If you had to categorize the components that
are the cause of most starter failures, the starter solenoid,
without a doubt would be on top of the list. The starter drive
may come in a close second.
Some large rebuilders may opt to rebuild
their starter solenoids in-house or have them built to their specifications,
thus having a better handle on quality and cost. Smaller rebuilders
and auto-electric shops, considering the economics and profitability,
by and large choose to buy their solenoids from manufacturers
or other distributors. They obviously shop around for the best
price, and at times, they get a product which may or may not perform
to their expectations.
In order to find out why starter
solenoids go bad or stick, let’s briefly review this component.

A starter solenoid that is used in automotive applications must
accomplish these TWO functions and in this order:
1) Work as a
solenoid, which by definition, is a device operated by electrical
energy to accomplish a mechanical motion, i.e. pulling the starter
drive into the flywheel.
2) After the above function is done and
the drive is engaged into the flywheel, the solenoid should work
as a high output relay and apply the battery power to the starter
motor.
It is also understood that the solenoid should be designed
and constructed so that a withdrawal from the flywheel will be
accomplished as soon as the starter switch is moved away from
the "START" to the "RUN" position. If this
is not done properly, we get a phenomenon known as STICKING solenoids
which will eventually destroy the starter and damage the flywheel.
To
accomplish the above functions, there are two windings used inside
the solenoid named PULL-IN and HOLD-IN coils.
The PULL-IN coil
has less turn but uses heavier gauge wires where as the HOLD-IN
coil has more turns but smaller diameter wires.
The operation of
the starter and the relationship between these two coils can be
explained in these three stages:
1) When the ignition/start switch
is turned to the "start" position, the current from
the positive side of the battery, through switch contacts, will
get to the "S" post of the starter. At this time the
two coils are in parallel, meaning the current has two paths to
go through. It will go from the "S" post, through the
PULL-IN coil, through the field winding and armature to ground
via the negative brushes. The current will also go through the
HOLD-IN windings to the ground. The combined electromagnetic force
generated by flow of the current through these coils that are
complementary to each other at this time, will be so strong that
the solenoid plunger will be pulled in and the drive will engage
the flywheel. See Figure 1 on page 40.
2) At the last stages of
the plunger’s travel, the contact disc will bridge the "BAT"
to the "MOTOR" terminal of the solenoid and the starter
motor will begin to rotate the flywheel. At the same time, this
action of the disc will apply battery voltage to the other end
of the PULL-IN coil that was going to the starter motor. Since
now both ends of the PULL-IN coil are receiving battery voltage,
the current will no longer be flowing in this coil. The plunger
is held in position by the magnetic forces of the HOLD-IN coil
only – thus the name. See Figure 2 on page 40.
3) After the engine
starts, the ignition/start switch is released from the "START"
position and there will be no current at the "S" terminal
of the solenoid. The starter motor, which is still turning, will
turn somewhat into a generator, inducing a current back into the
field coil because of an existing CEMF (counter electromotive
force), a rotating armature and motor principles. It’s more or
less the same as motor/generators in that when you apply power
it’s a motor and when turned fast enough, it is a generator. This
reverse current will feed back into the solenoid coils. At this
time the coils are in SERIES, but only the direction of current
in PULL-IN coil is reversed! See Figure 3.
The electromagnetic
forces generated at this time are opposite to each other. If the
coils were made magnetically balanced, meaning with the correct
amount of turns and wire size, these two magnetic forces would
be equal and cancel each other out. Then the solenoid spring is
able to return the plunger back, stopping the battery supply and
releasing the drive from the flywheel.
If the coils are not balanced,
the strength of the solenoid spring is not enough to pull the
plunger back and that’s when we encounter a starter that will
not disengage. It is very hard to detect unbalanced solenoid coils
during the free run or even during a load test. Short of using
specially designed solenoid testers that can electronically check
the coils for balance, you can do this very simple test that does
not require any special tools.
Check the coils for continuity
and amperage draw. Normally you will see twice as much amperage
draw in PULL-IN coil compared to HOLD-IN coil. You can find these
specs in service manuals or make your own figures from the OE
units. If the coils are okay and they do not smell of burned varnish,
hold the solenoid upright, put the appropriate plunger in the
solenoid, and with the help of your little finger prevent it from
coming off. Apply power to the "MOTOR" terminal and
ground the case (or the negative post in the case of a 40-MT type
solenoid). The meter (if used) will show a few amps draw, but
at the same time the plunger should move in or out of the solenoid
with no resistance. (The plunger will fall off the solenoid if
you let your little finger go). If you feel a little drag on the
plunger, the chances are good that the coils are not balanced
and may stick.
Obviously this procedure cannot work on solenoids
that have the contacts attached to the plunger assembly such as
early non-crimp cap Ford PMGR and 28-MT, Bosch (See Figures 4
and 5 on page 41) and others.
There are also solenoid testers available
that can thoroughly check for balance and performance.
It should
also be noted that the proper operation of the starter solenoid
in most Delco passenger car and intermediate applications depends
on the integrity of the stop collar, retaining ring and thrust
washer. If these parts are missing or broken, the starter drive
will travel further into the flywheel, causing the solenoid plunger
to go further back into the solenoid. After the ignition/start
switch releases, the plunger may have gone too far back, passing
a magnetic (center) point, not allowing the normal release from
the flywheel, even if the coils are well balanced. This can be
easily verified by looking into the core samples that have stayed
engaged.
Another type of problem with solenoids is a no-crank after
a hot engine that has been shut down and then tried to restart
again. (A hot-soak cycle). If the size of the wires and the number
of turns of PULL-IN coil windings have been compromised for the
sake of economy (such as using less wire or aluminum windings),
then there are more chances of a no-crank during a hot engine
start.
The heat of the engine compartment increases the resistance
value of the coils as well the wire leading to the "S"
post. This increase in resistance will limit the current flow,
thus reducing the magnetic forces generated to pull the plunger
in. Normally, as soon as the engine cools off the starter solenoid
will function properly.
A side note here: To overcome the no-crank
condition if the problem persists, you can install and wire up
an auxiliary relay such as a Ford type starter relay (Figure 4)
or a Bosch relay (Figure 5) by having it installed near the starter.
The
hot-soak voltage drop will have minimal effect on the operation
of the relay and enough power to the "S" terminal will
be supplied regardless of the temperature. The auxiliary relays
have been used extensively on RVs and applications where there
is a long distance between the ignition/start switch and the starter.

We have installed many such relays on turbo-charged engines where
the heat of the engine compartment is a great factor. It can also
be used to overcome an occasional CLICKING problem due to installation
of an alarm or a bad ignition switch where there is not enough
current available to pull the plunger in. (Excessive voltage drop
in key-start circuit).
And finally, if the starter has been clicking
but not turning (due to a worn-out solenoid contact or bad connections
at the BAT terminal), the chances of an ignition switch failure
will increase dramatically. If the solenoid clicks but the starter
does not turn, the sum of the currents of both coils will be passing
through the ignition switch contacts. This current which may exceed
30 to 35 amps, will burn the switch contacts where in a normal
application, this high current will be limited to a split second
and reduced immediately to less than 10 amps that is flowing through
the HOLD-IN coil only, depending on the type of solenoid and applications.
The
cost of a quality solenoid must be balanced against the cost of
warranty returns and claims. But the investment in properly performing
replacement parts is usually worth the expense up front.