Car Mechanics April 2014 www.greatmagazines.co.uk/carmechanics6

urprisingly, electric starters have not changed radically since they were first introduced to the car. The motor operates on a simple

brush-type principle and a cog (the pinion gear) resides at the armature shaft’s end. When power is supplied, the pinion meshes with the flywheel’s teeth and the engine is turned over under the motor’s power. When the engine is rotated at sufficient speed to start, the pinion disengages as power to the starter motor is cut. As the pinion is much smaller than the flywheel’s ring gear, the armature has to turn at a faster rate – a ratio of 15:1 is not uncommon on direct-drive types.

Due to the motor having to overcome opposing forces of compression (especially in a diesel) and friction, plus the drag caused by cold oil, starter motors can have a torque rating in excess of 200lb ft. Passing a heavy current through the ignition circuit is impractical – doing so would dictate very heavyweight cabling, otherwise the wiring would melt. Therefore, a solenoid utilises a low current trigger input from the ignition switch to close a pair of heavy-duty contacts that permit substantial current flow – at least 250 amps – to the starter motor.

Getting startedStarter motors differ in the ways their teeth engage with the flywheel. Cheaper cars used to employ inertia-type or Bendix Drive pinions. In virtually every case, the solenoid was mounted separately to the motor, often on the inner wing or bulkhead.

As the motor starts to turn, the pinion gear rides up the armature shaft on a threaded sleeve, until it meshes with the flywheel’s teeth. Once the engine fires, the action of it turning faster than the starter motor causes the pinion to disengage, due to the force of a powerful spring that bears against it. While inertia starter motors are inexpensive to produce, the method of pinion gear engagement is fairly aggressive, meaning that its teeth are prone to being chewed.

Inertia motors became obsolete as the 1980s progressed, but you will still find them on engines that power older cars, boats and other equipment. Electric-start lawnmowers, for example, tend to use inertia motors fitted with fibre pinions.

These days, pre-engaged starter motors serve almost every modern car. They differ from the cheaper inertia-type by

having their solenoids mounted on the top of the main motor casing. This serves a dual purpose: not only does it fulfill the switching criteria, but a plunger is operated electromagnetically to engage the pinion when the armature is either not turning or is just rotating very slowly, hence the term ‘pre-engaged’. A one-way clutch, which cannot be rebuilt separately, can be incorporated within the pinion. While pre-engaged motors are more expensive to produce, the pinion has a much longer life expectancy and the motor is significantly quieter.

Direct-drive starters have their pinions attached directly to the armature, while gear reduction starters use several gears internally, allowing the pinion to spin up to five times faster than the armature. This means a lighter and less bulky motor can be used by the car manufacturer.

As spare parts are so easy to obtain, ROB MARSHALL says you can save a significant sum by repairing a faulty starter motor at home.

STARTER MOTOR OVERHAUL

LEFT: An inertia motor can be identified by its simple construction and separate solenoid.

RIGHT: The pre-engaged motor has its solenoid mounted above the motor casing.

CIRCLIP

COLLAR

RUN-OFF

SPRINGTHREADED

SLEEVE

PINION

The inertia starter motor is also known as the Bendix Drive, named after its inventor, Vincent Hugo Bendix.

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TURN

Some starter motor designs also replace the field coil with a permanent magnet, meaning the battery does not have to energise both the field coil and armature simultaneously. However, the magnets attract iron particles and fragments that find their way into the bellhousing (caused by, for example, a failing dual mass flywheel), which can be sucked into the starter motor, thereby ruining it.

The introduction of stop-start and hybrid technology has seen the humble starter motor being put under even greater stress, but any worries about longevity have been resolved by strengthened brushes and pinions. Combined starter/alternators have been introduced, such as Valeo’s i-stARS (Starter Alternator Reversible System). It operates on the engine not via the flywheel, but by the alternator belt. A conventional starter motor tends to be

used as well, for the more difficult cold starts. Few of these units are in circulation and the aftermarket is only just beginning to repair them, because specialist calibration is required. These systems are not included within this feature.

