Air flow meter – hot wire waveform

4.2.3 Air flow meter – hot wire waveform

Figure 4.5 shows a micro mechanic mass airflow sensor from Bosch. This type has been in use since 1996. As air flows over the hot wire it cools it down and this produces the output signal. The sensor measures air mass because the air temper- ature is taken into account due to its cooling effect on the wire.

The voltage output should be linear to airflow. This can be measured on an oscilloscope and should look similar to Figure 4.6. The waveform should show approximately 1.0 volt when the eng-

Figure 4.5 Hot wire air mass meter (Source: Bosch Press)

ine is at idle. This voltage will rise as the engine is accelerated and air volume is increased pro- ducing an initial peak. This peak is due to the ini-

one screen, from idle, through acceleration and tial influx of air and drops momentarily before

back to idle again. The ‘hash’ on the waveform is the voltage is seen to rise again to another peak

due to airflow changes caused by the induction of approximately 4.0 to 4.5 V. This voltage will

pulses as the engine is running. however depend on how hard the engine is accel-

erated; a lower voltage is not necessarily a fault

4.2.4 Inductive crankshaft and

within the meter.

camshaft sensor waveform

On deceleration the voltage will drop sharply as the throttle butterfly closes, reducing the airflow,

The inductive type crank and cam sensors work and the engine returns back to idle. The final volt-

in the same way. A single tooth, or toothed age will drop gradually on an engine fitted with

wheel, induces a voltage into a winding in the idle speed control valve as this will slowly return

sensor. The cam sensor provides engine position the engine back to base idle as an anti-stall char-

information as well as which cylinder is on acteristic. This function normally only affects the

which stroke (Figure 4.7). The crank sensor pro- engine speed from around 1200 rev/min back to

vides engine speed. It also provides engine posi- the idle setting.

tion in many cases by use of a ‘missing’ tooth.

A time base of approximately 2 seconds plus In this particular waveform (Figure 4.8) we is used because this allows the output voltage on

can evaluate the output voltage from the crank

Oscilloscope diagnostics 43

Figure 4.6 Air mass meter waveform

sensor. The voltage will differ between manufac- turers and it also increases with engine speed. The waveform will be an alternating voltage signal.

The gap in the picture is due to the ‘missing tooth’ in the flywheel or reluctor and is used as a reference for the ECU to determine the engine’s position. Some systems use two reference points per revolution.

The camshaft sensor is sometimes referred to as the cylinder identification (CID) sensor or a ‘phase’ sensor and is used as a reference to time sequential fuel injection.

Figure 4.7 Crank sensor in position near the engine

This particular type of sensor generates its own

flywheel

signal and therefore does not require a voltage

Figure 4.8 Crank and cam sensor output signals

44 Advanced automotive fault diagnosis supply to power it. It is recognisable by its two

rotation (360° of camshaft rotation). The voltage electrical connections, with the occasional addi-

will be approximately 0.5 V peak to peak while tion of a coaxial shielding wire.

the engine is cranking, rising to around 2.5 V peak The voltage produced by the camshaft sensor

to peak at idle as seen in the example show. will be determined by several factors, these being the engine’s speed, the proximity of the metal rotor to the pick-up and the strength of the mag-

4.2.5 Coolant temperature

netic field offered by the sensor. The ECU needs

sensor waveform

to see the signal when the engine is started for its reference; if absent it can alter the point at which

Most coolant temperature sensors are NTC ther- the fuel is injected. The driver of the vehicle may

mistors; their resistance decreases as tempera- not be aware that the vehicle has a problem if the

ture increases (Figure 4.9). This can be measured CID sensor fails, as the drivability may not be

on most systems as a reducing voltage signal affected. However, the MIL should illuminate.

(Figure 4.10).

The characteristics of a good inductive cam- The coolant temperature sensor (CTS) will shaft sensor waveform is a sinewave that increases

usually be a two wire device with a voltage sup- in magnitude as the engine speed is increased and

ply of approximately 5 V. usually provides one signal per 720° of crankshaft

The resistance change will therefore alter the voltage seen at the sensor and can be monitored for any discrepancies across its operational range. By selecting a time scale of 500 seconds and con- necting the oscilloscope to the sensor, the output voltage can be monitored. Start the engine and in the majority of cases the voltage will start in the region of 3 to 4 V and fall gradually. The voltage will depend on the temperature of the engine.

The rate of voltage change is usually linear with no sudden changes to the voltage, if the sen- sor displays a fault at a certain temperature, it will show up in this test.