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The heartbeat of the transmission

How solenoid, hydraulic interaction achieves a specific gear and diagnostic tips
Tuesday, May 24, 2016 - 07:00
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When the solenoid influences its control valve, it regulates the valve to control line pressure to the clutch. For example, the 1-2-3-4-5 Reverse Control Solenoid (F) regulates its 1-2-3-4-5 Reverse Control valve in the valve body located directly in front of the solenoid. If a problem with this circuit fails, P0776 or P0777 may set. The system description for codes P0776 and P0777 provided by GM is as follows:

The transmission control solenoid 2 is a high pressure, normally low, variable force solenoid. The transmission control solenoid valve 2 regulates and directs pressurized oil to the 1-2-3-4-5-Reverse Clutch assembly to apply the clutch. The transmission control module (TCM) uses information from the speed sensors along with serial data information from other control modules to calculate the correct gear ratio and apply the correct clutch assemblies. The TCM compares the calculated ratio to the commanded gear ratio. The calculated and commanded gear ratios must match.

When they do not, depending on the failure, either P0776 or P0777 will set. These codes are defined as:

DTC P0776 – Transmission Control Solenoid Valve 2 Stuck Off – The 1-2-3-4-5-Reverse Clutch did not apply due to a hydraulic/mechanical condition. This results in a neutral condition.

DTC P0777 – Transmission Control Solenoid Valve 2 Stuck On – The 1-2-3-4-5-Reverse Clutch did not release due to a hydraulic/mechanical condition. This results in a tie-up condition.

Figure 5 Figure 6 Figure 7

These are type A codes which will cause the TCM to disable transmission adaptation and will freeze current adapt values. It will disable Tap Up/Tap Down control and converter clutch apply. It will also request the ECM to disable powertrain braking, neutral idle control, auto grade braking, and auto stop function. The TCM will command a default shift pattern and ignore all other requests. In this case, the command for P0776 is 6th gear and reverse only while P0777 will be 2nd gear and reverse only.

The order of diagnostics would be to first check the o-rings on the S2 (F) solenoid and then inspect the solenoid to see if it has failed mechanically being stuck in the low or high position (Figure 4). The next step would be to inspect the condition of the 1-2-3-4-5-Reverse Control Valve in front of the solenoid in the valve body to see if it is stuck or if the bore is wore (Figure 5). The next step would be to refer to a hydraulic to see if there are any other valves or check balls in the circuit which would require inspection. In this example, the pressure being sent to the 1-2-3-4-5-Reverse clutch from the regulating valve is also going to the 1-2-3-4-5-Reverse Clutch Boost valve. This too will also need to be inspected (Figure 6). If all these items pass inspection, it’s time to go into the transmission and check the clutch assembly and piston seals.

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