Theory Of Operation

GENERAL SYSTEM OVERVIEW:  The Variable Valve Timing (VVT) system continuously varies the Camshaft angular position. Based on the engine operating parameters, the Powertrain Control Module (PCM) calculates or models the optimum Camshaft angular position and moves the Camshaft to the desired position. The Camshaft Position (CMP) Sensor is used to monitor the position of the Camshaft. When the engine is in base engine valve timing the Camshaft hydraulic actuator (Phaser) is forced to the default lock pin position by a spring on the front of the Phaser. The PCM operates the VVT Solenoid through a Pulse Width Modulated (PWM) High Side Driver (HSD) circuit to control the movement of the Phaser.

GENERAL PHASER OPERATION:  The VVT Solenoid pushes against the plunger (spring loaded spool valve)  in the center of the Oil Control Valve (OCV) bolt which directs oil pressure to the Phaser on the Camshaft Sprocket. The OCV is driven to three different positions depending on which direction the Phaser is being moved. When the PCM commands the PWM voltage high (near 100%) the VVT Solenoid pushes the OCV in and allows oil flow into the chamber of the Phaser moving the Camshaft off of lock pin. When the PCM commands the PWM voltage low (near 0%) the VVT Solenoid allows the OCV to return to the resting position and allows oil flow out of the chamber of the Phaser moving the Camshaft back toward lock pin. When the Camshaft reaches the desired position the PWM signal is commanded to approximately 50% (approximately 6 volts) which holds the OCV in the center of the bore and prevents oil flow in either direction. This is done to hold the Phaser in the desired position.

DIAGNOSTICS:  Typical circuit diagnostics are performed on the HSD circuitry and VVT Solenoid as well as a VVT system performance diagnostics to detect a Target Error  and a Slow Response  of the camshaft, problems which could potentially increase tailpipe emissions. The performance diagnostic is primarily performed by comparing the actual Camshaft angular position to the desired Camshaft angular 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 angular position versus the desired position when the Camshaft is in a relatively static 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 Camshaft angular position is moving (phasing) in a particular direction, advancing or retarding. If the actual camshaft phasing operates slower than the diagnostic model by a calibrated threshold, it is considered failing. The model for the slow response diagnostic is created to pass for the worst possible case of a Camshaft Phaser that is still considered passing.

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

The table below calls out the names of the timing components and which Camshaft they are associated with. This information is intended to aid in diagnosing the proper component when performing DTC diagnostics.

2.0L GMET 4 Variable Valve Timing Component Locations
CAMSHAFT	CAM POSITION SOLENOID CALLOUT	CAM PHASER/OIL CONTROL VALVE CALLOUT	CAM SENSOR CALLOUT
INTAKE	VVT Intake Solenoid 1/1	Camshaft 1/1 Phaser or Oil Control Valve	CMP Bank 1 Sensor 2 (CMP Sensor 1/2)
EXHAUST	VVT Exhaust Solenoid 1/2	Camshaft 1/2 Phaser or Oil Control Valve	CMP Bank 1 Sensor 1 (CMP Sensor 1/1)