GMC 1989 Light Duty Truck, Repair Manual, Page: 642 / 998

6D1-2 SI SERIES GENERATORS
1. “ BAT” T erm inal
2. No. 1 T erm inal
3. No. 2 T erm inal
4. Test H ole
53. Part N u m b e r F-04478
OPERATING PRINCIPLES (Figure 3)
The battery is directly connected to the No. 2 term inal on
the generator. This supplies voltage through resistor R5 to
the base-emitter of transistor TR3. Transistor TR3 turns on
which then turns on transistor TR1 which allows field cur­
rent to flow through to ground. Also, resistors R2 and R 3are
connected to the battery through term inal No. 2, but the
discharge current of the battery is very low because of the
resistance values of R2, R3, R5, TR1 and TR3.
At this point, transistors TR3 and TR1 are turned on, but
no current is flowing through them. Now the engine control
switch is closed, allowing current to flow from the battery
through the indicator lamp to the No. 1 term inal on the
generator. From No.1 term inal, the current goes through
resistor R1, transistors TR3 and TR1 to ground, turning on
the indicator lamp. Resistor R6 carries some of the indica­
tor lamp current.
When the generator is spinning, AC voltages are genera­
ted in the “ Y ” stator w indings (delta w indings on the 17-SI).
The AC is converted to DC current in the diode trio, and is
then fed through to the field to transistor TR1 and to ground.
At the same time, the DC voltage from the diode trio causes
the indicator lamp current to stop, m aking the indicator
lamp go out.
Now the AC voltages are converted to DC in the rectifier
bridge. This DC voltage is then fed through the “ BAT”
terminal to the battery, thus keeping the battery charged
and supplying electricity to the vehicle electrical loads.
Figure 1— 12-SI Series 100 Generator
1. “ BAT” T erm inal
2. N o. 1 T erm inal
3. N o. 2 T erm inal
4. F ie ld G ro u n d H ole
A. “ Y ” S tator. (S ta to r is D e lta W o u n d
on th e 17-SI 100.)
F ig u re 2 — 1 7-SI S e ries 100 G e n e ra to r
F ig u re 3 — G e n e ra to r S c h e m a tic