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THE NUTS AND BOLTS OF ELECTRICAL DIAGNOSIS
Solenoid Testing and
Inductive Signature:
SPEAKER
by Mike Van Dyke
Electrical Testing to Find a Mechanical Fault
A
•
•
solenoid fault DTC will typically
fall into one of two main categories:
An electrical circuit fault, where
the computer detects the wrong
voltage or current in the solenoid
circuit.
A performance fault, where the
computer has determined the
function or event the solenoid is
controlling is wrong.
Some automakers, specifically Ford
Motor Company, have incorporated a technology into their PCMs that monitors the
mechanical operation of the solenoid itself,
by monitoring specific characteristics of
the current in the solenoid circuit it energizes. Ford calls this inductive signature
monitoring. In this issue of GEARS, we’ll
take an in-depth look at inductive signature
monitoring, and how we can apply its principles for testing solenoid operation.
Figure 1 shows a chart of Ford inductive signature DTCs and the solenoids they
relate to. You may have seen a solenoid
inductive signature fault DTC on a Ford,
or are just plain wondering what type of
failure it indicates. To have a better understanding of inductive signature monitoring,
let’s first take a closer look at the electrical
characteristics of a solenoid and what happens electrically when a typical on-off shift
solenoid energizes.
A Solenoid Has the
Property of Inductance
P1714
SSA (SS1) Mechanical Fault (Inductive Signature)
P1715
SSB (SS2) Mechanical Fault (Inductive Signature)
P1716
SSC (SS3) Mechanical Fault (Inductive Signature)
P1717
SSD (SS4) Mechanical Fault (Inductive Signature)
P1740
TCC Solenoid Mechanical Fault (Inductive Signature)
P1636
ISIG (Inductive Signature) chip communication fault. Internal
PCM failure, replace PCM.
Figure 1: Ford DTC’s for solenoid inductive signature fault indicate a
mechanical failure of the solenoid
Solenoid
Internal
Components
Figure 2: The armature is drawn to center of the windings with solenoid
energized. This movement of the armature creates the current notch,
or ‘Inductive Signature’.
A solenoid’s main electrical characteristic is that of an
inductor, in that it possesses inductance, which is the characteristic that opposes any change in current. This is why
current doesn’t immediately reach maximum when a solenoid
is energized; instead, the current rises at a steady rate until it
is limited by the DC resistance of the solenoid, as defined by
Ohm’s Law.
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22MikeVD-InduSig.indd 22
A Solenoid Is an Electromagnetic
Valve
An inductor — in this case a solenoid — stores energy
in the form of a concentrated magnetic field. Whenever current is present in an individual wire or conductor, a magnetic
field, however small, is created around the wire. The size of
the magnetic field has a direct relationship with the amount of
the current in the wire. With many turns of wire wound into
a coil, such as in a solenoid, the magnetic field becomes very
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8/14/06 11:20:03 AM
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Solenoid Testing and Inductive Signature
in the magnetic field will induce a change in current. Here
is the kicker; In the early 1800s, German physicist Heinrich
Lenz found that a changing current’s magnetic field induces
a voltage into an inductor that opposes the change in current;
this is known as Lenz’s Law.
Essentially the expanding magnetic field induces a counter EMF (ElectroMotive Force), or opposite voltage, into the
windings. This counter EMF works within the coil windings
to slow down the increase in current. The application of
Lenz’s Law gets deeply involved with electromagnetic theory,
so we’ll leave it right here. The main point is the principles
behind Lenz’s Law are the reason the current rises at a defined
rate, and cannot instantly go to maximum.
A solenoid has a moving magnetic armature in its core,
which performs the mechanical work; it operates the hydraulic
valve of the solenoid. The armature is offset from the center
of the magnetic field created by the solenoid windings (figure
2), so the magnetic field isn’t distributed evenly through the
Figure 3: The ‘dip’ in current indicates the armature has moved, armature. This allows the armature to be attracted toward the
verifying the mechanical operation of the solenoid.
center of the magnetic field as the magnetic field expands
(and current increases). This is where the magic begins.
concentrated, so we can now use this electromagnet to control
The Inductive Signature Current
a mechanical valve using an electrical signal.
Waveform
When the solenoid is energized, the current increases
Let’s look at what happens when our solenoid energizes:
from zero, heading toward maximum. As the current increasThe current increases, causing the magnetic field to expand
es, the magnetic field of the solenoid expands. The strength
until it becomes strong enough to move the armature. The
of the magnetic field has a direct relationship with the current
armature movement increases the concentration of the maggoing through the solenoid windings; any change in the curnetic field as the armature’s own magnetic mass moves farther
rent will change the size of the magnetic field, and any change
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GEARS September 2006
8/14/06 11:20:29 AM
Figure 4: A digital storage oscilloscope and a current clamp were used to capture
this properly functioning 5R55E shift solenoid C current waveforms.
into the magnetic field. Remember, a magnetic field changing
in the same direction of the current creating it will induce an
opposing voltage into the windings. Because the magnetic
field quickly expands when the armature strokes, it causes a
brief reduction in the current through the solenoid windings.
