|Code||Fault Location||Probable Cause|
|P0778|| Pressure control solenoid B -electrical malfunction |
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|Wiring, pressure control solenoid|
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What Does Code P0778 Mean?
OBD II fault code P0778 is a generic code that is defined as “Pressure control solenoid B -electrical malfunction”, and is set when the PCM (Powertrain Control Module) detects that the output voltage to the pressure control solenoid labelled “B” is either stuck at 0 volts, or is shorted to battery voltage when the solenoid is operating according to valid TCM (Transmission Control Module) gear shift calculations and strategies.
All automatic transmissions use varying degrees of hydraulic pressure to accomplish gear shifts, and to engage/disengage various transmission components such as clutch packs and the torque converter lock-up clutch.
To make gear shifts comfortable and efficient, the PCM/TCM controls a series of pressure control solenoids to redirect pressurized transmission fluid from one circuit to another. This process is roughly analogous to the way different gears are decoupled and engaged in different combinations in manual transmissions to bring about different gear ratios.
However, in automatic transmissions the process of establishing different gear ratios depends on how efficiently pressurized fluid is channeled between hydraulic circuits, and should one pressure control solenoid (or its control circuit) fail, pressurized fluid cannot be controlled effectively. When this happens the PCM or TCM will set code P0778, and may also illuminate a warning light.
The image below shows a typical transmission pressure control solenoid.
What are the common causes of code P0778 ?
Common causes of code P0788 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and or connectors
- Defective pressure control solenoid “B”
- Dirty, contaminated, or degraded transmission fluid
- Clogged fluid passages
- Dirty or clogged fluid filter
- Defective valve body
- Defective pressure pump
- Failed or failing PCM/TCM. Note that this is a rare event and the fault must be sought elsewhere before any control module is replaced
What are the symptoms of code P0778 ?
Note that while the symptoms of this code are largely make and model specific, some possible symptoms could include the following-
- Stored trouble code and possibly an illuminated warning light
- Other transmission related codes may be present, and especially codes that relate to the torque converter clutch
- Gear shifts may be harsh, erratic, or unpredictable
- Varying degrees of transmission slippage may occur under certain driving conditions
- Gear selection may be delayed
- Transmission may overheat
- Fuel consumption may increase noticeably
How do you troubleshoot code P0778 ?
NOTE: Note that code P0778 most commonly refers to an electrical malfunction in the circuit that controls the pressure control solenoid labelled “B” and only rarely to a condition where insufficient fluid pressure is being developed by the transmission’s internal pressure pump. However, be sure to refer to the manual to determine which control solenoid is labelled “B”, since manufacturers do not always follow convention when it comes to labelling parts, components, systems, and circuits. Testing the wrong circuit will result in a misdiagnosis, wasted time, and the almost certain unnecessary replacement of parts and components.
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.
Refer to the manual to identify and locate pressure control solenoid “B”, and its related wiring. Also determine the color-coding and function of each wire in the relevant harness.
Perform a thorough visual inspection of all related wiring. Look for damaged, burnt, shorted, disconnected, or corroded wiring and /or connectors. Make repairs as required.
If the code persists, perform resistance, ground, continuity, and reference voltage checks on all related wiring as per the instructions in the manual. Pay particular attention to the resistance values, since an abnormal resistance can indicate a short or open circuit. Compare all obtained readings with the values stated in the manual, and make repairs as required to ensure that all electrical values fall within the ranges specified by the manufacturer.
NOTE: Be sure to disconnect all wiring from the PCM /TCM during this step to prevent damage to the controller(s).
If all wiring checks out, refer to the manual to determine the correct procedure to follow to test the internal resistance of pressure control solenoid “B”, since its resistance is a reasonably good indicator of its overall condition. Replace the solenoid if its internal resistance does not agree with the value stated in the manual.
NOTE: Take note that while the pressure control solenoid is sometimes accessible from outside of the transmission, this solenoid is located inside the transmission on many applications. Since replacement of the solenoid in these cases requires partial disassembly of the transmission, the better option is to refer the vehicle to the dealer or other competent repair facility for professional assistance, and especially if the transmission has a “life-time” fluid fill.