Theory Of Operation

This diagnostic checks that the transmission fluid pressure is changing as expected. The transmission fluid pressure is continuously controlled using an electric pump. In order to maintain the fluid pressure at a value suitable for the normal system operation, the electric pump is activated when the pressure drops below a low pressure threshold and is deactivated when the pressure reaches a high pressure threshold. The fluid pressure then will continuously increase and decrease with a certain rate in the range defined by the two thresholds. Outside of this range, if the pressure were to fall below the lower threshold, the system will not be able to perform gear shift operation and other diagnostics will set a fault. Conversely if the pressure were to rise above the upper threshold, a mechanical relief valve will avoid the pressure becoming too high and the system will still continue to work properly. The Transmission Control Module (TCM) continuously stores the last measured transmission fluid pressure in its internal memory. The diagnostic compares the actual transmission fluid pressure to the stored value to be sure that the signal variation rate is above a minimum threshold. If the transmission fluid pressure does not vary as expected, the diagnostic will be considered failing.