Electrical Signals and terms

Posted May 7, 2010 by rostratransmission
Categories: automatic transmission components, how a solenoid works, solenoid terms, solenoids

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Activation voltage (also known as Pull in voltage)-The voltage required to make the solenoid armature move full stroke.  When testing solenoids,  use 8 volts, 8 volts at room temp =12 volts at hot temperature
Hertz-(or frequency) – A term used for PWM solenoids. Hz means cycles per second. A solenoid run at 64 hz will be turned on and off 64 times a second.
The duty cycle represents what % of time the solenoid is actually on during that 1/64th of a second cycle.
Peak and hold signal (modified PWM signal)– Solenoid is turned on for brief period of time, long enough to move armature.  Supply voltage is then dropped  to keep armature in the upstroked position. It takes much less energy to keep the armature upstroked, like pushing a car. Once it is rolling it takes less energy to keep it moving. This greatly reduces electrical consumption, and reduces solenoid heat generation. The hold portion of the signal will have a varying duty cycle to control solenoid output. For example, AX4S early model pwm 1.3 ohm solenoid would consume over 110 watts of power if treated like on/off valve. Try holding a 100watt light bulb in your hand!  Many early low ohm peak and hold solenoid have been converted to higher ohm replacements with slower activation signals.
Current Averaging Signals-Vary frequency and duty to keep the current supplied to the solenoid within a given range. Problems with vehicle voltage levels can cause havoc with EPC solenoids.

Peak and hold illustration

Peak and hold illustration

Peak and Hold Hydraulic response-What is happening in the VB.
The peak and hold signal is shown in the box upper left. Solenoid fired with 12 volts. Then held to 3 volts. The duration of the 3 volt hold time gradually increases with duty cycle.
Pressure Profile The pink line is the feed oil. The light blue line is the inductive trace of the solenoid itself. The yellow line is the pressure in the oil circuit controlled by the solenoid. This solenoid bleeds off pressure to the pressure control valve in the VB. Black line is the average pressure in the circuit.

How it works:
12 volts applied is indicated by the spikes. Yellow, oil pressure starts to fall cause solenoid is open Voltage drops to zero. (kink in curve shows when armature has closed the seat.) Pressure starts to rise again. As the hold portion of the signal gets longer, the armature stays open longer, letting more oil bleed out and the system pressure gets lower.


Profiling solenoids

Posted May 7, 2010 by rostratransmission
Categories: how a solenoid works, solenoid components, solenoid terms, solenoids, testing solenoids

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In Rostra’s hydraulic testing laboratory, valve body environments are reproduced within specially designed test fixtures. Control signals are measured on test vehicles and accurately reproduced using our custom electronic drivers. Data is collected on solenoid performance under all conditions to guarantee the solenoid will always work.
Illustration profiling solenoids

Profiling Solenoids: A Normally Closed solenoid

  • Response time-how long does it take solenoid to operate? ( in milliseconds)
    Measured by the current rise of coil (light blue). The dip in the light blue curve is when the armature bounces off the stop. ( Some vehicle computer measure this inductive spike to indicate solenoid operation.)
  • Source Pressure – Oil pressure value from pump (dark blue)
  • Feed Pressure – The pressure feeding the solenoid before the orifice restriction in the valve body separator plate.
  • Control pressure – The pressure measured between separator plate orifice, and solenoid port. This is the pressure that controls the shift valve. (pink)

Sequence of events:

  1. Solenoid is off. Feed pressure and control pressure are the same as the source pressure.
  2. Solenoid is turned on. Once the armature moves, as seen in the coil rise (light blue), the control pressure pressure falls.
  3. When solenoid is shut off, the control pressure builds back up
  4. .

Notice the problem here?
This solenoid had reduced stroke due to contamination, ball never fully off seat., all of the pressure could not bleed off.

