200 Quattro Wagon L5-2226cc 2.22L SOHC Turbo (MC) (1989)
Engine Control Module: Description and Operation
Engine Overspeed Fuel Cut-Off
Engine Overspeed Fuel Cut-Off
Engine overspeed protection is an ECU controlled function of the oxygen sensor frequency valve, and is determined by the engine speed sensor signal.
When the engine reaches a critical rpm (just before the engines "red-line" rpm), the ECU cuts off the fuel by cutting the duty cycle to the oxygen sensor
frequency valve. As soon as the engine speed drops below the critical rpm, the duty cycle is restored and fuel delivery returns to normal. The fuel is
alternately cut and restored many times each second, at a critical rpm that is just below the engines "redline" limit. This prevents engine overspeed while
maintaining torque and horsepower at the critical rpm.