Rebuild costsShould you treat your motor to a basic rebuild – replacing the brushes, their springs and the armature bushes only – the cost of parts is unlikely to exceed £20, which is a significant saving over the cost of a new motor. However, once the motor is disassembled, should you find that the solenoid, armature or field coils need replacing as well, the parts cost will inflate dramatically and might even eclipse the price of a guaranteed reconditioned exchange unit. This is in

addition to the inconvenience of having the car off the road for a longer period.

Starter motors can also be tricky to remove and not all of them are fitted in an accessible location, so check that removing the starter motor is within your capabilities. As carmakers tend to fit starter motors from different manufacturers, even for the same model, identifying which version is fitted to your car, either by registration or chassis number, might not be accurate.

The starter motor being demonstrated is a pre-engaged, direct-drive type, equipped with field coils. In the majority of cases, rebuilding a starter motor is straightforward. Other than a soldering iron and a multimeter, no special tools are required. However, should you run into difficulties, do not be worried about contacting your local auto electrician for assistance.

Combined starter/alternator units, which connect to the crankshaft pulley via a belt, are fitted to certain stop-start models and hybrid vehicles.

3As starter motors are damaged by excessive heat, on removal, try not

to bend any shielding out of shape.

1Most starter motor problems are caused by dirt affecting the internals, especially when the unit is submerged in an oil leak.

4Note the wiring’s position before disconnecting anything. Some motors

are supported by a rear bracket.

2Some starter motors can be tricky to see, let alone access. Ensure that removing it is within your capabilities.

IDENTIFYING PROBLEMSWhile fitted to the car, the starter can

manifest problems in different ways.

Reluctance for the motor to turn until

tapped with a suitable implement can

point to a worn solenoid, brushes or

armature. It can also indicate a blown

fuse or a problem with the car’s wiring.

Intermittent operation can be down

to the same components. A poor earth

can cause slow starter movement,

as can a shorted field coil. A grinding

noise can be caused by either a worn

pinion or sticky solenoid; do not use the

starter motor or else you can damage

the flywheel teeth. A high-pitched shriek

heard soon after the engine fires is

down to worn bushes that support the

starter’s armature. However, you should

check the battery and alternator before

suspecting the starter motor.

5Here, the starter came out from behind the engine. Some might

necessitate taking off a wheel or undertray.

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7Inspect the starter motor for any cracked or damaged casings, especially as the front plate tends to be cast from aluminium.6

Ensure that any locating dowels do not drop into the bellhousing. Retain any spacing shims, noting their position.

8Degrease the outer case with a solution that cannot dissolve the

glues and resins used on the armature.

12The small nuts that secure the solenoid to the motor’s front plate

can seize and snap easily, so be careful.

9If not done so already, note the identifiers stamped on the centre

casing and inform your spare parts supplier.

10While chipped or chewed teeth are unlikely to affect pre-engaged starter

motors, always replace inertia pinions.

11Disconnect the field coil supply lead from the solenoid, storing the bolt

and any washers for reuse.

13Withdraw the solenoid and spring. Be aware that some types need

unclipping from the arm within the motor.

14Drive out this steel pin. Try not to damage the soft aluminium front

plate casting. See Step 22.

15If fitted, remove the rear mount support bracket, retaining all nuts

and spring washers for reuse.

16If fitted, a dust cover not only protects the rear bush but also supports the

brush box. Note if a gasket is fitted.ST

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17A bolt is retaining the end-float washers here. Prevent the motor from turning by locking the pinion using… 18

…an implement such as a flat-bladed screwdriver. Remove the bolt and note the position of the end-float washers.

22Ease the armature from the front casting by working the nose out of the front bush. Lift off the pinion operating arm.

19Remove the retaining bolts holding the unit together. They can be mounted

either internally (as pictured) or externally.

20Slowly and carefully, pull the combined front casting and armature

away from the centre and rear casings.

21Most units have a rubber gasket and mounting plate between the

solenoid and front casing. Lift them away.

23Remove and store the other thrust washers fitted to the rear of the armature – they must be refitted later.

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24After lifting the rear plate, the brush box is uncovered. Here, the brush

springs are attached to the rear plate.

25Note the brush positions. prior to removal. Brush boxes can be damaged

easily – replacements tend to be available.

26Gently clean away oil and dust residue from all casings and parts,

before allowing everything to dry.

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27Once all the components have been allowed to dry after being cleaned in a suitable solution, lay them out on a clean surface. Inspect all of the parts for their serviceability and make sure that no major components need to be renewed.

