In most automatic transmissions, you can expect to find simple switch valves that control direction of fluid and regulator valves that control or vary the amount of pressure found in hydraulic oil circuits. As technicians, we commonly focus on the pressure regulator valve and the torque converter clutch regulator valve when evaluating a valve body or pump, and that’s good! But there’s one valve that dictates how well just about everything works in the transmission – the solenoid regulator valve. The solenoid regulator valve is also referred to as an actuator feed limit valve or a modulator valve (not to be confused with vacuum modulator valves). For example, GM’s Actuator Feed Limit (AFL) is just a fancy name for a solenoid regulator valve. In this article, I’ll refer to all of these valves as solenoid regulator valves. Let’s look at how important the solenoid regulator valve is in a few transmissions.
With the 6L80/6T70/6F50, the solenoid regulator valve does as its name implies; it limits the pressure to the solenoids in the transmission to about 130 psi. By doing this, the solenoids don’t have to be massive to handle full line pressure and the engineers have an easier job matching solenoid output pressure and volume to various solenoid amperages. Since the TCM and solenoids are expecting pressure feeding the solenoids to be between about 60 PSI (min line) and 130 PSI (limit pressure), the TCM is calibrated to expect a specific solenoid pressure when operating at a specific duty cycle. Imagine how tough it would be for the TCM to predict solenoid output pressure if the solenoids were fed the full range of line pressure of 60 to 200+psi, especially considering that most transmissions do not have any pressure sensor reporting the solenoid input pressure. This is not to say that it’s never done. As an example, the 10L series transmission uses full line pressure to feed the pressure control solenoid, torque converter clutch solenoid, and some shift solenoids, but it relies on linear, or electromechanical solenoids, to control the clutch pressures.
With the 6L/6T/6F series transmissions, the TCM modulates the solenoids to control individual pressure regulator valves for line pressure, driving and holding clutches, and the torque converter clutch. As seen in figure one, a failure in the solenoid regulator oil circuit will trickle downstream and affect overall line pressure, TCC, and clutch application. Too low of solenoid regulator pressure will result in low line pressure and low clutch pressure, which can cause slipping clutches and shift flares. Too high of solenoid regulated pressure will result in high line pressure and high clutch pressure, which can cause harsh engagements and shifts, binds, and broken components. Many transmissions use the same configuration as this 6L/6T/6F example, but there are variations and the way the hydraulic circuit is constructed plays a hand in what can happen when all is not well.
Valve Construction and Operation
The solenoid regulator valve is typically a simple regulator valve that utilizes a spring on one side of the valve that forces the valve in a position to allow full line pressure into the solenoid feed oil circuit as can be seen in figure two. On the opposite end of the valve, the newly created solenoid pressure pushes on the “reaction” portion of the valve and works against the spring pressure. When solenoid pressure builds high enough, the valve moves against spring force and eventually “cuts-off” the line pressure feeding into the solenoid circuit as shown in figure three. Not all manufacturers regulate the same pressure to their solenoids. Depending on the transmission, the pressure necessary to move the valve to the cut-off point could range from around 50psi (ZF 9HP) to 130 psi (6L/6T/6F).
The solenoid regulator pressure is determined by the force of the spring pushing the valve to the “open” state and the surface area of the reaction portion of the valve, which will allow solenoid pressure to move the valve to the “closed” state. It comes down to basic Pascal’s law, where Force is equal to pressure times area (F=P*A). Increasing spring pressure will also increase solenoid pressure, because it will require more solenoid pressure working on the reaction area to overcome the increased spring pressure. Replacing the valve with one that has less surface area on the reaction portion of the valve will also increase solenoid pressure, because it will require more solenoid pressure to generate enough force across that smaller surface area to move the valve against spring pressure.
Lastly, in case you were curious, that little balance orifice feeding the reaction side of the valve is used to dampen the pressure changes so the valve won’t overreact when solenoid pressure increases or decreases. Imagine the valve example in figure three with too large of an orifice, when pressure builds up, the pressure would push on the reaction area too quickly and cause the valve to overextend to the left. Then it would drop pressure rapidly and the spring force would overextend the valve to the right. This process would repeat and cause pressure oscillations and possible noise and rapid bore wear. If the orifice were too small, valve action would be delayed because pressure must first squeeze through that orifice before working on the reaction end of the valve. The engineers design this balance orifice to allow for smooth action of the solenoid pressure regulator valve. As a reference, I measured a few spacer plates that I had on-hand and the balance orifice sizes ranged from .033” on a 4L80 and 6F50, to about .040” on a 4L60 and ZF 9hp, and to a whopping .070” on the 5R55s.
The TCM expects the solenoid pressure to be within designed limits, and as mentioned, the TCM does not typically use a pressure sensor to monitor solenoid feed. So before you think about modifying the solenoid pressure by changing spring tension, realize that you could possibly push the TCM outside of its adaptive capability. Think back to figure one — if the TCM is expecting a maximum of 130psi feeding the solenoids (and it has no way of verifying that), it will operate the solenoids as if they are being fed 130psi. But if the solenoids were actually being fed 150 psi, as the TCM modulated the solenoids, more pressure than expected would push through the solenoids. In the case of our example 6L/6T/6F transmission, that problem would result in more pressure working on the clutch regulator valves and the clutches would receive high pressure at a faster rate during a shift. This will likely result in shift harshness and the TCM would work hard to adapt it out.
