Knock & Map Sensors
Today’s modern combustion engines are still relatively inefficient, despite over 150 years of development. Most engines will only convert approximately 35% of energy created in the combustion process into forward motion.
This is why ensuring an optimum combustion process is vital to achieving maximum performance and best economy. The engine needs to burn the air-fuel mixture in a controlled way and at a precise point in the piston’s stroke cycle. The modern ECU calculates the ignition timing in order for the combustion process to reach peak pressure and allow maximum energy recovery.
Ignition timing can be disrupted by engine knocking, a common phenomenon where pockets of the air-fuel mixture ignite away from the normal flame front and outside of the standard ignition cycle. Knocking takes energy away from the combustion process because the increase of pressure in the cylinder works against the compression stroke.
In its most severe form, knocking can cause serious engine damage. To detect and prevent this, engine manufacturers use a piezoelectric sensor mounted on the engine block to listen for engine knocking.
The Knock Sensor
The knock sensor acts like a microphone, transforming the vibrations caused by the sound waves from knocking into a voltage signal that can be read by the ECU. The ECU will analyse the sound from each cylinder and compare it to a reference noise based on the average noise level for a set period. If the sensor detects any noise exceeding the reference level, the ECU will identify it as knocking and begin to retard the ignition timing until the noise level returns to normal. This process happens continually to achieve optimum timing.
If a sensor fails and needs replacing, it is important to check that the surface of the mounting point is clean and free of contamination so the new sensor can be properly mated to the block. If not, the sensor may not operate correctly and could result in damage to the engine.
Garages should always use up to date diagnostic tools when dealing with any engine management issue. In the event of a knock sensor failure the following Generic P0 Series Fault Codes are likely to appear:
P0325 Knock Sensor Circuit Malfunction
P0326 Knock Sensor Circuit Range
P0327 Knock Sensor Low Input
P0328 Knock Sensor Input
P0329 Knock Sensor Input Intermittent
These should always be taken as a guide to the problem and followed with a thorough check of the engine management system which will help to identify underlying problems.
The MAP Sensor
The Manifold Absolute Pressure (MAP) sensor helps the ECU to determine the exact amount of fuel required to create the optimum air-fuel ratio for combustion. Pressure is measured at the inlet manifold to determine the air density and calculate the air mass flow into the engine. This measurement is used in conjunction with an air temperature reading to determine the correct amount of fuel required.
The MAP sensor is connected to the inlet manifold by a vacuum hose. The vacuum and pressure act upon the sensor diaphragm which the electronic control module converts into an electrical signal. The manifold absolute pressure is calculated using the following formula: Atmospheric Pressure – Manifold Pressure = Manifold Absolute Pressure
The map sensor also tests the functionality of the EGR valve, as part of OBD II standards, on vehicles fitted with a MAF (Manifold Air Flow) sensor. Following the introduction of Euro V and the upcoming Euro VI standards, emission control products are becoming increasingly important, leading to an increase in demand.
If the MAP sensor is suspected to be faulty, a simple test to measure the output voltage will establish if this is the case. A correctly working sensor will operate at approximately 1.5V at idle and increase to approximately 4V as the engine speed increases.