G643(E) Service Manual Chapter 5. Engine Management System (EMS) 160
There are multiple limp-home modes available with
electronic throttle control:
1. If the throttle itself is suspected of being
inoperable, the SECM will remove the power to
the throttle motor. When the power is removed,
the throttle blade returns to its “default” position,
approximately 7% open.
2. If the SECM can still control the throttle but some
other part of the system is suspected of failure,
the SECM will enter a “Reduced Power” mode. In
this mode, the power output of the engine is
limited by reducing the maximum throttle position
allowed.
3. In some cases, the SECM will shut the engine
down. This is accomplished by stopping ignition,
turning off the fuel, and disabling the throttle.
Picture courtesy of Robert Bosch GmbH
Figure 15. Throttle Body Assembly Exploded View
Ignition System Control
Spark-ignited engines require accurate control of
spark timing and spark energy for efficient
combustion. The MI-07 ignition system provides this
control. The system consists of the following
components:
• SECM
• Distributor with ignition module
• Ignition coil *
• Crankshaft position sensor *
• Crankshaft timing wheel *
• Spark plugs *
The SECM, through use of embedded control
algorithms and calibration variables, determines the
proper time to start energizing the coil and fire the
spark plug. This requires accurate crank/camshaft
position information, an engine speed calculation,
coil energy information, and target spark timing. The
SECM provides a TTL compatible signal for spark
control. The coil must contain the driver circuitry
necessary to energize the primary spark coil
otherwise an intermediary coil driver device must be
provided. The SECM controls spark energy (dwell
time) and spark discharge timing.
Figure 14. GM Distributor
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