WM 130/170/270 Repair Electrical
wc_tx000381gb.fm 93
11.7 WM 270 Basic Theory
See Graphic: wc_gr001824 and wc_gr001825
Revolution of the flywheel generates electricity on the primary side of
the ignition coil (1), and the base current I
1
flows to the power transistor
(2). Current I
1
turns the power transistor “ON” and the electric current
I
2
flows. This is the same situation when the contact breaker is closed
in a case of breaker point type ignition system.
At lower engine revolution, the low speed ignition timing control circuit
(3) operates to run the gate current I
3
to turn the control thyristor (5)
“ON”, thus the current I
1
bypasses the thyristor as current I
5
. At this
moment, the power transistor (2) turns “OFF” and the current I
2
is shut
off abruptly, resulting in high voltage generated in the secondary coil
(7) which produces spark at the spark plug. The ignition timing at lower
engine revolution is less advanced as shown in the chart.
At higher engine revolution (over 2,000 rpm), advancing control circuit
(4) operates to run the gate current I
4
to turn the control thyristor (5)
“ON”, thus the current I
1
bypasses the thyristor as current I
5
. At this
moment, the power transistor (2) turns “OFF” and the current I
2
is shut
off abruptly, resulting in the high voltage generated in the secondary
coil (7) which produces spark at the spark plug. At over 2,000 rpm,
ignition timing on each engine revolution is controlled by advancing
control circuit (4) that receives electrical information from revolution
sensing resistor (6). The advancing of ignition timing from lower to
higher engine revolution changes linearly as shown in the chart.
Reference: °BTDC = degrees before top dead center.
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