Join Date: Dec 2013
Vehicle: SLK 200 Kompressor ´05
Other Toys: Sold the kart, now it´s ****
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Engine Control Systems 101
Here comes a short explanation why "Inline Sensor tricking systems" don't work.
Todays diagnostic systems are built in a robust way, so you can in most of the cases continue to drive, even if a sensor or component fails.
Basically, you have following "Input sensors"
MAF, Mas Air Flow
MAP, Manifold Air Pressure
AT, Ambient Temperature
AP, Ambient Pressure
Then you have lot of system sensors, like Crank Position, CAM positions, Fuel pressure, Oil pressure, Oil temp, Lambda value etc.
Using values from those sensors, together with internal parameters like engine displacement, rpm, manifold displacement, crankshaft position etc.
You can determine the airflow trough the engine, and together with the Lambda sensor, adjust the fuel injection.
None of the components above are exact, so the ECU allows for an adaption of the value from the sensor.
In other words, a sensor that should have a nominal value=1.0 can actually have any value between 0.8 and 1.2
The ECU starts with value 1 as a reference, and then measures the outcome.
If it is not according to the expected outcome, it adjusts/adapts the sensor value by adding or subtracting 0.01 from the measured sensor value.
It will continue so, until the outcome is as expected. This takes several driving cycles, but normally you are within the correct range after 5-10 cycles.
(You can test this by lifting the battery for 10 minutes. You will probably get the feeling that the engine is more harsh, during the first one or two drive cycles)
This also allows for the sensors to drift during their lifespan. The ECU simply compensates for the drift.
Even if a component fails, it's value can be calculated by using the data from the remaining sensors, to create a backup value and allow you drive the car, often with reduced power and rpm, so you can take the car to the workshop.
So if anyone tries to fool the system, by connecting some other electronics to the MAF, MAP or AT, the engine will smell the rat, for instance by checking the Lambda value, and just compensate.
If you go outside the area of compensation, (0.79 or 1.21 in the example above), you will get a component failure lamp, and in worst case, end up in "Limp Home"