Euisun's Autotronics 2011: Day 16 - Oscilloscope Patterns to Capture

Single trace oscilloscope captures

MAF sensor
Make: Lexus
Model: ES300
Year: 2003

Volt/division: 1V
Time/division: 1S

Explanation of the graph:

The point A shows us that the normal idling voltage of the maf is 1.198V. When the engine accelerates, the signal changes to approx 2.6V(point B). Then during the deceleration, the signal drops sharply to the normal idling voltage which is 1.198V(point C).

Explain in detail an electrical fault that would make this unit operate incorrectly.

Dirt, oil can coat the sensing wire. The contaminated sensor often overestimates the amount of air entering the engine at idle and therefore causes the fuel system to go rich. At high engine speeds, the contaminaton can cause the sensor to underestimate the amount of air entering the engine. As a result, the fuel system will go lean.

ECT sensor
Volt/division: 0.5V
Time/division: 5S

ECT is a negative temperature coefficient. As the coolant temperature increases, the voltage drops. As shown above graph, the voltage of ECT is 1.439V. It means that coolant is half way between cold start and a warm idle condition.

If the ECT sensor has a poor connection(high resistance) at the wiring connector, the ECU will supply a richer-than -normal fuel mixture based on the resistance of the ECT sensor. Poor fuel economy can be caused by a defective sensor or high resistance in the sensor wiring. If the ECT sensor was shorted(low resistance), a leaner-than-normal fuel mixture would be supplied to the engine. A too-lean fuel mixture can cause driveability problems.

IAT sensor
Volt/division: 0.5V
Time/division: 5S

The IAT sensor is also a negative temperature coefficient(NTC) thermistor that decreases in resistance as the temperature of the sensor increases. The above graphs shows that the voltage decreases as temperature of the sensor increases.

If the IAT sensor is defective, it may be signalling the ECU that the intake air temperature is extremely cold when in fact it is warm. In such cases the ECU will supply a mixture that is much richer than normal.
If the wiring or the IAT sensor has excessive resistance, the ECU will supply a lower than normal fuel economy, and in serious cases, black exhaust smoke from the tailpipe during acceleration.

MAP sensor
Make: Toyota
Model: 4A-FE

Volt/division: 2V
Time/division: 1S

As the engine rpm increases, the intake manifold pressure rises and the voltage increases to 3.2V(point A).

When the accelerator is released, the engine vacuum rises and changes the MAP sensor voltage to 1.590V(point B).

Some dirt on the earth terminal can cause incorrect operation.

A defective vacuum hose to a MAP sensor can cause a variety of driveability problems including poor fuel economy, hesitation, stalling and rough idle.

Oxygen sensor
Make: Daihatsu
Model: YRV

Volt/division: 0.5V
Time/division: 1S

As shown patterns, approx 0.2V indicates a lean mixture and approx 0.8V indicates a rich mixture. It is important to check the cycle reaction time. Most lamda sensors will go from rich to lean in about 50 - 100ms and from lean to rich in about 75 - 150ms. If the oxygen sensor is taking slightly longer to reverse readings, this is an indication that is getting sluggish.

Normal closed loop Ziconia oxygen sensor voltage is -
Rich when high volts(Catalyic converter cleans up NOx)
Lean when low volts(Catalyic converter cleans up HC and CO)

TPS sensor
Make: Toyota
Model: 4A-FE

Volt/division: 1.0V
Time/division: 1S

Throttle position voltage is approx 0.3V(point B, throttle closed) at idle. Then as the throttle opens at 15 degrees, the throttle postion voltage reaches approx 1.0V(point A). The TP sensor voltage at idle is usually about 10% of the TP sensor voltage when the throttle is wide open, but can vary from as low as 0.3V to 1.2V, depending on the make and model of vehicle.

Any blind spots within the internal carbon track's swept area, will cause "flat spots" and "hesitations".

RPM sensor(cam or distributor)
Make: Toyota
Model: 4A-FE

Volt/division: 10V
Time/division: 20ms

This trace is an AC signal. When the trigger wheel is at the point where the air gap is the smallest, full magnetic saturation of the pickup coil has now been reached the induced voltage.(point A,peak voltage)
As the reluctor wheel pickup moves away, the air gap increases causing reluctance to magnetic field flow resulting in the magnetic lines of force collapsing across the trigger coil and inducing a voltage in the opposite direction. (point B, peak voltage)

Injectors(petrol)
Make: Toyota
Model: 4A-FE
Volt/division: 10V
Time/division: 10ms

Point A: Source voltage(Battery voltage: 14.4V) supplied to injector
Point B: Driver transistor turns on, pulling the injector pintle away from its seat, starting fuel flow
Point C: Peak voltage is caused by the collapse of the injector coil
Point D: Driver transistor turns off, ending fuel flow
Injector on-time is approx 3ms as shown above pattern.

Ignition primary
Make: Toyota
Model: 4A-FE
Volt/division: 50V
Time/division: 5ms

The ignition primary waveform measures the negative side of the ignition coil. There is no current in the coil's primary circuit until the dwell peirod(A-B). When the coil is earthed, the voltage drops to zero at point A. The induced voltage is produced by a process called magnetic inductance. At the point of ignition, the coil's earth circuit is removed and the magnetic field collapses across the coil's windings, this induces approx between 150 and 350 volts at point C(starting spark). At point D, the spark ends.

IAC(Idle air control)
Make: Toyota
Model: 4A-FE

Volt/division: 5V
Time/division: 200ms

An Idle air control controls idle speed by controlling the amount of air that passes around the throttle plate. More airflow results in a higher idle speed. The purpose of IAC systems is to stabilize idle speed during cold engine and after warm-up operations. As shown above patterns, it shows us that a duty-control rotary solenoid is used for IAC. By changing the duty ratio, a change in magnetic field causes the valve to rotate. Basically as duty ratio exceeds 50%, the valve opens the bypass passage and as duty ratio drops below 50%, the valve closes the passage.

Dual trace oscilloscope captures

MAP against Injectors(petrol)

Volt/division: A:2V / B:20V
Time/division: 50ms

Channel B shows Injector's pattern. At point A the injector is opening and the voltage is zero, because the injectors are earth triggered. When the injector is closed at point B, back EMF is produced.
Channel A shows MAP sensor's pattern. The voltage is approx 1.58V during idling.

The ECU uses the map reading to determing the fuel injector open time. The longer the injector is open, the more fuel gets into the engine.

RPM(distribution G) against Injectors(petrol)
Volt/division: A:2V / B:20V
Time/division: 50ms

Channel B shows Injector's pattern. At point A the injector is opening and the voltage is zero, because the injectors are earth triggered. When the injector is closed at point B, back EMF is produced.
Channel A shows RPM's pattern. When the trigger wheel is at the point where the air gap is the smallest, full magnetic saturation of the pickup coil has now been reached the induced voltage.(point C,peak voltage)
As the reluctor wheel pickup moves away, the air gap increases causing reluctance to magnetic field flow resulting in the magnetic lines of force collapsing across the trigger coil and inducing a voltage in the opposite direction. (point D, peak voltage)

The injector pulse is direclty related to the engine speed sensor input. As RPM increses, it needs to spray fuel faster.