Group
28
Ignition systems
Des
ign
and function - System descriptions
•
R
A
I
A
D
unit - Construction and signals
The above figures illustrate
the
construct
i
on of the
diagnost
i
c un
i
t
(
1
), i
ts
connections
,
and
the
sequence of events
which
occurs
i
n
the
c
ourse
o
f commun
i
cation between
the
un
i
t and
the
i
gn
i
tion
system contro
l
unit
(2
)
during fault
tracing
.
Construction and connections
(Picture
A
I
The diagnostic unit
c
onsists of
an LED
a
resistor
(
R
)
and a normally-open switch
(push
button
A)
.
Terminal
8
is
con
n
ected
to ground when the switch is
closed
.
A
co
nne
c
tion
point for the
coding cab
le
(3)
is provided between
the
resistor and switch. Socket No.6 is connected to
terminal
1
on
the ignition system
co
ntrol unit
.
Grounding of terminal
1
is
monitored by a
signal
transistor
(4)
(actually a transistor network, although symbolized
by
a
single component in
the figures) which
alternately
opens and
closes
the ground connection across control
unit
ter·
mina!
20
.
The
signal transistor
is controlled
by the diagnostic
circuit
(5)
in the
control
unit.
The battery
supplies
power to
control
unit terminal
5
and to the diagnostic
circuit
i
n the control unit
.
This ensures
that the
memory
i
s
not
erased
when the
ign
i
t
i
on is
switched
off
.
steps
(
Picture
B
)
Power is supplied
to
c
ontrol unit
term
i
nal
6
across ign
i
tion
sw
i
tch term
i
nal
15
when the key is turned to posit
i
on
II
.
Voltage
i
s present
at
termina
l
1
on the
control
unit when the
coding
is inserted in
socket No
.
6
on
the diagnostic
unit
.
Signal from diagnostic: unit
(Picture
C
l
The
control
unit must be supplied with
some
of signal
which of the test functions is
to
be activated.
This
information is
supp
lied
across the normally-open switch
(pushbu
tton
A).
(The
direction
of
the arrows
in
this picture
shows
that the
signal
flows from the
diagnostic
unit to the
cont
ro
l
unit
.)
The switch is
closed
by depressing the pushbutton
,
grounding terminalS
on
the diagnostic unit
.
Current then flows
from the battery
,
across ign
i
tion switch terminal
15
,
through the diagnostic unit and finally to ground across terminal
8
.
This
causes
the LED to
light
,
while the
voltage
normally present at control unit terminal 1 falls to 0
V
.
The control
unit determines whether the se
l
f-d
i
agnostic funct
i
on
or
funct
i
onal
test program has been selected by detecting
whether
the vo
l
tage
at
term
i
na
l
1
has
to
zero
once
or
twice
.
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