Rear View

Components On The Engine - Rear View:

1. Secondary air fan
USA vehicles are equipped with a secondary air system in order to reduce pollutants in the exhaust gas during the warm-up phase and to fulfil the emissions limits.
An electric air pump is mounted on the body in the engine compartment for this purpose. It is activated by the DME control module and, via air lines, injects the secondary air behind the exhaust valves.

2. Vacuum reservoir
Ensures the function of vacuum-operated components.

3. Pneumatic switch-over valve
A pneumatic switch-over valve is fitted between the air pump and exhaust valve. It is closed when the secondary air system is inactive and thereby prevents the induction of additional air as a result of flow conditions.

4. Check valve

5. Vacuum valve

6. Check valve

7. Throttle plate part

8. IACV (idle speed positioner)
A two winding positioner that is screwed directly to the throttle plate part without additional air guide hoses.

9. Throttle potentiometer
The throttle position is determined via a potentiometer fitted on the throttle plate shaft. The DME control module supplies the potentiometer with a controlled voltage (5 V). Depending on the position of the potentiometer (throttle plate angle), there is a greater or lesser voltage drop across the potentiometer. The DME also recognizes the smallest opening angle of the throttle via the voltage drop or the corresponding current. This is defined as the "idle position".

10. Hot film mass air flow sensor
A new hot film mass air flow sensor with the designation HFM 5 is used on the Boxster. Its housing shape permits installation in one direction only. It is fastened with M 5 Torx screws. As with the previous mass air flow sensors, the manufacturer performs laser calibration on the new HFM 5 in a so-called "master tube". In other words, a corresponding air mass produces an exactly defined voltage signal.

The hot film mass air flow sensor is fitted in the air cleaner housing at the outlet of the air cleaner. The operating voltage range is 9 - 7 Volt, and the output voltage of the mass air flow sensor (voltage to the DME control module) lies between 0 and 5 Volt. The actual measuring element is supplied with a controlled voltage of 5 Volt from the control module in addition to the voltage supplied by the vehicle electrical system to the mass air flow sensor. This prevents fluctuations in the vehicle voltage, e.g. when loads are switched on, from influencing the measuring process.

Intake air temperature sensor
The intake air temperature sensor is installed in the mass air flow sensor housing and is thus located in the intake air flow. It is supplied with voltage by the DME control module via terminal 43, and is connected together with the HFM 5 on the ground side. The intake air temperature sensor functions according to the NTC principle. The DME control module uses the signal from the intake air temperature sensor to calculate the substitute load signal if the mass air flow sensor fails. The load calculated by the control module, which depends on the throttle potentiometer, is corrected by the IATS signal. A negative correction is applied to the substitute load signal at high air temperatures.

At the same time, the decreasing air density is compensated. Furthermore, the risk of knocking increases with increasing intake air temperature. The ignition timing is therefore retarded at a high engine temperature (> 90 °C) and high intake air temperature (> 30 °C).

11./12. Oxygen sensors
These sensors are floating, i.e. the DME control module connects ground to the oxygen sensors. The sensors are located in the exhaust-gas flow ahead of the catalytic converter; two sensors are additionally installed behind the catalytic converter on OBD II vehicles.

Oxygen sensor heating
Besides being heated by the exhaust gas, the oxygen sensors are also heated electrically. For this purpose, the oxygen sensor, injection and ignition relay is activated and applies positive voltage to the oxygen sensor heaters after operation of the DME relay.
The DME control module connects ground to the heating resistors.

13. Pulse sender (reference mark sensor/rpm sender)
The DME measures the crankshaft rotation with an inductive sender. A pulse wheel is mounted on the flywheel for this purpose. This pulse wheel is a toothed ring stamped from sheet steel and spot-welded on the flywheel. It has 60 teeth. A gap produced by the omission of two teeth is defined as the reference mark and is located 84 ° ahead of TDC of the 1st or 4th cylinder. The pulse sender is fitted in a bore on the crankcase. The distance is not adjustable.