US Patent 7806093 Variable valve timing control

Engine control is provided for an engine having a crankshaft and at least one camshaft with variable valve timing in a rotation transmitting train between the crankshaft and the camshaft. A first value indicative of a cam angle at a home position at a first time is compared to a second value indicative of a cam angle at the home position at a second, earlier, time. Where a deviation in the compared values is detected that does not exceed a threshold value, the first value is stored for use in a subsequent comparison. Where a deviation in the compared values is detected that does exceed a threshold value, a failure condition is detected and the MIL light is illuminated. In this case, the value stored for comparison is not updated.