Failures, Diagnostics, and Solutions
I had a conversation about solenoid regulator failures with David and John from TransGo, and they mentioned that some solenoid regulator valves shouldn’t really be called regulator valves, because a true regulator valve will adjust pressure when it’s too high or too low. Some valves, like the solenoid regulator valve found on the 6L/6T/6F series transmission, shown in figure four, allow line pressure to pass through until it reaches a specific point where it will then cut-off its supply, effectively “regulating” the pressure. The problem arises when the valve bore wears or there’s cross-leakage which allows line pressure to sneak into the solenoid regulator oil circuit and build above 130psi. The valve continues to move over, but there is no exhaust circuit to allow pressure to release. Some solenoid regulator circuits, like the 4L80 shown in figure five, incorporate an exhaust circuit to regulate fluid pressure to a lower pressure if the solenoid pressure spikes high. Since the 6L/6T/6F series transmission doesn’t have a provision to regulate the pressure down, companies like TransGo have designed a pressure relief for the solenoid pressure circuit as shown in figure six. The pressure relief is not necessary on the second generation 6T35, 6T40, and 6T70 because GM has identified the troublesome design and added an exhaust circuit to the solenoid regulator.
With the huge responsibility of the solenoid regulator valve circuit, some techs feel it’s vital to verify valve, bore, and oil circuit integrity. This would include visually inspecting the valve to make sure the aluminum anodizing isn’t flaking or wearing, or any steel valves aren’t scored. With a clean valve body, it’s important to make sure the solenoid regulator valve moves freely in its bore. Some techs use a vacuum test station to check for excessive bore wear and reference vacuum test guides from suppliers like Sonnax. The vacuum test guides identify where to test the solenoid regulator valve, which will likely be at its reaction end.
A calibrated vacuum test station will pull 5” of vacuum through a 0.040” orifice and pull 25” of vacuum with a perfect seal (sea-level measurements). The vacuum test puts a value to the extent of the wear found in the valve body bore (figure seven). Wiggle tests, picking, and visual inspections are good, but they are also subject to interpretation. A vacuum test will provide repeatable results.
What’s a good vacuum test result? Well, that’s a topic for debate, because it all depends on the valve size, whether it is sealing one valve land or two, hydraulic circuit design, and what the oil circuit is feeding. The answers will vary greatly depending on who you ask. When discussing vacuum testing with Bob Warnke from Sonnax, he mentioned that it’s common to find low vacuum readings on worn valve body bores, but also measuring a very high vacuum reading, like 24” or 25” Hg, isn’t always a good thing. Valves do need clearance for oil and too high of vacuum could indicate a valve that’s too tight and might stick, but generally, the smaller the valve, the higher the vacuum results will be. He stated it’s not uncommon to pull 24” Hg on some small valves found on certain Toyota units. He also stated that solenoid regulator pressure is super critical on certain transmissions, such as the Toyota U660, 09G, TF81 to name a few. Solenoid regulator pressures that are off by as little as 3psi will start to exhibit changes in shift qualities.
Whether you’re new to vacuum testing or not, consider starting a log of vacuum test results from each overhaul, and with it, jot down a note describing the transmission’s symptoms and failures. If you need a place to start, opening the instructions for any particular valve at www.sonnax.com will provide the tech with a suggested vacuum test specification. The suggested specs aren’t found on the vacuum test guides, but they are included with the information found with the individual valves.
For another approach to handling solenoid regulator valve oil circuit issues, I spoke with Dean Mason from Superior Transmission Products, and he shared his philosophy and research on hydraulic designs, issues, and remedies relating to the solenoid regulator valve and transmission hydraulics in general. Dean mentioned that there are over twenty factors he considers when designing a valve and when reviewing Superior’s product catalog, you’ll see many of these valves carry the NO~STICK™ title. He describes one factor as “corrected bore clearance,” where he ensures that his valve design incorporates a “boundary layer” of fluid between the valve and bore. The boundary layer allows for valve lubrication and reduces bore wear. While measuring the diameter of a 5R55s stock valve and comparing it to Superior’s replacement valve, you will see that it’s about .0003” smaller to allow for this boundary layer of fluid as can be seen on figures eight and nine.
Another feature of the NO~STICK™ valves are the annular grooves across the length of the valve’s land, which reduces bore contact. He states these grooves also reduce valve drag and provide an area to collect lube oil and capture contaminates. As an example, while researching the cause for lockup and shifting issues, and erratic engagements on the 5R55n/ s/w, he discovered the necessity for stabilizing pressure regulation and reducing valve cycling distance and speed.
If the solenoid regulator valve and/or bore needs addressing, there are a few aftermarket options available.
- In some cases, you can use a “drop-in” valve that includes a valve that has been engineered to utilize the previously un-touched areas of a valve body bore or that has been re-engineered to prevent sticking. This process requires no reaming (figure ten).
- Another option involves placing a sleeve and a new re-engineered valve and spring in the worn bore. The new sleeve seals against the worn bore and the smaller valve and spring are engineered to produce original solenoid pressures. This is also considered a “drop-in” replacement, but it includes modified valves and springs. But, like example one, this option requires no reaming (figure eleven).
- There are options that involve reaming the bore and placing a sleeve with higher quality aluminum and a new valve. This option restores original operation, but replaces aluminum found in the casting with a high-grade aluminum that won’t wear out as quickly. A badly worn bore might make this option necessary (figure twelve).
In conclusion, don’t forget to closely evaluate the solenoid regulator circuit? It’s just too important and just about everything in the transmission is relying on it! With most popular transmissions, there are variety of options when dealing with worn valve bores ranging from drop in valves to reaming solutions. If there’s no option, then it’s time to source out a new or reman valve body.