BMW 5251 Service Manual page 353

L n i t i o n switch
4
The positive (+) power supply to the ignition coils comes to
the ignition switch via an unloader relay. When the ignition
switch is turned on, the relay closes, to provide voltage to the
ignition coils. The coils are then actuated at the negative side
by the engine control module.
NOTE-
Schematic diagrams shown are for illustrative purposes only.
See ELE Electrical Wiring Diagrams for engine manage-
ment specific wiring information.
WARNING-
Do not touch or disconnect any cables from the coils while
the engine is running or being cranked by the starter:
The ignition system produces high voltages that can be
fatal. Avoid contact with exposed terminals. Use ex-
treme caution when working on a car with the ignition
switched on or the engine running.
*
Connect and disconnect the DME system wiring and
test equipment leads only when the ignition is OFF
Before operating the starter without starting the engine
(for example when making a compression test), always
disable the ignition.
4
Knock sensors (arrows) monitor the combustion chamber for
engine-damaging knock. Six-cylinder models (shown) use
two sensors with each sensor monitoring three cylinders. V-
8
models use four sensors, each monitoring two cylinders. If
engine knock is detected, the ignition point is retarded by the
ECM.
I
Ignition advance
Ignition timing is electronically mapped and not adjustable.
The ECM uses engine load, engine speed, coolant tempera-
ture, knock detection, and intake air temperature as the basic
inputs for timing control. A three dimensional map similar to
the one shown is digitally stored in the ECM.
-
The initial ignition point is determined by the crankshaft
speed sensor during cranking. Once the engine is running.
timing is continually adjusted based on operating conditions.