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Variable Cam Timing: Description

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Variable Cam Timing (VCT) system allows exhaust cam to advance and retard at varying engine speeds. This is to reduce exhaust emissions and increase fuel economy. As exhaust cam retards in relation to crankshaft position, residual exhaust gases are left in combustion chamber. Residual gases cool combustion chamber and are inert when mixed with incoming fresh charge of fuel and air. This results in better fuel economy and lower Nitrogen Oxides (NOx) and Hydrocarbons (HC) emitted from the engine. VCT system eliminates the need for an Exhaust Gas Recirculation (EGR) system.

VCT system applies only to Escort and Focus 2.0L ZETEC (VIN 3) engines. VCT system consists of control solenoid, 5-tooth pulse ring (4+1) on exhaust camshaft, Intake Air Temperature (IAT) sensor, Engine Coolant Temperature (ECT) sensor, Camshaft Position (CMP) sensor, Mass Air Flow (MAF) sensor, Crankshaft Position (CKP) sensor and PCM. The following list of components and their specific operation corresponds to numbers in illustration. See Fig 1.

  1. PCM receives input signals from IAT sensor, ECT sensor, CMP sensor, MAF sensor and CKP sensor for determining operating conditions of the engine.
  2. VCT system is enabled by PCM when the proper conditions are met. PCM disables VCT system if a fault is detected.
  3. PCM calculates relative cam position using CMP sensor and data from (4+1) pulse ring mounted on exhaust camshaft. Relative cam position is calculated by measuring time between the rising edge of Profile Ignition Pickup (PIP) and the falling edge of VCT pulse.
  4. PCM continually calculates a cam position error value based on the difference between desired and actual position and a duty cycle is commanded for VCT solenoid valve. Engine oil is allowed to flow to VCT unit. For additional VCT solenoid valve information, see VARIABLE CAM TIMING SOLENOID VALVE .
  5. Oil flows to either side of the piston chamber changing a linear motion from piston to a rotation motion from the helical mechanism in VCT unit. During closed loop, PCM outputs a revised duty cycle to VCT solenoid valve to correct for cam position error. For additional VCT unit information, see VARIABLE CAM TIMING UNIT ASSEMBLY .

Fig 1: Identifying Variable Cam Timing System Components & Circuits
G00088435Courtesy of FORD MOTOR CO.
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