|Trouble Code||Fault Location||Probable Cause|
|P0978||Shift solenoid (SS) C -control circuit range/performance||Wiring, poor connection, shift solenoid, ECM/PCM/TCM|
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What Does Code P0978 Mean?
In an automatic transmission, gears are changed based on variances in pressure through the valve body. Pressure changes were modulated by spool valves previously, but on modern transmissions, spool valves have been replaced in large part with solenoid valves.
These solenoid valves are activated by the module that controls transmission shifts, typically the transmission control module (TCM) or the engine control module (ECM). When the gearshift position is changed, the TCM activates solenoid valves so the transmission can operate specific gears. For each gear that’s selected, there can be one or many solenoid valves that are activated, and as the gears change solenoids are deactivated and others activated.
The specific sequence that solenoid valves operate varies depending on the manufacturer. Each transmission manufacturer uses solenoid valves in different combinations based on their individual design. However, if code P0978 is present, the transmission will contain shift solenoid C, or SSC on the valve body in some form. Shift solenoids are numbered alphabetically, so shift solenoid C indicates it is the third shift solenoid on the valve body.
While the transmission is operating, the TCM checks each solenoid valve’s operation. If SSC fails its check more than twice in an ignition cycle, DTC P0978 is set in the TCM.
What are the common causes of code P0978 ?
There are a few common causes for DTC P0978:
Solenoid failure – DTC P0978 can be set from a malfunctioning shift solenoid. While it is not common to find an internal transmission shift solenoid failure, it is the most common cause of the shift solenoid range performance code.
Wiring problem – A poor connection, corrosion, or moisture at any of the wiring harness connectors from the TCM to the transmission can cause DTC P0978 to set. A common wiring issue is a break or short in the wiring caused by rodents or possibly damaged wiring from another repair.
Control module failure – It’s possible for DTC P0978 to set because of a fault in the transmission control module, though is seldom the only code present when the module has a fault. Check for multiple DTCs in the module to help assist in diagnosing module failure.
What are the symptoms of code P0978 ?
When DTC P0978 is set, the Check Engine light will be illuminated. Because the TCM is not able to determine if SSC is functioning as it should, it will enter failsafe operation, restricting functions such as adaptive controls and self-learning controls. The transmission may lock out certain gears related to SSC or may limit the shift range, so you may notice sluggish acceleration or uncharacteristic high revving. If SSC is used for torque converter lockup or as a pressure regulator to dampen harsh shifts, the transmission may operate through its full range of gears though there can be a noticeable harsh shift or your fuel economy may suffer.
How do you troubleshoot code P0978 ?
Code P0978 indicates a performance-related issue with shift solenoid C which can be attributed to a failed sensor, a wiring issue, or an ECM/TCM problem. To properly diagnose the SSC, you’ll need a vehicle-specific wiring diagram and a digital volt-ohm meter (DVOM) to check for resistance and continuity for SSC itself and the wiring harness.
1- Wiring harness inspection. Many electrical problems are related to wiring issues or poor connections. On your wiring diagram, determine which module communicates with SSC. Trace the wiring harness from the module to the transmission, inspecting for breaks in the wire, corrosion, or poor connections. Ensure each connector’s lock tab is fully seated or poor communication could result, setting a code for SSC performance in the module.
After ensuring the wiring is in good condition, remove and inspect each connector in the wiring harness for the transmission harness. Check for moisture in the connector, corrosion on the pins or terminals, bent or damaged pins, and dirt inside the connector. If there is any damage inside the connector, repair it and reconnect it, ensuring the connector is fully seated and secured with its lock tab.
Check continuity at the TCM harness using your DVOM. Disconnect the connector from the TCM module, then locate the signal circuit on the connector using your vehicle-specific wiring diagram. Test the signal circuit for continuity to chassis ground with your DVOM. Continuity should not be present. If there is, check the signal wire for a short to ground. Wiggle the wiring harness connectors and along the wiring harness, monitoring for fluctuations in continuity that may indicate an intermittent connection. Check the SSC ground circuit wire to chassis ground in the same way. Continuity should be present. If not, check for a break in the ground circuit for SSC.
Courtesy: Hyundai Motor Co.
Test the transmission’s operation to verify that the condition has been corrected. If the condition is still present, continue with the diagnosis.
2- Shift Solenoid C inspection
At shift solenoid C, test the continuity between the signal and ground circuit. Test for continuity with the engine off and ignition switch in the on position. You will need specifications for your vehicle to determine the correct resistance for your vehicle’s SSC. The continuity should be within the specified resistance. If it is not, SSC needs to be replaced.
Courtesy: Hyundai Motor Co.
3- TCM/ECM inspection
If the wiring passes inspection and shift solenoid C passes its resistance test, the control module may be to blame. Exchange the control module with a known good module, then test the transmission operation to verify the condition is repaired.
Because of the nature of electrical concerns, there may be an intermittent problem in any one of the possible causes. As well, after repairs are completed, it may be required to perform a relearn procedure for the transmission control module to determine gear positions.
Codes Related to P0978
- DTC P0972 Shift Solenoid “A” Control Circuit Range/Performance
- DTC P0973 Shift Solenoid “A” Control Circuit Low
- DTC P0974 Shift Solenoid “A” Control Circuit High
- DTC P0975 Shift Solenoid “B” Control Circuit Range/Performance
- DTC P0976 Shift Solenoid “B” Control Circuit Low
- DTC P0977 Shift Solenoid “B” Control Circuit High
- DTC P0978 Shift Solenoid “C” Control Circuit Range/Performance
- DTC P0979 Shift Solenoid “C” Control Circuit Low
- DTC P0980 Shift Solenoid “C” Control Circuit High
- DTC P0981 Shift Solenoid “D” Control Circuit Range/Performance
- DTC P0982 Shift Solenoid “D” Control Circuit Low
- DTC P0983 Shift Solenoid “D” Control Circuit High
- DTC P0984 Shift Solenoid “E” Control Circuit Range/Performance
- DTC P0985 Shift Solenoid “E” Control Circuit Low
- DTC P0986 Shift Solenoid “E” Control Circuit High
- DTC P0997 Shift Solenoid “F” Control Circuit Range/Performance
- DTC P0998 Shift Solenoid “F” Control Circuit Low
- DTC P0999 Shift Solenoid “F” Control Circuit High