Profiling solenoids: A Normally Open 3 way shift solenoid AX4S
Top Left Rostra sample:

  • Electrical response time (light blue)
  • Source from pump (dark blue)
  • Feed pressure to solenoid (TV pressure axode) (pink)
  • Control pressure to shift valve (red)
  • Sequence of events:
    1. Source Feed and control pressures are all the same.
    2. Solenoid is energized, and the control pressure bleeds to exhaust allowing shift valve to move.
    3. When solenoid is shut off feed pressure and control pressure rise simultaneously as the hydraulic circuit is re-charged with oil pressure

Top right: OE sample
Bottom right: Other after-market solenoid

Solenoid problems: Wear

Posted May 7, 2010 by rostratransmission
Categories: solenoid failure, solenoid problems, solenoids

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Wear Issues:
Wear is the second leading cause of solenoid failure.
An example of wear in a solenoid
Two types of wear:
Caused by repeated pounding of solenoid components. Accelerated by contamination, and material changes due to heat and chemicals in ATF.
Shows up in Seat areas. This solenoid face was originally flat. After 1 million PWM cycles the seat is badly worn. (3000 vehicle miles). A poor material selection was made by the MFG of this solenoid.
Spring pockets and areas of contact with the spring see considerable wear. Springs twist as the are compressed. This rotating, grinding action, as well as the repeated loading will cause the spring to dig its own pocket. Spring loads change, solenoid may loose it’s holding or closing ability.
Sealing faces, especially in EPC type solenoids. Spool valve edges get rounded pass too much oil, ability to regulate pressure goes south. Nissan EPC solenoids can wear out in as little as 40,000 miles. A lack of armature guidance causes the armature to hit not square wit h seat. Allowing for excessive oil leakage.
Seat areas, especially plastics, subject to creep, a gradual deformation and movement of plastic due to loss of material strength, from repeated loading. E40D on/off solenoids suffer this. Large ball, much surface area.
Springs loose their load handling ability with repeated cycles. Special materials should be used.

Some wear is normal.

Contamination will greatly increase wear.
Heat influences wear, especially in plastics. Impact resistance goes down with increased temp.solenoid wear: before and after
Proper material selection is required.
Solenoid testing in the shop can not determine the wear, or life expectancy of the solenoid.
Photos illustrate before and after shots.
Poor workmanship, tool drag mark on seat, gives a poor seal to begin with.
Same seat after cycling. Note out of round hole. Poor armature guidance, note wear, up to .015 a soft steel was used, good for magnetics, awful for seat wear.
Plastic vs metal
Plastics work well for on/off applications, not too good for PWM. Special hardened magnetic steels should be used in seat areas for best life. Also synergistic coating works well. Polymer impregnation into first few thousandths of metal
On/off 500,000 clean cycles…. 80 thousand miles
PWM 300,000,000 cycles… 80 thousand miles
Wear will result in:

  • Increased armature stroke
  • Longer solenoid response times
  • Sealing problems
  • Changes in solenoid performance
solenoid failure analysis graph

Solenoid failure analysis graph

The graph shows how excessive wear changes the solenoids ability to control output pressure. Poor material selection, and improper machining were the cause.

Understanding Solenoid terms

Posted May 3, 2010 by rostratransmission
Categories: automatic transmission components, how a solenoid works, solenoid components, solenoid failure, solenoid terms, solenoids

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Many different terms are used in describing solenoid function and operation. An understanding of these terms will give a firm knowledge of how the solenoid works and what may be wrong if the solenoid does not work.

diagram of solenoid porting

2 and 3 way, normally open and normally closed solenoids

Solenoid porting, Normally open, Normally closed, 2 way, 3 way :

A Normally Open solenoid is one which will allow fluid to pass through the solenoid orifice with no power applied to the solenoid. Energizing the solenoid will stop the fluid flow. Example: Toyota A 340 Lock up solenoid, 700-R4 lock-up solenoid.
diagram of normally on and normally off solenoids

A Normally Closed solenoid will not allow fluid to flow through the solenoid. The feed port will remain blocked until the solenoid is energized.

  • 2 Way solenoids usually vent to exhaust.
  • 3 Way solenoids control the flow directions of 3 separate circuits. Normal feed pressure enters into port #1 is directed to port #2. Port #3 will be open to exhaust. When energized, Feed prom port #1 will be directed to port # 3 and port #2 will be open to exhaust.

On/off function solenoid , Normally Open solenoid:
The Normally Open solenoid always exhausts fluid, keeping spool valve in the up position as illustrated. When energized, the solenoid closes, and causes pressure to build, thus moving the spool valve to the down position.

  • PWM solenoids will control the amount of fluid bleed off, to control the rate at which the spool moves, thus giving a smoother shift feel.
  • EPC solenoids will usually contain an internal spool valve. They use electrical energy along with hydraulic energy to balance the spools position, and thus regulate pressure.


