The variety and shape of solenoids used in modern transmissions changes 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 cost effective. For other models, the cost of either OEM or aftermarket replacement solenoids do make it worthwhile to test and only replace the ones that are worn or faulty.
Figure 1: Hydra-Test HT Sol Solenoid Test Machine
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. The Hydra-Test HT Sol has become the industry standard for solenoid testing with a range of adapters that can fit most any solenoid.
The machine comes with full documentation and can guide you through the testing process. For the purpose of this article, I am not going to go into detail on the HT Sol. I want to focus on the basic solenoid and testing terminology such as how you can identify solenoid types, and what kinds of a test methods you would use to accurately test the different solenoids. A good understanding of this will give you a good understanding on what you are trying to achieve when you are testing.
Let us 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 over temperature (for example between room temperature and 200F).
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 would allow 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. Sometimes these solenoids can be almost identical and will fit in the same hole on a valve body. You can imagine the shift issues that can be caused if this happens so be sure to double check.