Page 341
ELECTRONIC CONTROL UNIT
The control unit is the electronic part of the ignition system in which the pulse from the distributor is converted
and amplified. Control and regulation of the dwell angle
are automatic.
The control unit comprises a circuit board on which an
integrated circuit, transistors, condensors, diodes and
resistors are mounted.
The electronic control unit is mounted in front of the
left har:1dwheel housing.
BOSCH
ELECTRONIC CONTROL UNIT
11111111111111
0
1-♦-0-♦-1
0
11111
11111
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11111
11111
.......
11111
11111
1
ELECTRONIC CONTROL UNIT
1. Plastic cover
2. Power transistor
3. Printed circuit board
4. Base plate
SAMI
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342-13
Page 342
CHAINGING THE INDUCTION COIL
l
1. Remove the distributor.
2. Remove the distributor cap, rotor arm and condensate
trap.
3. Remove the cable contact retainer and withdraw the
contact.
4. Remove the vacuum control unit, the clip retainers
and the three screws in the impulse transmitter plate.
5. Remove the rotor circlip.
6. Lift the rotor and save the pin and the shim.
7. Remove the circlip on the impulse transmitter and lift
the latter.
S 5889
342-14
5'\AEI
Page 343
8. Remove the three screws securing the induction coil
to the plate.
Reassemble in the reverse order.
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SAM3
342-15
Page 344
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Page 345
Distributor
degrees
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S 5901
342-17
Page 346
'
SOME CONSIDERATIONS OF VITAL IMPORTANCE
TO WORK ON THE ELECTRONIC IGNITION SYSTEM
FAULT-TRACING
SYSTEM
IN THE ELECTRONIC IGNITION
1. When the engine is running, dangerously high voltage
which may prove fatal may be present in the primary
circuit of the coil and in all cables connected to terminal 1. This is because the spark energy in the system
is considerably higher than in conventional ignition
systems.
2. When the ignition is switched on, full primary current
will flow through the primary circuit of the coil. Consequently, always disconnect terminal 15 on the igni·
tion coil before commencing work with the ignition
on.
3. In conjunction with work on the screened impulse
cable between the impulse transmitter in the distri·
butor and the control unit, strict attention must be
given to polarity. Should the polarity be reversed, a
stable basic ignition setting will not be possible and
timing and dwell will also be affected.
The recommended steps must be performed one at a
time and in the stated order. If on inspection compo·
nents are found to be defective, these must be renewed
before any further fault-tracing is carried out. If a fault
is known to occur, for example, under certain tempera·
ture conditions, always attempt as far as possible to
trace faults under similar conditions. Thus, if trouble
with starting the car is experienced during cold weather,
fault-tracing should be carried out.on a thoroughly cold
car. Such procedure is necessary because defective elec·
tronic components may operate perfectly under normal
temperature conditions before breaking down completely.
Bad connections can also be affected by temperature.
Fault-tracing equipment
l~nition setting instrument (preferably with a 90° dwell
angle scale); volt-ohmmeter with scales for 15 V d.c.
and 5 V a.c., minimum sensitivity of 10 000 ohm/V, 0-5
ohm midscale, 0-5 kohm midscale.
1. Engine completely dead, fires without starting or is
difficult to start
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CHECK
I
1. Turn the engine over
on the starter and
check the length of
the spark between
the high tension leadl
from the coil and
ground
READING
CONDITION OF THE SYSTEM
PROBABLE FAULT
CHECK AND REMEDY AS
NECESSARY
More than 12 mm
Starter circuit and starter relay
probably working
Check 0.4 compensating
resistor by means of ohmmeter.
Check the ignition setting.
Check for flashover in coil isolator, distributor cap, rotor, ignition leads and plugs.
Check the fuel system.
Check that current flows from
the starter relay circuit to the
connection on the compen·
sating resistor. When the starter
is running, the voltage between
the common connection and
the + on the battery should be
0. If not, check relay and wiring.
Proceed to step 2.
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I Less than 12 mm
I or no spark
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2. Switch on the ignition. Before pro·
ceeding, check that
the battery charge
is at least 11 V.
!
342-18
'
Less than 11 V
Charge the battery; readings
taken with insufficient battery
voltage will give faulty values.
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I More than 11 V
Proceed to step 3.
Page 347
,
CHECK
READING
3. Measure the voltage
between terminal 15
ov
on the coil and
ground
Less than 6 V
6-8 V
CONDITION OF THE SYSTEM!
