Photo 1: The insulation between the commutator segments should be recessed about 1/32” beneath the commutator bars.
On June 15, 1911, Charles F. Kettering was awarded a patent
for an electric self-starter for automobile engines. Thinking out of the box,
Kettering designed a small, high-torque motor that would deliver a burst of
energy lasting only long enough to initiate the internal combustion cycle.
Fortunately for modern commuters, Kettering’s electric self-starter transformed
the automobile from a temperamental novelty item into a practical means of
From a historical view, it’s important to remember that
Kettering’s conventional field-coil starter required battery
power to create
the magnetic field needed to make the starter armature turn. During the 1980s,
field-coil starters were phased out in favor of “ferrite” permanent-magnet starters.
Photo 2: In this application, the starter armature is surrounded by six permanent magnets.
Since the fields in permanent magnet starters don’t require
battery power, permanent-magnet starters require much less current to crank an
engine. The result is a much lighter, far more efficient starter motor. But,
because permanent or “ferrite” magnets are made of a brittle ceramic material,
they are vulnerable to cracking caused by sudden impacts. Cracked magnets can
be tough to diagnose, which is why it’s usually better to replace the starter
as an assembly than to repair or rebuild it. See Photo 1.
Photo 3: This starter motor terminates into a planetary gear set similar to those used in automatic transmissions.
In addition, the rotating mass of the starter armature is
reduced to create a more compact starter motor assembly.
Photo 4: The secondary reduction drive gear contains an over-run clutch that disengages the starter motor as the engine speed increases.
As pictured above (see
), the armature on most modern starters terminates into a sun gear
mating with a set of planetary gears (see Photo 3
) provide the initial gear
reduction for the starter. A secondary reduction gear can also be used on
starters like the one used to illustrate this story. See Photo 4
The starter “solenoid” is actually a combination of an
electric relay and solenoid. The relay portion electrically connects the
starter armature to the battery. The solenoid portion mechanically engages the
starter’s drive pinion with the engine’s flywheel gear. While modern solenoids
usually incorporate two high-amperage terminals and one low-amperage, primary
activation terminal, some older designs might incorporate an additional
primary “by-pass” terminal that was originally designed to boost ignition coil
voltage during cranking. In some applications, the by-pass terminal is unused
and remains a vestigial remnant of past technology.
The starter over-run or one-way clutch is a simple
roller-type clutch that’s designed to release when the engine speed exceeds
cranking speed. In rare instances, the clutch will seize, which can cause the
starter armature to explode from centrifugal force as the engine accelerates.
In other cases, the clutch will simply wear out, which usually results in a
“whirring” sound, indicating that the starter motor is running, but not engaged
to the flywheel.
STARTER ACTUATION SYSTEMS
For safety’s sake, the starter’s primary circuit is routed
through a neutral safety switch on automatic transmission vehicles and through
a clutch safety switch on manual transmission models. With that said, current
practice is to reduce the electrical load on the ignition, neutral safety and
clutch switches by inserting a starter relay into the starter primary circuit.
In this case, the above switches activate the starter relay switch rather than
the starter’s primary solenoid circuit.
Keep in mind also that modern technology in some vehicles
has delegated the starter engagement process to the Powertrain Control Module
(PCM). In this system, turning the ignition switch or pressing the “start”
button simply commands the PCM to engage the starter motor. Failures in these
systems should first be diagnosed with a scan tool and by using diagnostic
techniques similar to those used in any other system controlled by the PCM.
The first step is to make sure that the battery terminals
and cables are free of corrosion. Next, determine the battery state of charge
(SOC) and condition by testing with a conductance or variable-load, carbon
pile battery tester. Recharge or replace the battery as required. Voltage drop from
the battery to the starter can be measured by attaching a voltmeter in parallel
to the positive battery terminal and to the solenoid B+ terminal.
Photo 5: Corrosion at the lower solenoid terminal on this starter caused an intermittent “clicking,” no-cranking complaint.
