Contents - Volkspage

5B.2 Ignition systems
1 The Bosch Mono-Motronic and Digifant
systems are self-contained engine
management systems, which control both the
fuel injection and ignition. This Chapter deals
with the ignition system components only -
refer to Chapter 4A or B for details of the fuel
injection system components.
2 The ignition system comprises four spark
plugs, five HT leads, the distributor, an
electronic ignition coil, and an Electronic
Control Unit (ECU) together with its
associated sensors, actuators and wiring. The
component layout varies from system to
system but the basic operation is the same for
all models.
3 The operation is as follows: the ECU
supplies a voltage to the input stage of the
ignition coil which causes the primary
windings in the coil to be energised. As the
engine rotates, the ECU is triggered to
interrupt the coil primary supply voltage by
the distributor-mounted Hall sender. This
results in the collapse of a primary magnetic
field, which then induces a much larger
voltage in the secondary coil, called the HT
voltage. This voltage is directed by the
distributor, rotor arm and HT leads, to the
spark plug in the cylinder on its ignition
stroke. The spark plug electrodes form a gap
small enough for the HT voltage to arc across,
and the resulting spark ignites the fuel/air
mixture in the cylinder. The timing of this
sequence of events is critical and is regulated
solely by the ECU. The basic ignition setting
can be adjusted by turning the distributor
body on its mountings - refer to Section 5 for
greater detail.
4 The ECU calculates and controls the
ignition timing and dwell angle primarily
according to engine speed, crankshaft
position and inlet air volume flow rate
information, received from sensors mounted
on and around the engine. Other parameters
that affect ignition timing are throttle position
and rate of opening, inlet air temperature,
coolant temperature and on certain systems,
engine knock. Again, these are monitored via
sensors mounted on the engine.
5 On systems where knock control is
employed, the knock sensor is mounted on
the cylinder block - this has the ability to
detect engine pre-ignition (or ‘pinking’) before
performance is affected or engine damage is
caused. If pre-ignition occurs, the ECU
retards the ignition timing of the cylinder that
is pre-igniting in steps until the pre-ignition
ceases. The ECU then advances the ignition
timing of that cylinder in steps until it is
restored to normal, or until pre-ignition occurs
again.
6 Idle speed control is achieved partly by an
electronic throttle valve positioning module (or
auxiliary air valve, depending on system type -
see Chapter 4A or B for details) and partly by
the ignition system, which gives fine control of
the idle speed by altering the ignition timing.
7 On certain systems, the ECU has the ability
to perform multiple ignition cycles during cold
starting. During cranking, each spark plug
fires several times per ignition stroke, until the
engine starts. This greatly improves the
engines cold starting performance.
8 It should be noted that comprehensive fault
diagnosis of all the engine management
systems described in this Chapter is only
possible with dedicated electronic test
equipment. Problems with the systems
operation that cannot be pinpointed by
following the basic guidelines in Section 2
should therefore be referred to a VW dealer
for assessment. Once the fault has been
identified, the removal/refitting sequences
detailed in the following Sections will then
allow the appropriate component(s) to
renewed as required.
Note: Throughout this Chapter, vehicles are
frequently referred to by their engine code,
rather than by engine capacity - refer to
Chapter 2A for engine code listings.
Warning: Extreme care must be taken
when working on the system with the
ignition switched on; it is possible to get a
substantial electric shock from a vehicle’s
ignition system. Persons with cardiac
pacemaker devices should keep well clear
of the ignition circuits, components and
test equipment. Always switch off the
ignition before disconnecting or
connecting any component and when
using a multi-meter to check resistances.
General information
1 Comprehensive fault diagnosis of all the
engine management systems described in
this Chapter is only possible with dedicated
electronic test equipment. Problems with the
systems operation that cannot be pinpointed
by following the basic guidelines described in
this Section should therefore be referred to a
VW dealer for assessment. Once the fault has
been identified, the removal/refitting
sequences detailed in the following Sections
will then allow the appropriate component(s)
to renewed as required.
2 Most ignition system faults are likely to be
due to loose or dirty connections or to
‘tracking’ (unintentional earthing) of HT
voltage due to dirt, dampness or damaged
insulation, rather than by the failure of any of
the system’s components. Always check all
wiring thoroughly before condemning an
electrical component and work methodically
to eliminate all other possibilities before
deciding that a particular component is faulty.
3 The old practice of checking for a spark by
holding the live end of an HT lead a short
distance away from the engine is not
recommended. Similarly, no attempt should
be made to ‘diagnose’ misfires by pulling off
one HT lead at a time. Not only is there a high
risk of an electric shock, but the HT coil
and/or exhaust catalyst could easily be
damaged.
4 Problems with the systems operation that
cannot be pinpointed by using the guidelines
in the following paragraphs should be referred
to a VW dealer for assessment.
Engine will not start
5 If the engine either will not turn over at all,
or only turns very slowly, check the battery
and starter motor . Connect a voltmeter
across the battery terminals (meter positive
probe to battery positive terminal), disconnect
Hall sender multiway connector from the
distributor, then note the voltage reading
obtained while turning over the engine on the
starter for (no more than) ten seconds. If the
reading obtained is less than approximately
9.5 volts, first check the battery, starter motor
and charging systems (see Chapter 5A).
6 If the engine turns over at normal speed but
will not start, check the HT circuit by
connecting a timing light (following the
manufacturer’s instructions) and turning the
engine over on the starter motor: if the light
flashes, voltage is reaching the spark plugs,
so these should be checked first. If the light
does not flash, check the HT leads
themselves followed by the distributor cap,
carbon brush and rotor arm using the
information given in Chapter 1.
7 If there is a spark, check the fuel system for
faults referring to the relevant part of Chapter
4 for further information.
8 If there is still no spark, then the problem
must lie within the engine management
system. In these cases, the vehicle should be
referred to a VW dealer for assessment.
Engine misfires
9 An irregular misfire suggests either a loose
connection or intermittent fault on the primary
circuit, or an HT fault on the coil side of the
rotor arm.
10 With the ignition switched off, check
carefully through the system ensuring that all
connections are clean and securely fastened.
11 Check that the HT coil, the distributor cap
and the HT leads are clean and dry. Check the
leads themselves and the spark plugs (by
substitution, if necessary), then check the
distributor cap, carbon brush and rotor arm as
described in Chapter 1.
12 Regular misfiring is almost certainly due to
a fault in the distributor cap, HT leads or spark
plugs. Use a timing light (paragraph 4 above)
to check whether HT voltage is present at all
leads.
13 If HT voltage is not present on one
particular lead, the fault will be in that lead or
in the distributor cap. If HT is present on all
leads, the fault will be in the spark plugs;