diagnosing this code.
Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
NOTE: Pay particular attention to all and any additional codes that may be present- both active and pending. Transmission related codes that precede P0746 may have caused, or contributed to P0746 being set, which means that these codes must be investigated and resolved in the order in which they were stored, before a diagnosis of P0746 is attempted. Failure to do this will almost certainly result in a misdiagnosis, wasted time, and the unnecessary replacement of parts and components.
If the application does not have a “filled-for-life” transmission (refer to the SPECIAL NOTES at the top of this guide), check the level of the transmission fluid as per the instructions provided in the manual as a first step, since low fluid levels is a major cause of this code.
However, before topping off the fluid level (if required) be sure to inspect the fluid for signs of discoloration, degradation, contamination, or the presence of metal or other types of wear particles. Fluid that is dark in color, has a tarry consistency, or has a burnt odor can also cause this code. Thus, if the transmission fluid is in a less-than-perfect state, replace it with fluid of the correct type and grade as per the instructions provided in the manual before continuing with the diagnostic procedure.
WARNING: If the transmission fluid contains an excessive amount of metal wear particles, do NOT replace the fluid, or top off the fluid level before finding, and correcting the problem that is causing the wear particles. Excessive wear particles indicate a serious mechanical problem or failure that can be the cause of one or more transmission related codes / problems, and in many cases, the transmission has to be repaired or replaced before these codes can be cleared.
If however, the transmission fluid is clear and has a “fresh” smell, a low fluid level can only be the result of a fluid leak than in turn, can cause all manner of transmission related codes and problems. Thus, if the fluid level is low, find and repair all fluid leaks on the transmission before topping off the fluid level, or continuing with the diagnostic procedure.
NOTE: Be aware that many fluid leaks on automatic transmissions cannot be easily repaired on a DIY basis, since repairing some leaks requires the removal, and even partial disassembly of the transmission.
Consult the manual, or the diagram that indicates all the pressure-testing points on the transmission, and attach the pressure gauge to the point that will allow you to obtain a reading of the pressure that the pump develops during normal operation in way that will not cause pressure to be lost.
The object of this step is to see if the pressure pump works as it should, so be sure to follow the directions provided in the manual to do this correctly, and safely. Compare the obtained reading(s) with those stated in the manual. Allowable deviations will be stated in the manual so if this test yields values below the allowable deviations, the pressure pump is defective. In these cases, the wisest option is to refer the vehicle to the dealer or other competent repair shop for professional assistance.
If the transmission fluid is in good condition, the fluid is up to the “FULL” mark, the pressure pump delivers the correct pressure and there are no fluid leaks present, refer to the manual to locate the relevant wiring harness. This harness usually connects to a connector on the transmission casing, but use this time to also determine the function, and color-coding, of each wire in the connector.
Once the proper harness is identified, thoroughly inspect the full length of the harness (up to where it connects to the PCM or TCM for signs of damage, discoloration, corrosion, short circuits, or disconnected wires and or connectors. Make repairs as required if any of the above is found.
Clear all codes after repairs are complete, and operate the vehicle for at least one complete drive cycle to see if any codes return.
If the code persists, disconnect the transmission harness from the PCM / TCM (to prevent damage to the controller(s) and perform resistance, continuity, and ground connectivity checks as per the instructions provided in the manual on all circuits. Compare all obtained readings to the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.
Clear all codes after repairs are complete, and operate the vehicle for at least one complete drive cycle to see if any codes return.
If the transmission harness checks out, suspect an electrical problem in the transmission, or a failure of the pressure control solenoid itself. To confirm this, perform resistance and continuity checks on the transmission’s internal wiring from the connector on the transmission. Compare all obtained readings to the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.
If discrepancies are found, the only way to gain access to the wiring is by removing the transmission’s oil pan. However, do NOT remove the oil pan on a transmission with a “lifetime” fill- refer to the SPECIAL NOTES at the top of this guide.
If it is possible to remove the oil pan, do so safely, by preventing fluid spills when the fluid is drained off. Remove the oil pan, and inspect it for the presence of excessive amounts of wear particles, or wear particles that seem abnormally large. These articles could be metal, or from destroyed friction material, but regardless of what they are, they indicate a serious mechanical issue that must be resolved before the vehicle is put back in service.
If however, the oil pan is clear of wear particles, inspect the internal wiring. Bear in mind that due to the presence of metal in the transmission fluid, repairing the wiring is NOT recommended. If damaged wiring is found, the entire internal harness must be replaced to prevent possible short circuits caused by metal in the transmission fluid.
If the internal harness checks out, use the scanner to command the pressure control solenoid to cycle repeatedly to eliminate intermittent breaks in continuity as the cause of the code. The scanner will display the status of the solenoid- if the status does not change, the solenoid is defective, and must be replaced with an OEM part to ensure proper operation.
However, this is not always possible (or advisable) to do this on a DIY basis. If there is any doubt about your ability to remove and replace the solenoid, refer the vehicle to the dealer or other competent repair shop for professional assistance to prevent the possibility of causing damage to transmission components.
In cases where the manual provides clear instructions on how to remove/replace the pressure control solenoid, follow the instructions exactly to avoid issues later on. Replace the transmission filter with an OEM replacement, and use a new gasket when replacing the oil pan to prevent leaks.
Refill the transmission with fluid of the correct type and grade as per the instructions provided in the manual. Double check both the fluid level, and the transmission for leaks, clear all codes, and operate the vehicle normally before rescanning the system to see if the code returns.
At this point, it is unlikely that the code will return, but if it does return, suspect a defective PCM or TCM. If this is suspected, refer the vehicle for professional diagnosis, since most generic code readers cannot identify corrupted programming or other issues with control modules.
Codes Related to P0746
- P0745 – Relates to “Pressure Control Solenoid A Malfunction”
- P0747 – Relates to “Pressure Control Solenoid A Stuck On”
- P0748 – Relates to “Pressure Control Solenoid A Electrical”
- P0749 – Relates to “Pressure Control Solenoid A Intermittent”
BAT Team Discussions for P0746
I have a 1999 Olds Delta '88 LS. Apparently, the car is double shifting where it jerks very hard when shifting gears. My tranny guy says a code appears P0746. I had him replace the tranny with a remanufactured one from the GM dealer. He claims that it is not the transmission, but the computer. ...