Other Articles - August - 2020

Solenoid Test Methods

The variety and shape of solenoids used in modern transmissions change with each new model. On any rebuild, solenoids are a suspect part and should either be tested and verified for proper operation or replaced. Some shops opt to just replace all the solenoids on the valve body. On some models, that makes sense as the replacement solenoids are fairly inexpensive. For other models, the cost of the solenoids makes it worthwhile to test and only replace the ones that are worn or faulty.

When you test a solenoid, a hydraulic solenoid testing machine with specific adapters for each solenoid is by far the best and most accurate way to test. Most of these machines will have documentation to guide you through a testing process. For the purpose of this article, I am not going to go into detail on machines. Rather I’ll focus on some of the basic terminologies, how you can identify solenoid types, and what kinds of test methods you would use to accurately test the different solenoids.

Let’s start with some characteristics and terminology used when talking about solenoids.

SOLENOID RESISTANCE

One of the key measurements of a solenoid is to use a multimeter to measure the resistance of the coil. This is a quick and easy check where you can find if you have a short circuit, open circuit, or the possibility of a partially shorted solenoid coil. An important thing to remember is that when you measure the resistance, you are measuring the resistance of the coil, which is a very, very long piece of wire. Unlike a true resistor, where the resistance stays constant over a wide temperature range, the resistance of a solenoid coil will measurably change based on temperature (room temperature compared to 200°, as an example).

NORMALLY OPEN OR NORMALLY CLOSED

This refers to the hydraulic state of the solenoid when it is off with no electricity applied. A normally open solenoid allows oil to flow from the inlet to the outlet. A normally closed solenoid would block oil between the input and output. Applying power to the solenoid will switch it to the opposite state.

ELECTRICAL SOLENOID ACTIVATION

This refers to how the solenoid is electrically driven when it is energized. The simplest method is to energize it with ignition voltage and allow it to draw full current. The next most common method is to use pulse width modulation (PWM). PWM is pulsing the solenoid on and off many times a second, and the amount of on-time vs. the amount of off-time varies to change the amount of average current the solenoid will draw. There is one other method called “peak and hold.” I’ll detail that in another article.

Generally, solenoids can be categorized into two types. The first is an on/off where the solenoid either allows full pressure at the outlet or blocks pressure to the outlet. The second is a pressure regulating solenoid where the output pressure from the solenoid is regulated to the desired valve by the varying the current draw of the solenoid.

ON/OFF SOLENOIDS

  • On/off solenoids are generally smaller in construction.
  • On/off solenoids have higher resistance. If you measure the resistance, it is usually greater than 10 ohms.
  • Continuous current through these solenoids is around 0.4-0.7 Amps. They are designed to handle that amount of current indefinitely.
  • These solenoids were the primary used as shift and torque converter clutch solenoids in early applications. As transmissions and shift strategies have become more complex, they have been replaced by regulating solenoids.
  • These solenoids cannot regulate pressure and will apply full pressure to the outlet.

TESTING ON/OFF SOLENOIDS

The testing of on/off solenoids is straightforward. Pressure should be applied to the inlet, and if it is normally closed, you should not see pressure at the outlet. Once energized, you should see pressure at the outlet. For a normally open solenoid, the opposite is true. Pressure applied at the inlet should be seen at the outlet. When it is energized, there should be no pressure at the outlet.

One of the most important aspects of testing an on/off solenoid is that there is no leakage through the solenoid when it is in the closed condition. While the solenoid may appear off, wear or cracks internally could allow a small amount of fluid to leak past. If you see leakage, the solenoid should not be reused.

REGULATING SOLENOIDS

  • These are larger in constructing and incorporate an internal regulating valve.
  • These have various names such as Electronic Pressure Control (EPC), Linear Solenoids, or Trim Solenoids.
  • These have lower resistance. If you measure the resistance, it is generally less than 10 ohms.
  • Continuous current through these solenoids is varied and can range from 0 to just over 1 amp.
  • These solenoids are the most common type used in transmissions today and allow for the smooth application of clutch to clutch shifting as well as the smooth control of the torque converter clutch.
  • These solenoids regulate pressure at the outlet. The pressure is proportional to current and is repeatable.

TESTING REGULATING SOLENOIDS

The testing of regulating solenoids is a bit more complex. The pressure is applied to the inlet, and outlet pressure should be observed as the current is varied to the solenoid. Pressure should either increase or decrease with current depending upon if the solenoid is normally open or normally closed. These solenoids will have a maximum outlet pressure. Applying excessive inlet pressure is not needed and could yield incorrect test results if the solenoid is over pressurized.

The most important aspect when testing a regulating solenoid is that pressure changes smoothly as the current is varied and that the outlet pressure is always the same at a given current. Solenoids that are worn will show a difference in outlet pressure at a given current. These solenoids can also stick and have a dead spot where even though the current changes, the outlet pressure remains the same. Lastly, if the outlet pressure is not steady and fluctuates rapidly at a given current, then this is an indication that the solenoid may be having trouble regulating.

The best way to visually see how a regulating solenoid is performing is to sweep the current and graph this against pressure. A sweep from zero to max (1-1.3 amps) and back to zero should reveal a smooth graph where pressure changes as the current increases and decreases. One key item to note is that the pressure at a given current value is nearly the same as you increase and decrease current and then come back to the same current valve. This difference is called hysteresis. There will be some difference in this pressure value (or hysteresis), but it should be minimal. As regulating solenoids wear, the hysteresis increases noticeably.

One last item that is important about regulating solenoids is that some have an adjustment screw. That allows adjustment of the pressure vs. current relationship, i.e., you will measure more or less pressure for all current values as you adjust the screw in or out. This adjustment screw should not be set to an arbitrary setting, but rather set to match a known “calibrated” solenoid. The TCM in the vehicle has an expectation of what this pressure vs. current relationship should be, and you can easily adjust this outside its expected range. When adjusted outside the expected range, the solenoid technically functions properly, but the pressure vs. current is now beyond what the TCM expects, and this can cause shifting problems and fault codes.

There’s a lot to digest and understand when it comes to solenoid operation and testing. We’ll take it a little at a time and cover more in future articles.


Garrett Herning is the Director of Technical Support and Sales for Hydra-Test USA. He is an electrical and mechanical engineer with a background in automotive testing and test equipment design with companies such as Axil-line, Zoom Technology, and Power Test. He resides just outside of Milwaukee, WI with his wife and two children, ages 11 and 2.