Understanding Solenoid terms

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.

Explore posts in the same categories: automatic transmission components, how a solenoid works, solenoid components, solenoid failure, solenoid terms, solenoids

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