Test Point 5 - Air Mass Sensor
TEST POINT 5 - Air mass sensor - Fault code 1_21Possible Causes, Elimination, Remarks
a) Power supply (V)
- Disconnect the plug at the air mass sensor and connect the voltmeter to plug terminals 2 and 4 using the test leads.
- Use the test leads to connect terminals 17 and 21 on the LH control unit plug after it has been disconnected.
Display: battery voltage
No display:
- Remove LH relay XXV and bridge terminals 87 and 30 at the relay base.
Display on voltmeter connected to air mass sensor plug: battery voltage.
If there is a battery voltage display, check the LH relay and its activation.
If there is no display, check leads and plug connections according to wiring diagram.
b) Checking resistance of hot wire circuit at the air mass sensor (Ohms)
- Disconnect the plug connection at the air mass sensor. Connect terminals 3 and 5 at the air mass sensor with the ohmmeter. Display: 3.6 - 4.1~
c) Checking the hot wire signal (V)
- Connect the plug to the air mass sensor. Take out LH relay XXV and bridge terminals 87 and 30 at the base of the relay. Connect terminals 6 and 7 at the LH control unit plug with a voltmeter. Display: 1.6 V
- Blow on the hot wire in the air mass sensor and watch the voltmeter. The voltage reading must change. (1.6 - 5 V)
d) Checking the hot wire burn-off circuit (visual check)
- Run the engine with the air mass sensor installed and connected. When an engine temperature of > 60°C (140°F) is reached, raise engine speed above 2000 rpm and then stop the engine by switching off the ignition. After a delay of approx. 4 seconds, the hot wire must glow for approx. 1 Sec. (burn- off function).
e) Emergency operating program
- The emergency operating program stored in the LH control unit is activated if the air mass sensor fails, so that the car can be driven to the nearest authorized repair shop.
- With engine warm and at engine speeds < 2000 rpm, ti = 3.5 ms - With engine warm and at engine speeds > 2000 rpm, ti = 6.3 ms
(ti = injection time)