After the armature strokes, the current continues on its normal
upward path to its maximum level.
The result is the current waveform in figure 3. Notice the
prominent dip in the rising portion of the current waveform.
By monitoring the current increase and detecting this dip,
the PCM can determine whether the solenoid armature has
Tech at The Speed of Life
moved, and verify the mechanical operation of the solenoid.
Putting It to the Test
This brings us to a quick, useful solenoid test you can
perform using a current clamp with a digital storage oscilloscope (for more information on the inductive current clamp
see Using an Inductive Current Clamp, GEARS August
2005). You can check a solenoid’s mechanical operation simply by connecting the current clamp anywhere in the circuit,
energizing the solenoid, and examining the waveform.
Figure 4 shows a 5R55E shift solenoid current waveform;
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25
8/14/06 11:21:35 AM
Solenoid Testing and Inductive Signature
notice the characteristic current notch in the rising slope. This
is a handy test to check a solenoid without having to disassemble the transmission, and it’s another way to test solenoid
performance on the dyno or solenoid testing machine. The
main point you want to verify is the current notch should
occur in the rising slope, not after. On a typical solenoid, it
will occur at about 70% of maximum current.
When the solenoid is turned off, there’s also a hump in
the falling slope of the current waveform, created when the
pintle strokes back to its rest position (figure 4). The falling slope current is easiest to see in solenoid driver circuits
that use a clamping diode, which allows current to circulate
through the solenoid circuit as the solenoid’s magnetic field
decays after it’s turned off.
Induced Voltage Also Indicates
Mechanical Operation
Negative voltage "hump" induced
when the solenoid pintle seats.
Figure 5: The characteristic ‘hump’ on this voltage
waveform taken from a Nissan Maxima EPC solenoid. This
Voltage waveforms can also indicate a solenoid’s
indicates the armature has moved to the closed position.
mechanical function, particularly with solenoid driver circuits that don’t use a clamping diode. Solenoids are driven
in this manner to help them close
in a faster, more controlled manner,
for precision control of oil flow and
valve movement.
On these circuits, you’ll see a
negative-going voltage spike when
the solenoid de-energizes. This is
due to Faraday’s Law of Induced
Voltage. Around the same time
Heinrich Lenz was discovering the
principles of Lenz’s Law (opposition to change in current), English
physicist Michael Faraday discovered that when the magnetic field
around an inductor changes, regardless of what caused the change, it
induces a voltage into the inductor.
Normal voltage waveform
This is the principle on which an Broken armature parts in the solenoid
caused the extra voltage spike
with good solenoid
ignition coil works.
Faraday’s Law is also at work
Figure 6: This is the damper clutch control solenoid waveform on a Mitsubishi
in our solenoid circuit; when the
KM transaxle, showing a mechanical failure of the solenoid.
solenoid turns off, the current drops
quickly, and the magnetic field collapses. The collapse of the magnetic field induces a voltage
Ford’s inductive signature monitoring strategy makes
into the coil, which is the opposite polarity of the voltage that
us think about some of the details of solenoid operation and
was sustaining it, causing a negative voltage spike.
testing. Having a simple understanding of inductance and
As the magnetic field becomes weaker, the solenoid
electromagnetism can go a long way in waveform analysis
armature returns to its rested position. As the armature moves
of solenoid control circuits. As you can see here, knowing
in the collapsing magnetic field, it induces a small voltage
what to look for when analyzing solenoid voltage and current
into the coil (figure 5). This indicates the solenoid is functionwaveforms can help you verify not only the integrity of the
ing mechanically. You can see the same type of characteristic
electrical circuit, but the solenoids’ mechanical operation as
in some fuel injector circuits, where it’s often referred to as
well.
the pintle hump because it’s induced when the injector pintle
Don’t miss Mike Van Dyke’s class on Reprogramming
seats.
and Scan Tool Diagnosis on Saturday, October 7th at the 2006
Figure 6 shows the same type of solenoid and driver cirPowertrain Expo in Orlando.
cuit with an internal mechanical failure of the solenoid. The
armature broke apart, and pieces of it were bouncing around
every time the solenoid turned off, inducing the extra voltage
spike you see in the waveform.
26
22MikeVD-InduSig.indd 26
GEARS September 2006
8/14/06 11:22:00 AM
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