There are some common processes used regardless if you are a shop rebuilding a transmission on the bench or are a large reman with assembly lines for high volume production of units. In either case, there is a process of disassembly and determining what parts are good and can be reused, and which ones are bad and must be replaced. Many hard parts that can simply be inspected for wear, cracks, or other damage and determined very quickly if they are useable or not. Many soft parts are automatically replaced as they are not reusable. Between these two types of parts are the ones that require some level of testing to determine if they can be reused or if they need to be replaced. Solenoids are a prime example of such a part since a hydraulic test is required to determine if they are still functioning properly.
When we talk about testing, we not only need to understand why we are testing it, but more importantly, the how do we test it. The “why” is fairly straight forward, as we want to determine if it is still functioning properly and that we can reuse it. The how to test it might not be as clear, and there are multiple methods of how to test. The basis of testing involves giving the unit under test some type of a stimulus and measuring the response. Then we examine the response and see if it is within an acceptable range or not to determine if it passes or fails. The proper stimulus and what the acceptable range are is things might get tricky.
So how do we test a solenoid? The obvious answer might be to apply a hydraulic pressure, apply an electrical current, and observe the output pressure. In the case of a variable pressure solenoid, different current points should produce different output pressures and these pressure points should either increase or decrease with current depending upon what type of solenoid it is, right? Well, that answer is partially correct.
I was working with a customer once and we were testing a Ford 5R55E EPC solenoid. The controller we had was able to vary the current to the solenoid and you would set it to a value and then check on a pressure gauge the output pressure of the solenoid. I started with it off, then set it at 0.3 amps, then 0.7 amps, then 1.0 amps. The pressure decreased each time. I came back to 0.7, then 0.3, and finally back to 0, and the pressure increased with each test point. At each test point I stopped for about 20 seconds so we could check and read the pressure and talk about it. The pressure readings matched what we expected. So, we could conclude that this solenoid was fine, operating properly, and ready to be put onto the valve body, right? Wrong!
This same machine also had an option where we could ramp the current to the solenoid from 0 to a maximum and back to zero over time and then graph the response on the computer screen. We set the max current to 1.0 amp and our total time for the ramp up and down to about 20 seconds and ran the test. When we did this, we saw a vastly different picture. The solenoid would drop pressure as expected as current increased, however as the current decreased, the solenoid would stick for about 5 seconds at minimum pressure and then snap to about half pressure as we continued decreasing current. This repeated each time we cycled the solenoid as we ran it over and over again. From the graph it was obvious we had a problem, which we could only see as we continually increased or decreased the current, rather than just holding it steady at certain current points.
This gets to the very root of the how do we test it issue. We also need to understand why one method indicated that everything was fine and the solenoid was working properly, while the other showed us a problem that would cause drivability issues, would definitely give codes, and possibly damage to the transmission under certain conditions as our line pressure would be much lower than expected in the unit.
Let us think about how this solenoid functions in the vehicle during normal operation. We want to test in the same way and try to mimic the real-world operating conditions as closely as possible. If we think about an EPC solenoid, especially one that is controlling line pressure as in this case, how does it operate as the vehicle as it goes down the road? Does it sit at a constant current or is the current constantly varying as the driver changes throttle position, if we are shifting gears, or if we apply the brakes? If you answered that it is constantly varying, you are correct. There may be times when it will be constant (say a long stretch of highway where you keeping a constant speed and throttle), but each time we change an input, it ramps the current up or down over time, just like when we ramped the current and observed it on the graph. If we only look at a few points where the current is constant, we completely miss the solenoid momentarily sticking before it snaps into place due to wear, internal contamination, or damage.
The takeaway from this is that a tool (in this case a solenoid test machine with a variable current and graphing capability) is only as good as the operator and how they are using it. It is important to think about and understand how you are testing. Proper testing yields good results and will save you both time and money, as well as the frustration of chasing a bad part that tested “good” because of incomplete testing.
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.