Hot Film Mass Air Flow Sensor; Oxygen Sensors; Pulse Sender; Intake Air Temperature Sensor - Porsche Boxster Technical Manual

24 DME diagnosis Boxster
8
10. Hot film mass air flow sensor high air temperatures.
At the same time, the decreasing air density is
A new hot film mass air flow sensor with the des- compensated. Furthermore, the risk of knocking
ignation HFM 5 is used on the Boxster. Its hous- increases with increasing intake air temperature.
ing shape permits installation in one direction The ignition timing is therefore retarded at a high
only. It is fastened with M 5 Torx screws. engine temperature (> 90 °C) and high intake air
As with the previous mass air flow sensors, the temperature (> 30 °C).
manufacturer performs laser calibration on the
new HFM 5 in a so-called "master tube". In other 11./12. Oxygen sensors
words, a corresponding air mass produces an
exactly defined voltage signal. These sensors are floating, i.e. the DME control
module connects ground to the oxygen sensors.
The hot film mass air flow sensor is fitted in the The sensors are located in the exhaust-gas flow
air cleaner housing at the outlet of the air clea- ahead of the catalytic converter; two sensors are
ner. The operating voltage range is 9 -7 Volt, additionally installed behind the catalytic conver- 8
and the output voltage of the mass air flow sen- ter on OBD II vehicles.
sor (voltage to the DME control module) lies be-
tween 0 and 5 Volt. The actual measuring ele-
ment is supplied with a controlled voltage of
5 Volt from the control module in addition to the Oxygen sensor heating
voltage supplied by the vehicle electrical system
to the mass air flow sensor. This prevents fluctua- Besides being heated by the exhaust gas, the
tions in the vehicle voltage, e.g. when loads are oxygen sensors are also heated electrically. For
switched on, from influencing the measuring pro- this purpose, the oxygen sensor, injection and ig-
cess. nition relay is activated and applies positive volt-
age to the oxygen sensor heaters after operation
Intake air temperature sensor of the DME relay.
The DME control module connects ground to the
The intake air temperature sensor is installed in heating resistors.
the mass air flow sensor housing and is thus lo-
cated in the intake air flow. It is supplied with volt- 8
age by the DME control module via terminal 43,
and is connected together with the HFM 5 on the 13. Pulse sender (reference mark sensor/
ground side. The intake air temperature sensor rpm sender)
functions according to the NTC principle.
The DME control module uses the signal from The DME measures the crankshaft rotation with
the intake air temperature sensor to calculate an inductive sender. A pulse wheel is mounted
the substitute load signal if the mass air flow sen- on the flywheel for this purpose. This pulse wheel
sor fails. The load calculated by the control mod- is a toothed ring stamped from sheet steel and
ule, which depends on the throttle potentiometer, spot-welded on the flywheel. It has 60 teeth. A
is corrected by the IATS signal. A negative cor- gap produced by the omission of two teeth is
rection is applied to the substitute load signal at defined as the reference mark and is located
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24 -D 20 24 01 DiagnosisjTroubleshooting DME
98624D1 Printed in Germany -4, 1996