Solenoids – Flushing & Cleaning

Over the years I have heard several different philosophies when it comes to flushing and cleaning solenoids. Most hydraulic solenoid test machines have either flush mode or an external cleaning system for solenoids. It is easy to think of it like you are putting the solenoid in a dishwasher of sorts and using hot ATF or solvent to blast away the dirt and debris. During my time at Zoom Technology, I visited several customers who would put solenoids in their Answermatic Solx machine and run them in flush mode for thirty minutes or more for each solenoid. Initially you might see a black puff of dirt come out, but after a few cycles the fluid remained “clean” coming out of the solenoid, but the thought was if some was good, more was better. In this article I want to take an honest look at flushing and cleaning of solenoids.

First, where does the dirt and debris come from that we are trying to flush out. When the transmission was initially assembled everything was clean and new, and all of the parts, the fluid, and the filter were clean and assembled in a clean environment. The “dirt” comes from normal wear of the parts inside the transmission. The obvious place would be from bands and clutches. If these slip or burn up, a lot of that material will come off will circulate throughout the unit. A second place would be any parts in the rotating assembly, bearings, bushings, and steels. Anything that is moving will exhibit wear over time and wear is basically material being removed that will again, circulate inside the unit. A third place would be from the valve body. We are all familiar with valve body wear and again as valves and valve bores wear, this is material being removed and circulating inside the unit.

Outside of a catastrophic transmission failure, all of this debris is basically from normal wear of the transmission. Much of it will be caught by the filter or settle in the pan. We can break this material down into three types. The first is non-metallic (clutch and band material). The second is non-magnetic metallic material (aluminum and brass). The last is magnetic metallic material (and wear from steel parts such as bearings and steels). Think about a typical GM transmission pan with the donut magnet in one corner. When you clean the pan the first two items that are not caught in the filter will tend to coat the pan or gather to areas that does not have constant fluid flow during operation. The third item (the steel) collects on the magnet.

At its very basic form, what is a solenoid? Think back to grade school science class when you took a nail, wrapped a coil of wire around it, and applied a current to it. You made an electromagnet. The current in the wire wrapped around the nail magnetized it and you could pick up steel. The more wraps of wire and the higher the current, the stronger the magnet. The nail had to be steel as a piece of aluminum or brass would not do anything. A solenoid is basically the same thing. We have a spool of wire and a steel (or iron) core. When we energize the solenoid, rather than pick things up, we use this magnetic force to


Figure 1: An Electromagnet from Wire and a Nail

move a ball against a seat or move a valve as part of a regulating solenoid. Increasing current in the case of a regulating solenoid increases the force and allows us to control pressure with a high degree of precision. Think back to science class one more time when we made the electromagnet with the coil of wire and a nail. When you turned the current off the nail would lose most of its magnetism. It would, however, retain a very weak magnetic field even when the current was off. If we were lucky it would maybe pick up a staple, but this concept of becoming weakly magnetized is important to remember for the discussion on flushing and cleaning.

Figure 2: Shift Solenoids Assembly

I want to breakdown a simple shift solenoid. In Figure 2, you can see each part of a typical shift solenoid assembly. The small pin makes contact with the

ball. This pin is held towards the coil by a spring and moves to push the ball against the seat when the solenoid is energized and hence blocks flow. All of the fluid flow happens inside the plastic end that houses the ball and seat. Outside of the sleeve shown next to the coil, all the other items between the plastic seat and the coil are made of steel. The outside housing that holds this all together is not shown.

I want to identify three things when it comes to flushing and cleaning this solenoid.

1 – Most solenoid machines run a max temperature of around 160F. This is about the safe limit for an operator. In the vehicle, the transmission will generally run at 200F or more. While we are flushing with “hot” fluid, it is actually slightly colder than the normal operating temperature of the solenoid.

2 – With most solenoid machines I am able to vary the input pressure to the solenoid during a flush on the machine. This gives me the ability to set it higher than it normally sees in operation and the higher pressure can help to flush away accumulated dirt and debris in the normal paths of flow.

3 – If we look at the flow path of the solenoid, we can see with this one that we are only able to flush around the ball and seat area. The pin, spring, or the core inside of the coil is not in a flow path that would provide any real flushing. No matter what we do, these parts cannot be flushed and anything that has accumulated there, will remain.

The takeaway from this is that we have a limit to the flushing and cleaning ability for this solenoid. An initial burst of high-pressure fluid will clear out what it can in the ball and seat area, but as we can see anything further is not going to provide any additional flushing.

