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A P1870 code means the transmission is seeing a slip condition. Sometimes it points to the TCC clutch not working properly. Sounds like the transmission needs to come out. See if you can get hold of a GM diagnostic chart for this code and it will make more sense to you.
CIRCUIT DESCRIPTION:
The Powertrain Control Module (PCM) monitors the Torque Converter Clutch (TCC) slip by calculating the difference between the engine speed and the transmission output shaft speed. When the PCM detects an excessive amount of slip when the TCC is engaged, then DTC P1870 sets. DTC P1870 is a type A DTC.
CONDITIONS FOR SETTING THE DTC:
No Throttle Position (TP) DTCs P0122 or P0123
No Vehicle Speed Sensor (VSS) DTC P0502
No 1-2 solenoid performance DTC P0751
No 1-2 solenoid electrical DTC P0753
No 2-3 solenoid performance DTC P0756
No 2-3 solenoid electrical DTC P0758
No TCC Pulse Width Modulation (PWM) solenoid electrical DTC P1860
No TCC enable solenoid electrical DTC P1864
The engine speed is greater than 500 RPM for 5 seconds.
Not in fuel shut off
The throttle angle is 9.5-35%.
The transmission is not in 1st gear.
The transmission gear range is D4.
The transmission fluid temperature is 20-1300C (68-2660F).
The engine torque is 70-270N.m (50-200 ft lbs.).
The Manifold Absolute Pressure (MAP) is 0-105 kPa (0-15 psi).
The TCC is commanded ON and at maximum apply for 5 seconds.
The TCC slip is greater than 200 RPM for 8 seconds.
The above conditions must occur three times with the TCC commanded OFF between each occurrence.
ACTION TAKEN WHEN THE DTC SETS:
The PCM inhibits TCC operation.
The PCM inhibits 4th gear, if the transmission is in the hot mode.
The PCM illuminates the Malfunction Indicator Lamp (MIL).
CONDITIONS FOR CLEARING THE MIL/DTC:
The PCM turns OFF the MIL after three consecutive ignition cycles without a failure reported.
A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.
The PCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.
DIAGNOSTIC AIDS:
Ensure that the vehicle's final drive ratio matches the PCM's calibration.
Ensure that the transmission will shift through all four gears with the gear select lever in the D4 range.
Good luck!
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