The purpose of this test is to evaluate the voltage output from the internal track of the Air Flow Meter during engine idle, WOT and over-run conditions.
Plug a BNC test lead into Channel A on the PicoScope, place a large black clip on the test lead with the black moulding (negative) and a Back-pinning Probe on the test lead with the red moulding (positive).
Clip the large black clip on the battery negative terminal and probe the air flow sensor's output terminal with the Back-pinning Probe as illustrated in Figure 1. If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout box or lead if you have one available.
When testing the air flow meter, it may take several attempts to centre the waveform when capturing the output.
With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings. Snap the accelerator quickly from idle to full throttle and observe the waveform.
The voltage output from the internal track of the Air Flow Meter (AFM) should be linear to flap movement. This can be measured on an oscilloscope and should look similar to the example shown.
The waveform should show approximately 1.0 volt when the engine is at idle, rising an initial peak as the engine is accelerated. This peak is due to the natural inertia of the air vane and drops momentarily before the voltage rises again to a peak of about 4.0 to 4.5 volts. This voltage will however depend on how hard the engine is accelerated, so a lower voltage is not necessarily a fault within the AFM. On deceleration the voltage will drop sharply as the wiper arm, in contact with the carbon track, returns to the idle position. This voltage may in some cases dip below the initial voltage before returning to idle voltage. A gradual drop will be seen on an engine fitted with an idle speed control valve as this will slowly return the engine back to base idle as an anti-stall characteristic.
A time base of about 10 seconds or more is used, enabling the operator to view the AFM's movement on one screen from idle, through acceleration and back to idle again. The waveform should be clean with no drop-out in the voltage, as this indicates a lack of electrical continuity. A good example of this is shown on the 'Faulty 12 volt AFM' example waveform. This is common on an AFM with a dirty or faulty carbon track. The problem will show as a flat spot or hesitation when the vehicle is driven, and is typical on vehicles with high mileage that have spent the majority of their working lives with the throttle in one predominant position.
The hash on the waveform is due to the vacuum change from the induction pulses as the engine is running.
This type of Air Flow Meter is probably the most popular version used and has been used on systems such as Bosch L, LE, LE3 and Motronic, and Ford EEC IV. Several Japanese manufacturer's have also based their systems on this tried and tested unit. The air vane meter works by having the air flowing into the engine passing through the metering unit, past a spring-loaded flap which moves in proportion to the rate of air entering the engine. The air vane's movement is recorded by a wiper arm moving across a carbon track, and the voltage output from this is reported back to the Electronic Control Module (ECM) which then delivers the correct amount of fuel for the airflow measured.
The AFM can have a varying number of electrical connections. The common ones are listed below.
Four-terminal units have: a voltage supply, an earth connection via the ECM, an output from the air temperature sensor and the output from the air vane meter.
Five-terminal units have: as above, but with an additional output from a Carbon Monoxide (CO) potentiometer.
Seven-terminal units have: as the four-terminal unit plus an extra wire to the air temperature sensor and two terminals to the fuel pump contacts. These contacts close and complete the circuit on the fuel pump when the engine is cranked, and the incoming air moves the flap by approximately 5°. This is a typical AFM as fitted to certain Range Rovers.
The voltage output from the internal track should be linear to flap movement. This can be measured on an oscilloscope and should look as detailed on the example shown above.
The AFM also has an internal compensation chamber that stabilises the movement of the flap and avoids erratic movement from induction pulses. The co-mixture content adjustment is via an internal air bypass or a potentiometer, depending on the version.
Figure 2 shows an air-vane air flow meter. This unit is from a Bosch LE3 system and the control unit is mounted on top of the air flow meter body.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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