PROBABLE FAULT
Break in compensating resistor
or leakage from terminal 15 on
ignition switch
Short-circuit
ing of coil
in primary wind-
Primary coil winding and com-
CHECK AND REMEDY AS
NECESSARY
I
Check compensating resistor by
means of ohmmeter. Check that
voltage from the ignition switch
is present at the single terminal
on the 0.4 ohm resistor. If not,
check ignition switch and wiring
Check resistance of primary
winding (0.95-1.4 ohm)
Proceed to step 4.
pensat ing resistor sound
8-12 V
12 V
Bad ground connection
Break in primary coil winding
Control unit not conducting
4. Measure the voltage
between terminal 1
on coil and ground"\
ov
Short-circuit
0.5-2 V
Power transistor in control
unit sound
12 V
Control unit not conducting
in control unit
Check using ohmmeter that contact pin 31 on the control unit
connector has good ground connection.
Check using ohmmeter
(0. 95-1 .4 ohm)
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··1't't'1't'1
111111
111111
111.♦.'.♦.'1
I
Check for battery voltage at contact pin 15, using voltmeter
342-19
'.♦.'.♦.'.'
111111
11111
111111
Proceed to step 5.
Change control unit. Check for
111111
11111
111111
11111
.1...♦•••♦1
Exchange control unit
faulty insulation on cables to
terminal 16 on the control unit.
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Page 348
CHECK
READING
5. Measure resistance
in +ho
.cot:!or-.do.-v
... ---- --------·
Should be
CONDITION OF THE SYSTEM,
PROBABLE FAULT
CHECK AND REMEDY AS
NECESSARY
Appreciable deviation
Defective secondary winding in
coil
Change the coil
Proceed to step 6
60-80°
(65-90 %)
Control unit and impulse trans·
mitter probably without fault
Maximum reading
on scale
Dwell meter range insufficient
(many dwell meters have maximum reading of 70°) and consequently no information is
obtained
Connect a voltmeter across pins
7 and 31 d of control unit cable
connector. At starting speed
(100 rev/min), a minimum reading of 1 V a.c. should be obtained. This indicates that im·
pulses of sufficient strength are
being generated. If there is no
voltage or the voltage is too low,
check the screened impulse
cable. Thereafter, check the impulse transmitter in the distributor using an ohmmeter (8951285 ohm). Check the air gap be
tween the rotor and the stator
and, if necessary, adjust to 0.25
mm using a non-magnetic feeler
gauge. If the fault persists,
change the impulse transmitter.
The control unit does not
react to impulses from the
impulse transmitter
Change the control unit
r:;_r:;_g
-,-
1, ohm
- .. - .
coil winding
6. Connect a dwell
meter
Check dwell angle
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2. Poor running
Poor running of the engine is unlikely to be caused by
faulty electronics.
Check the following items first:
1. Good connections throughout ignition system.
2. Ignition setting and centrifugal and vacuum advance.
3. General condition of spark plugs.
342-20
4. Rotor: check operation, insulation (burns, dirt) and
contacts.
5. Distributor cap: insulation (cracks, flashover burns,
dirt) and contacts.
6. Ignition leads.
7. lgnisiton coil: spark length at starting revs 12 mm
minimum; insulation (burns, dirt).
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Page 349
8. Connect a dwell meter and check that the dwell angle
varies with the speed as shown on the chart. At less
than 2,000 rev/min, the dwell angle is largely regulated by the control unit. At speeds greater than 2,000
rev/min, the dwell angle is largely regulated by the
shape of the impulses from the impulse transmitter.
Thus wide deviations in the dwell angle can be traced
either to the control unit or the impulse transmitter.
Deviation of± 10° is to be regarded as normal.
9. The fuel system,
In the case of faults occurring under extremes of temperature, the control unit may be suspected. Poor soldering
and defective components probably become most evident
at extremes of temperature. The conventional ignition
condensor is incorporated in the control unit and may
be faulty just as frequently as in conventional ignition
systems - in other words, very rarely.
Closing angle
Closing
angle%
degrees:
100%
-
\
80%
\
~
60%
--
,_-70°
,_
--
_500
40 %
- _300
,_ -200
20 %
0%
0
1000
2000
3000
4000
5000
6000
U/min KW
Motorvarv/min
S 6309
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342-21
Page 350
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