THE NUTS AND BOLTS OF ELECTRICAL DIAGNOSIS …Automatic Drive P.O. Box 440 Bellows Falls, VT...

22 GEARS September 2006

AA solenoid fault DTC will typically fall into one of two main catego-ries:

• 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 tech-nology into their PCMs that monitors the mechanical operation of the solenoid itself, by monitoring specific characteristics of the current in the solenoid circuit it ener-gizes. Ford calls this inductive signaturemonitoring. In this issue of GEARS, we’ll take an in-depth look at inductive signature monitoring, and how we can apply its prin-ciples for testing solenoid operation.

Figure 1 shows a chart of Ford induc-tive 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 under-standing of inductive signature monitoring, let’s first take a closer look at the electrical characteristics of a solenoid and what hap-pens electrically when a typical on-off shift solenoid energizes.

A Solenoid Has the A Solenoid Has the Property of Inductance Property of Inductance

A solenoid’s main electrical characteristic is that of an inductor, in that it possesses inductance, which is the char-acteristic 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.

A Solenoid Is an Electromagnetic A Solenoid Is an Electromagnetic ValveValve

An inductor — in this case a solenoid — stores energy in the form of a concentrated magnetic field. Whenever cur-rent 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

Solenoid TestingSolenoid Testing andand Inductive Signature:Inductive Signature: Electrical Testing to Find a Mechanical Fault

THE NUTS AND BOLTS OF ELECTRICAL DIAGNOSIS

by Mike Van Dyke

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)

P1636ISIG (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

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’.

SolenoidInternalComponents

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concentrated, so we can now use this electromagnet to control a mechanical valve using an electrical signal.

When the solenoid is energized, the current increases from zero, heading toward maximum. As the current increas-es, the magnetic field of the solenoid expands. The strength of the magnetic field has a direct relationship with the current going through the solenoid windings; any change in the cur-rent will change the size of the magnetic field, and any change

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 coun-ter 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 armature. This allows the armature to be attracted toward the center of the magnetic field as the magnetic field expands (and current increases). This is where the magic begins.

The Inductive Signature Current The Inductive Signature Current WaveformWaveform

Let’s look at what happens when our solenoid energizes: The current increases, causing the magnetic field to expand until it becomes strong enough to move the armature. The armature movement increases the concentration of the mag-netic field as the armature’s own magnetic mass moves farther

Solenoid Testing and Inductive Signature

Figure 3: The ‘dip’ in current indicates the armature has moved, verifying the mechanical operation of the solenoid.

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GEARS September 2006 25

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

moved, and verify the mechanical operation of the solenoid.

Putting It to the Test 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 oscil-loscope (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 sim-ply 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;

Figure 4: A digital storage oscilloscope and a current clamp were used to capture this properly functioning 5R55E shift solenoid C current waveforms.

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notice the characteristic current notch in the rising slope. This is a handy test to check a solenoid without having to disas-semble 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 fall-ing 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 Induced Voltage Also Indicates Mechanical OperationMechanical Operation

Voltage waveforms can also indicate a solenoid’s mechanical function, particularly with solenoid driver cir-cuits 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 (opposi-tion to change in current), English physicist Michael Faraday discov-ered that when the magnetic field around an inductor changes, regard-less of what caused the change, it induces a voltage into the inductor. This is the principle on which an ignition coil works.

Faraday’s Law is also at work in our solenoid circuit; when the solenoid turns off, the current drops quickly, and the magnetic field col-lapses. The collapse of the magnetic field induces a voltage into the coil, which is the opposite polarity of the voltage that was sustaining it, causing a negative voltage spike.

As the magnetic field becomes weaker, the solenoid armature returns to its rested position. As the armature moves in the collapsing magnetic field, it induces a small voltage into the coil (figure 5). This indicates the solenoid is function-ing mechanically. You can see the same type of characteristic in some fuel injector circuits, where it’s often referred to as the pintle hump because it’s induced when the injector pintle seats.

Figure 6 shows the same type of solenoid and driver cir-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.

Ford’s inductive signature monitoring strategy makes us think about some of the details of solenoid operation and testing. Having a simple understanding of inductance and electromagnetism can go a long way in waveform analysis of solenoid control circuits. As you can see here, knowing what to look for when analyzing solenoid voltage and current waveforms can help you verify not only the integrity of the electrical circuit, but the solenoids’ mechanical operation as well.

Don’t miss Mike Van Dyke’s class on Reprogramming and Scan Tool Diagnosis on Saturday, October 7th at the 2006 Powertrain Expo in Orlando.

Solenoid Testing and Inductive Signature

Figure 5: The characteristic ‘hump’ on this voltage waveform taken from a Nissan Maxima EPC solenoid. This indicates the armature has moved to the closed position.

Figure 6: This is the damper clutch control solenoid waveform on a Mitsubishi KM transaxle, showing a mechanical failure of the solenoid.

Negative voltage "hump" inducedwhen the solenoid pintle seats.

Broken armature parts in the solenoidcaused the extra voltage spike

Normal voltage waveformwith good solenoid


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THE NUTS AND BOLTS OF ELECTRICAL DIAGNOSIS …Automatic Drive P.O. Box 440 Bellows Falls, VT...

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