The rule of thumb is that voltage drop shouldn’t exceed 0.5
volts during cranking. The voltage drop on the negative ground terminal can
similarly be measured by attaching the voltmeter lead to a clean area on the
engine block and to the battery B- terminal. Here again, the voltage drop
shouldn’t exceed 0.5 volts. See Photo 5.
STARTER CURRENT DIAGNOSIS
Figure 1: A lab scope display of the relationship between voltage and amperage can provide valuable information about the condition of the starter and battery.
Most starter-related electrical failures can be diagnosed by
measuring current flow into the starter. Actual current flow to the starter
can be measured by attaching a 600-amp inductive current probe to the battery
positive or negative cables. The probe can be attached to a multimeter with a
minimum/maximum (min/max) recording feature or to a two-channel lab scope. To
illustrate how a starter works on a vehicle in good condition, I’ve included a
lab scope recording of battery terminal voltage and starter amperage draw. See
The amperage draw begins from the “zero” point at the left.
The initial amperage drawn by the solenoid primary circuit occurs at 70
milliseconds (ms). If the voltage
remains at zero, it’s likely that the system has a bad neutral or clutch safety
switch, or that the starter relay is defective. If the solenoid amperage
remains at 2-3 amps, the solenoid doesn’t have continuity to the starter. Bad
solenoid contacts, worn starter brushes or an open-circuit armature can be the
cause. In this case, the primary symptom will be a clicking noise as the
solenoid primary circuit activates. Any of the above failures can result in an
intermittent starter engagement complaint.
Once the solenoid closes the circuit at 100 ms, the amperage
draw increases to 311 amperes at the trigger point. As the engine cranks, the
amperage draw declines until approximately 300 ms. At about 300 ms, amperage
rises slightly as the torque load on the starter is momentarily increased due
to a possible variation in fuel delivery or spark advance.
Figure 2: At 2.5 milliseconds, available battery voltage drops from about 13.0 volts to 9.19 volts as the starter is engaged. After the engine starts at 6.3 milliseconds, the alternator begins recharging the battery at 13.8 volts.
Similarly, battery terminal voltage spikes down to nearly
8.0 volts at 100 ms as cranking amperage is suddenly drawn from the battery.
The battery terminal voltage begins to rise to about 10.0 volts at 200 ms as
the starter amperage begins to stabilize. As the engine begins to crank, 10.0
volts should be considered the minimum voltage. If the battery won’t maintain
10.0 volts during cranking, the PCM might fail to process data or activate the
injector and ignition system drivers. See Figure 2.
BATTERY VOLTAGE GRAPHING
Graphing available battery terminal voltage also provides a
direct insight into battery condition. Charging voltage should be achieved
approximately two seconds after the engine starts. If charging voltage doesn’t
increase within that time interval, it’s likely that the battery doesn’t have
enough remaining plate capacity to fully support starter current draw. In any
case, using a lab scope to display available battery voltage and amperage
discharge is an easy way to quickly evaluate battery, starter and starter
A LOOK AT IDLE/STOP TECHNOLOGY
We’re beginning to see “idle/stop” or “stop/start”
technology enter the non-hybrid import market, with fuel savings ranging from
an estimated 5 to 15% in normal driving. Although a version of idle/stop
technology was popularly introduced in a European version of Volkswagen in
1983, the technology has a number of issues, including how to power the HVAC
and lighting systems while the engine is stopped.
Because idle/stop technology obviously requires a rapid
discharge/recharge cycle, the absorbed glass mat (AGM) battery most closely
meets those requirements. Similarly, idle/stop engine cranking systems include
integrated starter/generator systems mounted at the flywheel or connected to
the front of the crankshaft by the drive belt. Others use an “enhanced” starter
motor system that is built to withstand repeated cranking cycles. With the
advent of direct fuel injection and electronic valvetrains, some manufacturers
have explored using fuel and spark timing alone to initiate the internal
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