CASING

BOLT

FIELD COIL

RETAINING

SCREWARMATURE BRUSH

REAR

BUSH

DUST

COVER

BRUSH

SPRING

REAR

PLATE

THRUST

WASHERS

THRUST

WASHER

& BOLT

FIELD

COILS

OUTER

CASING

CASING BOLT

FRONT

BUSH

FRONT CASING

PINION

ASSEMBLY

PINION RETURN

SPRING

PINION

OPERATING

ARM

SOLENOID

28As they are inexpensive, renew the carbon brushes regardless. The length of the new brushes here was 15.2mm.

30A more comprehensive overhaul can include renewing the field coils,

armature, pinion and solenoid.

33The new bush must be pressed into the aperture evenly, using either an

old bush or a small socket.

31Allow the bushes to soak in engine oil for 24 hours prior to fitting. They

absorb the lubricant and will last longer.

34After the replacement bush is in position, replace any brush springs

if they are fitted to the end cap.

32Working on the rear cap, press out the old bush using either a small

socket or a screwdriver.

35To renew the front plate’s bush, repeat Steps 32 and 33, taking extra

care not to damage the aluminium casting.

29As a bare minimum, renew brushes and their springs, as well as the two bushes that support the armature at each end.

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36The pinion tends to be retained by either a collar or a circlip. Remove it and slide the pinion from the armature shaft. 37

Check your replacement pinion matches that of the original. Slide it onto the shaft and replace the retaining circlip.

38Check the surface, on which the brushes bear, for heavy scoring. A

commutator lathe can remove the high spots.

39Do not sand the commutator. Polish it with wet-and-dry sandpaper and

clean using contact cleaner and a toothbrush.

40Check the commutator’s individual segments for continuity and for

any short circuits.

41Check also for continuity between each field coil connection and that no winding is shorting out to the earthed body. 42

Should either fault be detected, renew the field coils (if fitted) by removing the screws on top of the casing.

43Should you not renew the field coils, cut off the old brushes from the

post. Leave some copper wire exposed, so that you can solder new brushes to it.

44Solder the new brushes to the exposed wire. In some cases, you’ll

find it’s hard to solder directly to the post as the solder just runs off.

45Reassemble the solenoid’s pinion operating arm (see Step 22) and

locate the armature’s nose into the front housing’s bush.

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53Refit or replace the gaskets (see Step 21), solenoid and return spring. Refit the field coil lead to the original

terminal, if fitted (see Step 11).

54Test the starter motor and solenoid before refitting it to the car. Do not connect live power leads directly to the terminals

because a high current’s spark will erode the threads. Use a switching medium, such as a jump-pack.

THE COSTIn the case of this Valeo D9E motor,

renewing all the parts at home would

cost £92.93+VAT – which is more

than a new aftermarket replacement,

which Wood Auto Supplies prices

at £79.20+VAT. However, replacing

the bushes, brushes and one or two

major components, if needed, will

realise a saving. The component

prices below exclude VAT, and, while

specific to the unit being dismantled

in this feature, give an idea of which

parts cost the most.

Solenoid ..........................................£16

Brush set .................................... £2.32

Brush spring set ..................... £1.12

Armature .................................. £25.60

Pinion .........................................£12.16

Field coils .................................£35.20

Bushes (x2) ............................... £0.53

SPARE PARTS (MAIL ORDER)

Wood Auto Supplies Limited

Colne Road

Huddersfield

HD1 3ES

Tel: 01484 439000

Email: sales@woodauto.com

Web: www.woodauto.com

51Ensure that any rubber grommets, which surround a protruding field

coil lead, are fitted and located correctly.

52Consulting Steps 15 and 16, refit the dust covers, their gaskets and

the rear supporting bracket.

46Drive the operating arm’s pivot pin back into the casing, but check if it

needs to pass through the aperture.

47Position the main casing, which incorporates the field coils/permanent

magnets, into the front housing.

48Fit the brushes and, if necessary, secure the brush box. Ensure that the

thrust washers are present on the armature.

49Refit the bolt or renew the securing washers. Ensure that the other set

of thrust washers is refitted.

50The rear plate’s bush may be reluctant to locate easily into the

armature. Tighten the locating bolts evenly.