In this technical article, we will cover the design, function and how to test and diagnose issues with Hot Film Mass 5 Air Flow Sensors, also known as HFM5 sensors. These sensors are responsible for reporting air-mass volume to the engine control module for precise air fuel control.
For precise control of the air fuel ratio, it is key for the air mass entering the engine to be measured correctly. To do this, hot film mass air flow sensors measure a small portion of the air entering the engine. The sensor takes into account positive and negative intake pulses caused by intake and exhaust valves opening and closing. Intake air temperature or ambient pressure changes have no effect on air mass measurement.
HFM5 sensor housing and measuring tube are designed to accommodate engine measuring volumes from 370 to 970 kg/h. The tube is designed in a way to ensure airflow is uniform. The airflow rectifier can be constructed of a plastic mesh with a straightening action and a wire mesh or a wire mesh on its own. The measuring tube is installed in an intake duct close to but downstream of the intake air filter.
Within the measuring tube is the measuring cell and the integrated evaluation electronics. The HFM5 measuring cell consists of a semiconductor substrate with a sensitive diaphragm surface with incorporated temperature sensitive resistors. Measuring tube shape provides smooth airflow over the substrate, without swirl effects.
The HFM5 sensor is a thermal sensor with a centrally located heating resistor on the measuring cell that heats a sensor diaphragm and maintains it at a constant temperature. Temperature drops across each side of the controlled heating zone (on each side of the sensor). As air flows over the measuring sensor, the uniform temperature profile at the sensor changes. On the inlet side, temperature drops faster since the air flowing past this area cools it off. On the opposite side of the sensor, the temperature drops slightly, because the incoming air has been heated by the heater element. The change in temperature distribution leads to a temperature difference between the two measuring points. The temperature difference is a measure of the airflow mass.
The sensor electronics convert the resistance differential at the two sensor measuring points into an analog voltage signal ranging from 0 – 5 volts DC. The engine control module is programmed with a sensor curve map to convert the voltage into a value representing the air-mass flow. The shape of the curve helps with fault detection such as, sensor plausibility and open / shorted circuit. Some engine control software systems will require an additional ambient air temperature sensor integrated into the HFM5, however this sensor is not necessary for proper sensor function.
HFM5 sensors can begin to detect incorrect air-mass readings if the sensor diaphragm is contaminated with debris, water or oil. A C-shaped bypass duct helps to keep this to a minimum but it is still something to consider when diagnosing sensor faults. Dirty or contaminated sensor should be replaced, not cleaned, as cleaners can damage the sensitive electronics.
Testing HFM5 Sensors
When testing a mass air flow sensor, wide open throttle (WOT) snap the or perform a WOT test drive (accelerate from a stop and accelerate WOT until max RPM is reached. Usually by the second gear shift). Do not perform this test during unsafe or on roadways with other vehicles. Do not exceed posted speed limits. You will want to monitor the sensor signal under all RPM ranges of the test cycle.
You can verify sensor signal using a scan tool. Use the chart below to determine estimated sensor output at specific load and RPM ranges.
If available, use the graphing function of your scan to monitor sensor data over time.
You can also test using a scope. It will show a similar pattern as the scan tool data when graphed.
The signal below tested using the ATS eScope shows a sensor that could not reach peak voltage. We expect to see peak voltage over 4.0 volts, on most vehicles.
This what to expect from a good pattern when performing the snap throttle (WOT) test with a scope.
Of course you can use a digital volt ohm meter (DVOM), however the reading displayed will be too slow to determine if there is a signal dropout or spike.
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