Air Mass Meter (HLM)

Air Mass Meter (HLM):

Check voltage supply HLM:

U-Batt supplied between Pins 2 and 4 at the HLM.


Check HLM signal:

•D• 2 - The load signal TL provides an indication of the measured air flow rate in the HLM. When idling, the value should be 2.0 ms +/- 20%. In the case of a break in the voltage supply or in the signal line, the value is a constant 2.20 ms (substitute value) at idle speed and 0 ms under part load and full load.

The load signal is formed dependent upon the voltage (< 5 V) applied at the hot wire of the HLM. The load signal is displayed in the BMW Service Tester only when the engine is in operation.

The voltage value of the hot wire can be measured between Pin 14 (sensor ground) and Pin 41 (signal HLM) of the DME control unit. The measurement can also be conducted with the ignition ON (nominal value approx. 1.4 V).

The load signals of the two rows of cylinders must scarcely deviate from each other. Check the burn-clear function if the load signal is permanently too low. A load signal which is too low tends to produce a lean mixture, thus reducing the engine output.

Check burn-clear function:

Increase engine speed once to above 2000 rpm (engine temperature > 60° C). Approx. 5 seconds after switching off the engine, the hot wire can be seen to briefly light up (burn-clear) at the HLM (air cleaner cover removed).

Within 5 sec after switching off the engine, a voltage of approx. 3.5 V is applied at Pin 13 for 0.5 sec to bum clear the hot wire. This signal can be measured between Pin 13 and vehicle ground.