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Sensory Overload: The First Step in Vehicle Diagnostics

Today’s automobiles rely on highly-sophisticated electronic systems that manage virtually every aspect of the vehicle. In our latest Building Blocks article, we discuss these systems and how they interface with the ECU to silently run the whole operation.

After processing input from an array of sensors located throughout the powertrain and chassis, the ECU must perform millions of split-second operations based on constantly changing variables. This article will highlight the sensors that control the engine and explain their basic functions so you better understand how each one affects and interacts with the engine management system as a whole.

Air Flow Meter (AFM) / Mass Air Flow (MAF) Sensor

The MAF sensor measures the amount of air (mass flow rate) entering the engine so the ECU can inject the correct amount of fuel (air to fuel ratio).

Manifold Absolute Pressure (MAP) Sensor

A MAP sensor serves the same basic function as a MAF, but instead calculates the density of air entering the engine to determine the engine’s air mass flow rate.

Crankshaft Angle Position Sensor

As you probably guessed, the crank position sensor measures the position or rotational speed of the crankshaft and directly affects the ignition timing. Due to the wider availability of precision electronics, crank angle sensors have begun to replace traditional distributor-based ignition systems as standard equipment in newer automobiles.

Camshaft Position Sensor

Like the crank sensor, the cam position sensor is a type of Hall Effect sensor that measures the position and rotation of the camshaft. This information governs ignition timing and fuel injection; it also tells the ECU the positions of the valves and pistons, which is crucial to engines with variable valve timing.

Water (Coolant) Temperature Sensor

The ECU relies on the coolant temperature to ensure the engine is running safely and efficiently. Severe damage can occur if an engine overheats, while water temperatures that are too cool result in poor engine performance and decreased fuel economy. Most engines are designed to operate within a specific temperature range for optimal performance.

Intake Air Temperature (IAT) Sensor

The IAT sensor simply tells the ECU the temperature of the air entering the engine; colder air is denser and requires more fuel.

Throttle Position Sensor (TPS)

As the accelerator is pressed, the TPS relays the exact position of the throttle plate (0-100%) to the ECU. This sensor is typically mounted on the butterfly valve of the throttle body.

Idle Air Control (IAC) Valve

While not an actual sensor, the idle air control valve regulates the amount of air entering the engine to maintain a constant idle speed. Without it, the driver would have to keep the accelerator slightly pressed at all times to prevent the car from stalling.

Vehicle Speed Sensor (VSS)

This one is pretty self-explanatory. The VSS is a tachometer that measures the wheel speed of a vehicle. It is typically used in anti-lock braking systems and for traction control purposes.

Oxygen (O2) / Sensor

Located in the exhaust stream, the O2 sensor is responsible for measuring the air/fuel ratio (in Lambda) of the engine and dictates its overall performance, fuel economy and emission control.

Knock Sensor

Detonation, pre-ignition, knock and pinging are all words used to describe the same harmful condition that can destroy an engine. The knock sensor is located in the engine block and “listens” for sounds within the frequency range associated with detonation. The ECU will then compensate, usually by retarding the ignition timing to lower cylinder temperatures.

These sensors represent the first step in understanding the on-board diagnostics systems found in modern vehicles. In the next article, we will discuss the progression of these diagnostics systems and explain how the sensors are used to pinpoint problems so they can be more quickly and easily fixed.


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