Starter circuit
7.26.1 Starter circuit
current test technology to detect the full range of battery failure modes including bad cells, sul-
In comparison with most other circuits on the phation, internal short circuits, and other chem-
modern vehicle the starter circuit is very simple. ical and physical failures. Testing takes less than
The problem to overcome, however, is that of volt
5 seconds and will even work on batteries dis- drop in the main supply wires. A spring-loaded charged down to as low as one volt.
key switch usually operates the starter; the same Some of the key features of this tester are:
switch also controls the ignition and accessories.
The supply from the key switch, via a relay in Automated system test of battery, alternator many cases, causes the starter solenoid to operate and starter in under a minute.
and this in turn, by a set of contacts, controls the Detailed test data: alternator ripple, internal heavy current. In some cases an extra terminal resistance, starter draw, state of charge, char- on the starter solenoid provides an output when ging amps, and volts.
cranking, usually used to bypass a dropping resis- Tests discharged batteries down to one volt.
tor on the ignition or fuel pump circuits. The Impedance/current (IC) test technology.
basic circuit for the starting system is shown in Wireless printer option.
Figure 7.59. The problem of volt drop in the main Integrated high and low amp probe options. supply circuit is due to the high current required
MicroVAT uses a fan cooled 50 A load and by the starter particularly under adverse starting integrated amp probe to test the quantity and
conditions such as very low temperatures. quality of alternator output with an alternator
A typical cranking current for a light vehicle ripple test. Many late model computer-controlled
engine is in the order of 150 A but this may peak charging systems virtually shut down under no
in excess of 500 A to provide the initial stalled load conditions.
torque. It is generally accepted that a maximum Diagnostic tests that can be carried out with
volt drop of only 0.5 V should be allowed this tester, when an amps probe is also used, are
between the battery and starter when operating. as follows.
An Ohm’s law calculation indicates that the
160 Advanced automotive fault diagnosis
Heavy current wire
Starter switch
Supply terminal
Solenoid switch
Light current wire
Battery
Motor
Buffer spring
Pinion
Figure 7.60 Inertia type starter
Figure 7.59 Starter circuit
not applied until the pinion is fully in mesh. They prevent premature ejection as the pinion is held
maximum allowed circuit resistance is 2.5 m ⍀, into mesh by the action of a solenoid. A one-way when using a 12 V supply. This is a worst case
clutch is incorporated into the pinion to prevent situation and lower resistance values are used in
the starter motor being driven by the engine. An most applications. The choice of suitable con-
example of a pre-engaged starter in common use ductors is therefore very important.
is shown in Figure 7.61.
Figure 7.62 shows the circuit associated with
7.26.2 Inertia starters
operating this type of pre-engaged starter. The basic operation of the pre-engaged starter is as fol-
In all standard motor vehicle applications it is lows. When the key switch is operated a supply is necessary to connect the starter to the engine ring
made to terminal 50 on the solenoid. This causes gear only during the starting phase. If the connec-
two windings to be energised, the hold-on winding tion remained permanent, the excessive speed at
and the pull-in winding. Note that the pull-in wind- which the starter would be driven by the engine
ing is of very low resistance and hence a high cur- would destroy the motor almost immediately.
rent flows. This winding is connected in series with The inertia type of starter motor has been the
the motor circuit and the current flowing will allow technique used for over 85 years, but it is now
the motor to rotate slowly to facilitate engagement. becoming redundant.
At the same time the magnetism created in the The starter engages with the flywheel ring gear
solenoid attracts the plunger and via an operating by means of a small pinion. The toothed pinion
lever pushes the pinion into mesh with the flywheel and a sleeve splined on to the armature shaft are
ring gear. When the pinion is fully in mesh the threaded such that when the starter is operated
plunger at the end of its travel causes a heavy-duty via a remote relay, the armature will cause the
set of copper contacts to close. These contacts now sleeve to rotate inside the pinion. The pinion
supply full battery power to the main circuit of the remains still due to its inertia and, because of the
starter motor. When the main contacts are closed screwed sleeve rotating inside it, the pinion is
the pull-in winding is effectively switched off due moved into mesh with the ring gear.
to equal voltage supply on both ends. The hold-on When the engine fires and runs under its own
winding holds the plunger in position as long as the power the pinion is driven faster than the arma-
solenoid is supplied from the key switch. ture shaft. This causes the pinion to be screwed
When the engine starts and the key is released, back along the sleeve and out of engagement
the main supply is removed and the plunger and with the flywheel. The main spring acts as a
pinion return to their rest positions under spring buffer when the pinion first takes up the driving
tension. A lost motion spring located on the plun- torque and also acts as a buffer when the engine
ger ensures that the main contacts open before throws the pinion back out of mesh.
the pinion is retracted from mesh. During engagement if the teeth of the pinion hit