The 21-SI Delcotron® Generator is a one-wire
configuration. One-wire refers to the minimum
number of lead wire connections necessary at the
generator for operation. Some applications may
operation, and/or an additional ground lead
The one-wire type requires only that the generator
output ("BAT") terminal be connected to the battery
positive terminal and that a ground path be
provided between the generator housing and the
battery negative terminal (refer to Figure 2). "R"
and/or "I" terminal connections are optional and do
not affect generator operation.
Figure 3. 21-SI Electrical Terminals
Figure 2. Basic One-Wire System
indicator light is connected in series with this
External connections to the 21-SI generator are
terminal, the light will be on whenever there is a
made to terminals as shown in Figure 3. The BAT
voltage difference between the "positive" side of
terminal may be 1/4", 5/16", or M6 (metric) size,
the field circuit and the system voltage at other
depending on the application requirement.
side of the indicator light. During normal generator
Optional connections to the 21-SI series include
operation, the light will be off since the diode trio
"R" (relay) and "I" (indicator light) terminals, and a
output voltage equals the system voltage.
ground lead connection to the generator housing.
The "R" and "I" terminals are available in either
A Relay terminal may be located either counter-
threaded (10-24 or M4) or pin types.
clockwise from the BAT terminal where an R is
A threaded 1/4" or 5/16" hole in the slip ring end
molded into the casting, or at an opening
(SRE) frame is provided to connect a ground lead
clockwise from the BAT terminal and labeled
if used; otherwise, the ground path is through the
"RELAY" by a permanent adhesive label next to
mounting hardware and brackets to the engine.
the terminal. On a replacement unit, there will be a
round push-on label on the terminal itself. This
Some applications use a debris shield on the
terminal may be used to operate a charge
outside of the SRE housing of the generator. Such
shields are added by the engine manufacturer to
device by providing voltage pulses at about half of
reduce the amount of airborne debris that enters
system voltage and at a frequency of 1/10 of the
the generator in severe environments. A stud
mounted in the "I" terminal hole may be used to
attach the debris shield to the generator. The stud
When an "I" terminal is present it will be located
is not connected electrically inside the unit.
where there is an "I" molded into the casting,
clockwise from the BAT terminal. An "I" terminal is
connected internally to the field circuit. If an