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How to test E30 Bosch AFM (Bench test)

July 9, 2024

There is only one correct way to test an E30 AFM, and that’s by using an oscilloscope.

The most common mode of failure is the wearing of the carbon strip within the AFM. In simple terms, the AFM is one big variable resistor. It takes a +5V supply, and the carbon strip provides variable resistance as it opens.

This is called the LMM (Luftmassemesser) signal and is indicated as a Up/Uv value:

Up/Uv value (Up = potentiometer voltage, Uv = power supply voltage 5V): Indicates the voltage relation resulting from the sensor plate position in LMM. At idle speed, the value should be 0.2 to 0.3; at full load, it rises to 0.9.
Only an oscilloscope will work because you need to find momentary dropouts in the signal due to worn spots in the carbon track. A multimeter is not enough. An oscilloscope is essentially a multimeter that graphs the output over time.

Though it might sound intimidating, it’s rather easy. All you need is a handheld oscilloscope from Amazon, a connector from Aliexpress, and a power supply. Any benchtop power supply will do, as almost all of them can provide at least 5V:

You will also need a 5-pin EV1 connector to connect to the AFM to easily apply the probes:

A handheld oscilloscope is essential:

Now, connect everything as indicated:

When you move the flap inside the AFM, you should see a nice smooth signal. When you open the door more, the voltage increases; when you close the door, the voltage decreases.

A bad result, where voltage is dropping out (indicating wear in the carbon track), would look like this:

Lastly if you’re chasing down an idle issue, make sure that the idle bypass screw has not been tampered with; most factory AFMs will have a plastic plug over this screw.

This bypass is set at the factory and should not be adjusted.

If the plug is missing and it has been adjusted, you can set it back to factory by adjusting the screw depth to match the number stamped on the AFM case. Each AFM has a different number stamped, as every AFM is adjusted differently from factory.

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