Mass Air Flow (Maf) Sensor; Checking And Replacing - BMW 5251 Service Manual

Fuel Injection
I
Mass air flow (MAF) sensor,
checking and replacing
4
The hot film mass air flow (MAF) sensor (arrow) sends a
varying voltage (approx. 0.5
-
4.5 vdc) to the ECM represent-
ing the measured intake air. The air mass input signal is pro-
duced electronically by the MAFsensorusing a heated metal
film in the air flow stream.
The ECM relay provides the operating voltage to the MAF
sensor. As air flows through the MAF sensor, the film is
-
cooled. To maintain the film at a constant temperature, addi-
tional heating current is necessary. It is this additional current
that is the basis for the input signal.
If there is no output signal from the MAF sensor, the ECM op-
erates the engine using throttle position and engine rpm in-
puts. A faulty MAF sensor illuminates the MIL.
A faulty MAF sensor can produce the following problems:
Difficult to restart when engine is hot.
Engine starts then stalls.
Engine starts and runs only with accelerator pedal de-
pressed.
Disconnect electrical harness connector from MAF sensor
(arrow).
-
Turn ignition on and check for voltage and ground at connec-
tor.
There should be ground at pin 1 and battery voltage at pin
3.
If any faults are found, check for wiring breaks between
MAF sensor and ECM and between MAF sensor and ECM
relay.
NOTE-
Visually inspect the sensor for damage, missing or blocked
screens. The screens affect air flow calibration. Also inspect
the sealing O-rings where the sensor inserts into the air filter
housing and intake boot. Ensure the pin connections are
clean and tight.
4
Connect a BMW-compatible scan tool or an OBD II generic
scan tool. Check air mass signal (load signal) with the engine
at idle. Raise idle speed and check that load signal changes
with rpm.
CA
UTIOI\C
Be sure ignition is OFFbefore disconnecting or reconnecting
r
1
DME svstem components.
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