Theory Of Operation

The Variable Camshaft Timing (VCT) system continuously varies the Camshaft target position based on the engine operating parameters. The Powertrain Control Module (PCM) calculates or models the optimum Camshaft angular position and makes adjustments using the Variable Camshaft Timing (VCT) Solenoid. The VCT Solenoid is energized to move the internal Oil Control Valve to direct the flow of oil to a hydraulic actuator (Phaser) on the Camshaft Sprocket to change the Camshaft position relative to the Crankshaft position. The CMP Sensor is used to monitor the position of the Camshaft.

A VCT diagnostic is performed to detect a Target Error  and a Slow Response  of the Camshaft, problems which could potentially increase tailpipe emissions. The diagnostic is primarily performed by comparing the actual Camshaft position to that of a calculated model for the Camshaft position. This diagnostic is continuous once the enable conditions are met, however the Slow Response and Target Error conditions are not performed simultaneously.

• The Target Error  diagnostic evaluates the actual Camshaft position versus the modeled position when the Camshaft is in a relatively stable position. The difference between the two is considered to be the amount of "error". If the error amount is greater than a calibrated threshold, it is considered failing.
• The Slow Response  diagnostic is only performed when the modeled Camshaft position is moving (phasing) in a particular direction, advancing or retarding. The modeled Camshaft phasing position is created to match only the worst possible case of a passing camshaft position. If the actual camshaft phasing operates slower than the model by a calibrated threshold, it is considered failing.

If either of these diagnostics fail, the Camshaft Position Slow Response DTC will set.