  • Measured in ohms
  • PWM, EPC have low ohm coils 1-3 ohms.
  • On/off solenoids have higher ohms 15-25 ohms
  • Resistance increases with temperature.
  • Less electricity can pass through the coil as its resistance grows.
  • The magnetic force gets lower with temperature.
  • Solenoid speed will get slower with temperature.
  • Weak solenoid may fail when it is hot.
  • For a solenoid to operate with 12 volts, at 120 C, you should derate the activation voltage to 8 volts at room temperature.
illustration of stroke

Illustration of stroke

Stroke, Air gap:

  • Defined as the distance armature travels in solenoid before hitting the stop.
  • On/off solenoids typically have large strokes of.020/.030.
  • PWM Solenoids have smaller strokes of 010/.018.
  • Heat will reduce the solenoid stroke stroke.
  • Contamination will reduce the solenoid stroke.
  • Wear and abrasion will increase the solenoid stroke.

Example of Normally closed stroke issues;

  • Reduced stroke means ball won’t get far enough from seat, may obstruct flow and cause delays in venting the feed pressure.
  • Increase stroke means the armature has to travel farther to open the solenoid seat. This may result in excessive flow and too fast of a pressure drop in the controlled circuit.
  • Excessive stroke will lead to solenoid malfunction as the air gap gets too large to attract the armature. The solenoid may also begin to leak if the spring can not hold the ball in its seat.

The magnetic strength of the armature is measured by the force it exerts in the solenoid.
The force is typically measured on force vs. stroke curve. As the armature gets closed to stop, the force increases. Similar to the experiment of holding two magnets close together.
2 different styles used:

  • Flat face, low initial force, rapidly increases as armature reaches stop.
  • Matching faces, higher initial force, does not increase as rapidly as flat face.

These values determine what pressures the solenoid will operate at. Pressure =Force/Area… PSI = Armature force/ Orifice area. What happens if you drill the orifice open more? The solenoid will not operate at higher pressures.

  • Normally closed magnetic force force must overcome spring force holding the solenoid shut.
  • Normally open magnetic force must overcome hydraulic pressure into solenoid.
  • Different solenoids work in different area of these curves.

As always, I like to refer you to Rostra Transmission’s corporate web site for information.

Solenoid Components

Posted April 21, 2010 by rostratransmission
Categories: how a solenoid works, solenoid components, solenoids

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This post details the components used in a typical solenoid design.

Solenoid Coil and Magnet wire

Solenoid Coil and Magnet wire

Solenoid construction: Coil & Magnet wire

Strength of the solenoid comes from its coil.

Coil is made of magnet wire, nearly pure copper wire with a very thin insulation. The insulation is usually less than .001 inch thick.

Many different insulations are used in magnet wire. For a transmission solenoid ,the insulation is polyester with a nylon overcoat, rated at 200 c.

The resistance of magnet wire decreases with size.

Low ohm coils use larger wire, and have lower resistance. PWM, EPC solenoids 1-3 ohms may have wire of.020 while higher ohm 15- 20 on/off solenoids use wire at .010 dia.

Because more electricity can travel through the larger wire, the low ohm solenoids are stronger, faster, also give off more heat.

Solenoid construction: Bobbin

The bobbin holds the magnet wire in place. It also provides a location to anchor the lead wires and to provide strain relief. The bobbin will often locate and guide the moving components of the solenoid.

The bobbin will also help determine or control the solenoid stroke.

Special engineering thermal plastics are used in molding the bobbin. The molded bobbin must withstand the harsh chemical environment, heat, and provide for a durable low friction surface.

A solenoid can (or enclosure)

A solenoid can (or enclosure)

Solenoid construction: Solenoid can

The solenoid can will provide for the means of attachment to the transmission valve body.

The can will locate all of the internal components and securely hold them together.

The solenoid can details will assist in determining the solenoid stroke. Close tolerances are needed to guarantee the concentricity of internal components.

The material composition and can geometry’s will carry and direct the magnetic flux to the proper areas of the coil for optimum operational performance.

Solenoid construction: Port Assembly and Seat

Port, locates and seals solenoid in VB.

Controls the flow of fluid. NC design, ball stays shut against port, until solenoid is energized. And lets fluid flow through.

Sealing area on the back of the seat prevents debris from entering sensitive areas of the solenoid.

Durable materials are used in the seat area. The material must be impact resistant and hold up well to ATF and high temperatures.

Often on/off solenoid seats will be plastic. PWM are usually metal This will depend on the solenoid cycle rate and expected service life. Rostra solenoids are designed for a minimum of 80,000 miles operation. Most are tested to the equivalent of 120,000 miles.

