When I was younger, a lot younger, one of the first college classes I attended was a physics class. I learned tons about our world and why things work as they do. One of the areas of emphasis was in regard to a 17th century French physicist by the name of “Blaise Pascal”. Without Blaise Pascal and his discoveries, none of us would have the jobs that we do. Pascal is credited with what is known today as “Pascal’s Law.”
Pascal’s law states that “pressure that is applied to a confined liquid transmits its force equally in all directions, even at right angles.” So, if you place fluid under force in a piston chamber, it will apply the force equally across the base of the piston and throughout the chamber. In other words, it is the foundation of hydraulic systems in use in numerous industries around the world including the automatic transmission.
A couple of other hydraulic fundamentals to go along with Pascal’s law include:
- Liquids, for all practical purposes, are not compressible, so they can be used to transmit motion and force.
- If you force liquid through a restriction such as an orifice or valve, there will be a difference in pressure from the inlet side of the orifice/valve to the outlet side of the orifice/valve as long as flow is present. This fundamental is why orifices and valves are used to help control the rate of clutch apply in a transmission. Once the flow stops across the orifice/valve, the pressure on both sides of the orifice/valve will equalize, and Pascal’s law will again be in effect.
- Pumps DO NOT create pressure. Pressure is created by a resistance to flow. The pump creates flow, and the restriction to that flow creates the pressure. Just think of the nozzle on the end of your garden hose. The pump provides the flow, and the pressure regulator valve provides the resistance to that flow resulting in hydraulic pressure being developed in your transmission.
This past week I was working with a major remanufacturer on several issues they were having. One of the issues had to do with failed dyno tests due to line pressure issues on 6L80s. That led me to study in depth how the PR system in a 6L works, and it was enlightening enough that I thought I would share some of my “enlightenment” with you.
As all of you know, the 6L series of transmission have plenty of pump and pressure regulator valve issues. The 6L pumps are variable displacement type of units that use the pressure regulator valve to control the position of the slide, controlling the pump volume, which ultimately controls the system pressure.
The valve utilizes four lands. Land one is located furthest away from the spring and is used to control the position of the regulator valve based on balance oil supplied to that land. In addition, land one helps to keep the valve from rocking in the bore, which can lead to other valve lands being damaged. Land three is where most of the action is. Land three controls the pressure into the slide’s decrease chamber, which controls the slide’s position and, ultimately, the system pressure.
Being at a build center, an endless supply of pumps and valves were available to work with. The first thing that popped out at me is that vacuum testing the valve and bore only tests land one, and not land three. Land three is not vacuum tested because there is no feasible way to actually test it due to the pump channeling design. This leaves you with a visual inspection, light testing, or a wiggle test for that portion of the valve and bore.
What this means is that the valve can pass a vacuum test but not perform as needed in the transmission. With access to a lot of pumps, we were able to select any pump we wanted for our testing. We tried to select the most worn pump we could based on a visual inspection of the bores. We used the same pump for all of the testing and simply changed out the pressure regulator valve.
To highlight this, let’s look at the following:
Figure 1: We used three separate worn regulator valves for our testing. Valve A has a worn land one and land three but the other lands are in good shape. Valve B has lands one and three worn. Valve C has land one worn.
Figure 2: The valve with both lands one and three worn (Valve B) was selected for testing. As you can see it failed the vacuum test. This valve will cause pressure issues when installed in the transmission.
Figure 3: The valve with land 1 worn (Valve C) was selected. As you can see it to failed the vacuum test, but the transmission pressures were not an issue like it was with valve B as previously outlined.
Figure 4: The valve with worn land 3 (Valve A) was selected. In this instance, Land one is in good shape, but land three is worn. The vacuum test passed, but the valve will cause pressure issues when installed in the transmission.
A lot of shops ask what I think of the drop-in PR valves that are available. If you look at the valves available, they have some things in common. For the most part, the valves have extended the lengths of land one, and some also extend the length of land three. (Figure 5) Since we did not find one pump that had bore issues in all the countless pumps we checked, I would not have any issues installing a drop-in or a good stock regulator valve. In either case, we checked the drop-in valves against a good stock regulator valve. The vacuum tests were good, and the pump operated as expected. The advantage of the drop-in valve is longevity, as the increased land length will help to stabilize the valve for long-term use.
No matter which direction you choose to go, always inspect the land one and land three bores for damage. The land three bore gets beat up from the valve not being held in the proper position in the bore, so that is the one that you will typically see an issue with. (Figure 6).
Well, there you have it. 6L80/90 pressure regulators are high-wear items and it is something you need to pay attention to. In addition, cleanliness and proper pre-lubrication of the valve is important if you want to avoid issues. Until next time remember, “Hard work beats talent when talent does not work hard.”
