Circuit Description

WARNING: This page is about a different car, the 2002 GMC Yukon XL, 2002 GMC Sierra, 2002 Chevrolet Suburban, and 2002 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The PCM monitors the engine speed and the transmission output shaft speed. The PCM calculates turbine shaft speed and Torque Converter Clutch (TCC) slip speed by using inputs from the Input Speed Sensor (ISS), Output Speed Sensor (OSS), and other transmission components. The forward clutch housing is used as the ISS rotor. Whenever the TCC is engaged, engine speed and turbine speed will closely match, indicating low TCC slip speed. In D3 with the TCC engaged, calculated transmission component slip can only occur in the torque converter. In D4 with the TCC engaged, transmission component slip can occur in the TCC or the 4th clutch assembly.

If the PCM detects an excessive TCC slip speed in D4, when the TCC should be engaged, then DTC P1870 sets. For California emissions vehicles, DTC P1870 is a type "B" DTC. For all other vehicles, DTC P1870 is a type "C" DTC.