Solenoid flux ring

Solenoid flux ring

Solenoid construction: Flux ring

This washer shaped ring fits snuggly into the can, and adjacent to the port.

The flux ring carries the magnetic flux from the can to the center of the solenoid.

The gap between the flux ring and the armature must be carefully controlled. Too wide, not enough flux will jump the gap. Too small, armature will not want to move.

Speed and reaction time of the solenoid can be optimized by varying the design details of the flux ring.

Proper materials must be selected that can carry the magnetic flux.

Next add the wave spring washer:

Washer absorbs dimensional changes due to heat/ cold. Keeps components properly located.

Solenoid armature

Solenoid armature

Solenoid construction: Armature

The armature will be used to hold ball in place.

It will move upwards when solenoid is energized.

The design sizes, location & tolerances are critical.

Must move freely under all conditions, and temperatures.

It is desirable to balance the mass with the magnetic saturation of the steel.

Controls the sealing of the solenoid.

The armature is directly responsible for determining solenoid stroke.

Solenoid spring

Solenoid spring

Solenoid construction: Spring

The return spring keeps the armature pressed against the ball and provides a force that will keep pressure from opening up the solenoid seal ( Zero Flow).

Non-magnetic materials desired (300 stainless, slightly magnetic often used)

Must be designed to overcome spring fade.

Great for trapping debris.

solenoid insulator

Solenoid insulator

Solenoid construction: Insulator

Protects lead wires from an electrical short to the solenoid can

Diodes are often used to provide transient spike protection. When a magnetic field collapses, a voltage spike of 700 Volts or more may be created. The diode dampens this voltage spike to prevent computer damage.

Solenoid construction: Stop

The stop directs magnetic flux into solenoid, and provides the crimping surface for the solenoid can.

It is fixed in place and does not move.

The armature is attracted to the stop, and will halt the motion of the solenoid. This is the click you hear when turning on a solenoid.

The stop sets the working gap (stroke) of the solenoid.

The stop is often vented to remove debris, or extra fluid from the solenoid internals.

Some stops are adjustable to fine tune solenoid performance, then locked in place.

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Magnetic theory, finite element analysis:

Posted April 16, 2010 by rostratransmission
Categories: how a solenoid works, magnetic theory

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Rostra utilizes special design software to optimize the magnetic circuit in each solenoid. The operation and strength of a solenoid can be predicted prior to prototyping.

A plot of the magnetic flux density

A plot of the magnetic flux density

Shown in the picture is a plot of the magnetic flux density, along with coil design parameters.

The following posts will detail the components used in a typical solenoid design. Key design features and operational characteristics will be described in detail.

With the knowledge of how and why things work, it is often easier to determine when something is not working properly.

For information on over 200 electronic parts covering more than 500 different vehicle applications

Posted April 14, 2010 by rostratransmission
Categories: how a solenoid works, solenoids

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These first few posts will focus on the design theory behind transmission solenoids:

Solenoid theory:

  • A solenoid coil creates magnetic energy.
  • The strength of the solenoid comes from its coil.
  • Coil is made of magnet wire.
  • The magnetic energy is referred to as magnetic flux.

Magnetic flux theory:

When electricity flows through a conductor, a very small, magnetic field is generated around the wire. This magnetic field is measured in terms of its’ magnetic flux (measured in a unit called Maxwells). This magnetic flux will travel in small circles around the wire.

Now if several wire are placed together, with the current flowing through them, the magnetic field will begin to encircle all the wires. When the flux measured around any cross sectional area it is then termed magnetic flux density. (The unit is Gauss) .

The closer the wires, and the more wires that are present will result in a higher magnetic flux density.

A diagram showing magnetic fields around a solenoid
Magnetic flux travels about the entire solenoid.

Magnetic theory, the solenoid:

Looking at a solenoid coil, the magnetic flux travels about the entire solenoid as shown in the picture.

The strongest magnetic flux density is in the center where the line are closest together. All of the magnetic flux is forced to flow through this area. When no more flux can pass through the center, it has become magnetically saturated.

Each component in the solenoid gets polarized with it’s own north and south pole. This creates the magnetic attraction between solenoid components.

The amount each part gets magnetized depends on the components material composition. i.e. steels get magnetized, brass, does not.

The solenoid enclosure, can, helps to direct these lines of flux to the proper areas of the solenoid.