Figure 3: Regulating Solenoid Assembly

Let’s breakdown the regulating solenoid like we did the shift solenoid. As you can see in Figure 3, there are similarities to the shift solenoid. The key difference is that instead of a ball and seat, we have a pressure regulating valve that is controlled by current. As the current is varied to the solenoid coil, the diaphragm moves back and forth which causes the pressure regulating valve to move. There are also three distinct ports on this solenoid, the inlet, the outlet, and the exhaust port. Inside this solenoid there are also two distinct flow paths, one between inlet and outlet and one between inlet and exhaust. There are three things I’d like to identify when it comes to flushing and cleaning this solenoid.

1 – There are screens on all ports of the solenoid. Pressure regulating solenoids are generally regulate pressure, not flow. Flow is low through these solenoids and the screens will act as a flow limiter if you try to force fluid through with higher pressure. Unlike the shift solenoids we cannot force a large amount of fluid through to flush this solenoid.

2 – Take a look at the debris inside this solenoid. This was taken from a vehicle that had come in for a rebuild. It was a normal service, no catastrophic failure. The metal core that is part of the diaphragm is steel and (going back to our science class again) is going to be magnetic when current is applied and hold a weak magnetic field when off. Look at the debris stuck inside attracted by magnetism. Outside of cutting this apart, there no way you can realistically “flush” this debris out of this solenoid. Also, this debris is more or less held in place by the magnetic field anytime the solenoid is energized. Even with the solenoid off we will still have a weak magnetic field acting upon it.

3 – If we look at the flow paths of this solenoid, everything is happening on the end opposite as the coil. We have movement of the core inside of the coil up against the diaphragm, but there is not a flow path that would be able to provide any real flushing. This area of the solenoid cannot be flushed.

The takeaway from this is that flushing is probably even less effective than the shift solenoid. Most areas where debris will tend to collect cannot be reached or cleaned. We are also fighting against the magnetism and low flow through the solenoid even if we use a higher pressure to flush the solenoid.

I want to comment on degaussing. You can demagnetize (or degauss) steel by applying an AC current to either the wire wrapped around it or as a separate field. There have been several degaussing tools sold over the years that look like a circular wand and the concept was you used these to remove the magnetism on the internal steel components caused by the normal operation. Once you did this, any magnetic material inside would flush right out as we removed the magnetic field holding it in place. There are two things to keep in mind in regards to this.

1 – Magnetism and electromagnetics is complex. The principles and design are complicated, and a one size fits all tool that can degauss any solenoid regardless of how much or how little steel it has in it is a lofty task.

2 – Even if I have a degaussing tool that does the trick, the only way I can truly flush the solenoid is to move it from closed to open position. As soon as I apply a current to the solenoid to open or close it, I have reapplied the magnetic field and we are back to where we started, negating any degaussing effect.

Should we conclude that flushing and cleaning of solenoids are not worth the effort? Should we be wary of reusing them because there really is not a way to completely clean them? The fact is solenoids are designed to operate with some level of debris that accumulates over time. Between the transmission filter and the screens on the solenoid, anything inside is very fine and generally is going to collect in areas not critical to its function. What we should be looking for is that we do have proper solenoid function. Has this solenoid been compromised in a way that it no longer is able to function inside the transmission as intended? A ball and seat can wear or crack, a regulating solenoid can wear and either stick or not be able to repeatedly regulate pressure.

The best advice when it comes to flushing and cleaning of solenoids would be as follows:

1 – Clean the outside of the solenoid. As you can see in Figure 4, this regulating solenoid attracts debris on the connector end. We certainly do not want to introduce this into our freshly rebuild transmission or into our clean solenoid testing machine when we perform a solenoid test. This is one of the reasons why Hydra-Test offers a separate ultrasonic cleaner so that solenoids can be pre-cleaned before placing them in the HT Sol.

Figure 4: Dirty Regulating Solenoid

2 – Perform a flush as best we can. We will push some debris out, but letting it sit in a flush mode for more than a few cycles is not going to give much of an added benefit. The Hydra-Test HT Sol allows you to quickly test at a high pressure the first couple of cycles before dropping down to your test pressure and removes the need for a separate flush cycle.

Figure 5: Ultrasonic Cleaner option on the HT Sol

3 – Test the solenoid against a know good standard. Does it function properly, and does it repeat each time we test it. This will identify any wear issues inside the solenoid and allow us to quickly determine if it can be reused or if it is time to recycle it. Also, if this solenoid is adjustable or needs calibration, you can perform this and bring the solenoid back into its factory specification. This is the reason why the HT Sol provides a graphical comparison so you can quickly determine if the solenoid you are testing matches a known good graph or not. Since it can be set to continuously cycle, you can bring an adjustable solenoid back into specification while it is running and watching for the graphs to overlay as it cycles.

This approach should give the best use of both your time and your equipment when working with reclaimed solenoids.


Figure 6: HT Sol Graphical Comparison




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