Functional Description of EGM
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Status September 2003 (EvoBus-Service / AFT) Page: 34 of 83
2.4.4 Turbocharger diagnosis
By comparing the target boost pressure and the actually measured boost pressure, the EGM can deduce
that there are malfunctions in the turbocharger circuit.
Two monitoring functions are realised in the EGM.
• Overpressure detection
If the actual boost pressure is above the permissible value of 2.35 bar for longer than 15 seconds,
then the event "Boost pressure too high" is detected and the fault code 1820 is entered. This limit
pressure check takes place cyclically every 40 ms. An active overpressure fault is first reset in the
engine state "Engine stopped" with the engine stopped. If the boost-pressure only briefly exceeds the
currently permissible value, the fault debouncing begins again from the start the next time the limit is
exceeded.
• Overpressure detection
If the actual boost pressure is above the permissible value of 2.35 bar for longer than 15 seconds,
then the event "Boost pressure too high" is detected and the fault code 1820 is entered. This limit
pressure check takes place cyclically every 40 ms. An active overpressure fault is first reset in the
engine state "Engine stopped" with the engine stopped. If the boost-pressure only briefly exceeds the
currently permissible value, the fault debouncing begins again from the start the next time the limit is
exceeded.
• Plausibility of turbocharger circuit
From the speed of 1,400 rpm and a torque request greater than 1,000 Nm, the boost pressure must
lie within a defined range [350 mbar < (p_LLuf – p_atmosphere) < 1,500 mbar] above the
atmospheric pressure, i.e. the turbocharger must have built up a defined gauge pressure. If no limit
function (e.g. engine protection, end limitation) is effective, checking is carried out for a maximum
and a minimum permissible overpressure. If under the above operating conditions the boost pressure
fails to reach the expected minimum pressure following a debounce time of 5 seconds, or if the boost
pressure exceeds the maximum permissible overpressure, a boost circuit fault (fault code 1818) is
entered in the fault memory. In the first case a fault can, for example, occur in the air guidance
system (e.g. air hose defective), and in the second case, for example, a fault can occur on the
turbocharger (wastegate).
The turbocharger circuit check is carried out every 40 ms. The test is only carried out when both the
boost-pressure and the atmospheric-pressure sensor are electrically okay and supply plausible values.
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