|Code||Fault Location||Probable Cause|
|P0784|| Gear selection, 4 -5 -shift malfunction |
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|Wiring, TR sensor, shift solenoids, transmission mechanical fault|
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What Does Code P0784 Mean?
OBD II fault code is defined as “4-5 Shift Malfunction” and is set when the PCM (Powertrain Control Module) detects a malfunction or abnormality in the hydraulic (or sometimes electrical) circuit that controls gearshifts between first and second gear on fully automatic transmissions.
Gearshifts on fully automatic transmissions are accomplished by a microprocessor that controls shift solenoids that manage the effective transfer of pressurized transmission fluid between various hydraulic circuits. By switching pressurized fluid from one circuit to another, different gear ratios are established by the reconfiguration of the planetary gears in the transmission. This reconfiguration has the same effect as changing gears on a manual transmission to produce different gear ratios.
Automatic transmissions use an internal pump that is driven by the engine through the torque converter to maintain the pressure required by the transmission clutch packs to transmit power through a planetary gear set. Therefore, when a shift solenoid fails, or inhibits the free movement of pressurized fluid from one circuit to another, there is insufficient pressure in the affected circuit to engage a gear, or to maintain the selected gear ratio.
Although design specifics vary greatly from one manufacturer to the next, all applications use input data such as the engine load and speed, transmission turbine speed, throttle position, and road speed to calculate the optimum gear ratio to suit the reigning operating conditions. By always ensuring that the selected gear ratio is appropriate for current operating conditions, engine performance is maximized, and the least fuel is used.
Code setting parameters also differ between manufacturers, but in general, code P0784 will be set when the PCM detects a mismatch between the desired gear ratio, and the gear ratio that the transmission is actually in at the time. When this happens, a trouble code will be stored, but the CHECK ENGINE warning light may, or may not illuminate, since some applications require several failure cycles before any warning lights will illuminate.
The image below shows the typical location of shift solenoids in an automatic transmission. Note the healthy, bright red color of the transmission fluid.
What are the common causes of code P0784 ?
The most common causes of code P0784 are low transmission fluid levels, and old, degraded transmission fluid. Other possible causes include-
Incorrect, or unsuitable transmission fluid
Blocked, or clogged internal fluid passages.
Open, shorted, or damaged wiring and corroded connectors.
Defective shift solenoids.
Mechanical failures inside the transmission.
TCM/PCM failures, but this is very rare.
What are the symptoms of code P0784 ?
In some cases, there may be no symptoms other than a stored trouble code, and an illuminated CHECK ENGINE light. Other possible symptoms could include-
The presence of other shift-related trouble codes, such as P0780 – “Shift Malfunction”
Failure to engage any gear.
Harsh, erratic, or unpredictable shifting.
Increased fuel consumption.
The transmission may enter into a “limp” mode.
How do you troubleshoot code P0784 ?
NOTE: It is important to note that electrical control circuit failures can cause code P0784 to be set as well. Do not automatically assume that the 4-5 shift solenoid is faulty; perform a thorough inspection of all associated wiring and connectors to rule out electrical issues as the cause of code P0784. It is also recommended that you have a good repair manual for the vehicle being worked on at hand to determine the location, color-coding, and routing of all associated wiring.
Extract all fault codes present, and record all freeze frame data for future reference should an intermittent fault be identified later on.
Check all associated wiring for continuity (disconnect all control modules before starting continuity checks), ground, and resistance, and compare obtained readings to the values stated in the manual. Repair wiring as required.
If no electrical issues are found, check the transmission fluid to verify that it is at the correct level, that the fluid is not dirty and/or contaminated, and that it does not have a burnt smell.
NOTE #1: It may not be possible to do this check on all vehicles, since many newer vehicles do not have transmission dipsticks. If needs be, it may be necessary to draw a sample of transmission fluid from the fluid cooler lines. Consult the manual to locate the cooler lines, but be careful not to introduce dirt into the fluid cooling lines.
NOTE #2: Transmission fluid that appears dark, or has an abnormal smell must always be considered as inadequate, and must be replaced. Bear in mind that a recent episode of engine overheating can degrade transmission fluid, so make sure that the engine has not overheated to any degree in the recent past.
If the transmission fluid is suspect in any way, it must be replaced. However, this may present a problem on vehicles with lifetime transmission fills, since it is sometimes impossible to know when the fluid in these transmissions is at the correct level. Too much fluid is just as bad as too little, so follow the directions on fluid replacement in the repair manual, or remove the vehicle to a dealer for further diagnosis and repair.
On vehicles where a fluid replacement does not present major problems, remove the transmission oil pan, and check for the presence of large metal fragments, clutch friction material, or other signs that the transmission had suffered some sort of mechanical failure.
If any such debris is found, it is often more cost effective to replace the entire transmission with a rebuilt unit than to attempt repairs. However, if no debris is found, perform a thorough inspection of all internal wiring and connectors. Consult the manual to identify the affected shift solenoid, but take note that the solenoid itself forms part of the electrical control circuit, and it must therefore also be checked for continuity, and resistance.
NOTE: Do NOT attempt to repair any burnt, shorted, or otherwise-damaged wiring, and or solenoids found during the inspection. Most, if not all, applications require the replacement of the entire internal wiring harness to ensure a reliable repair.
If the internal wiring appears to be normal, or after replacing the internal harness, connect a code reader and attempt to activate the affected solenoid manually. The scanner will inform you if the solenoid is working or not; if it does, activate it repeatedly to check for intermittent faults. If the solenoid does not work, and all wiring and connectors is in perfect condition, replace the solenoid. Retest the system after the solenoid replacement to ensure that it works properly.
Replace the transmission oil pan (fitting a new filter and gasket ), fill the transmission with fluid of the correct type and grade, and test the vehicle with a scanner connected to monitor the system during normal operation.
There should be no adverse readings, and the code should not return. However, if the fault or any symptom(s) do return, it is very likely that the operating pressure in the transmission is too low. Consult the manual on the correct procedure to check the pressure delivered by the pump.
NOTE: It is important to follow the directions in the manual exactly in order to obtain an accurate result. For instance, some applications require that the transmission fluid be at a specific temperature during a pressure test, so consult the manual on this point.
If the transmission operating pressure does not fall within the range specified in the manual, make repairs as required, or refer the vehicle to a dealer for further diagnosis and repair.
If despite all repair attempts the code, and/or symptoms persist, suspect a defective TCM. Bear in mind though that control module failures are rare, and the fault must be sought elsewhere before any control modules are replaced, which requires that the replacement must be programmed.
In some cases, there may be an intermittent fault present, which you may have to allow to worsen before an accurate diagnosis and definitive repair can be made.
Codes Related to P0784
NOTE: In some cases, code P0780, “Shift Malfunction”, may be present along with the above, related codes.