Downloaded from www.Manualslib.com manuals search engine 6.1 Distributor bypass operation
At engine speeds below 450 r.p.m. the ECU does not control
the firing angle. The ECU keeps the bypass line to terminal
C of the distributor low for engine speeds below 450 r.p.m.
The spark advance for the vehicle is set to 10ø BTDC by the
distributor, while the engine speed is below 450 r.p.m..
Above 450 r.p.m. the ECU takes the bypass line high (+5
volt) and takes over control of the spark advance as de-
scribed below.
6.2 Engine speed and crankshaft position measurement
Engine speed is calculated by measuring the period from one
negative edge to the next negative edge of the distributor
reference signal. The reference signals are separated by
180 degrees.
The distributor generates a signal as shown in the reference
and ignition waveform diagram. The time taken for the crank-
shaft to rotate from 10ø BTDC to the next 10ø BTDC marker
is measured. Knowing the time it takes the crankshaft to
rotate through 180ø it is possible to calculate the current
engine speed in degrees per second (ø/s). Crankshaft posi-
tion is obtained by sensing the falling edges of the refer-
ence signal.
