The heart of the matter

March 22, 2016
The quality of the filter, pump and related seals are critical to the proper operation of the transmission and its length of life.

It is no secret that having a good working heart is a necessary element towards enjoying quality of life. The same is true for an automatic transmission. The quality of the filter, pump and related seals are critical to the proper operation of the transmission and its length of life. Its veins reach out to apply clutches and brakes as well as lubrication and cooling circuits. A failure with the pressure circuit is no doubt an undesirable event.

An interesting aspect to pumps is the design manufacturers choose to use in their transmission. This couldn’t be anymore evident than with GM and Ford in their joint venture to build a six speed front wheel drive transmission. As mentioned in a previous Powertrain Pro article called “The Twists and Turns in Keeping Up with the Changes,” GM went with a crescent style pump while Ford went with a gerotor pump.  GM has a long history of using both a crescent style pump and a variable displacement vane pump (VDVP). Ford has had a history of using crescent style pumps too, but gerotor style pumps as well. The article mentioned that gerotor style pumps are less expensive to manufacture than crescent style pumps and can package nicely in a small diameter space. The same is true with “compact vane type pumps,” which have become widely used in a variety of applications.

The positive displacement rotary vane pump has history as far back as the late 1800s. The variable displacement vane pump showed up about a half-century later. The first time I ever saw a rotary vane motor was in my impact wrench back in the early 70s.

Today we are seeing this style of pump being referred to as a “twin-pipe-vane pump” or “twin-stroke vane pump.” General Motors uses this style pump in their new 8L90 transmission (figure 1), which they call a “Binary Displacement Vane Type Pump.” 

Figure 1
Figure 2

There are several advantages for using this style pump. Its compact size allows it to be positioned in an “off-axis” location. An “on-axis” style pump is typically located in the front of the transmission. The torque converter can then be used as an input drive member of the pump. This style set-up however, extends the length of the transmission.

An off-axis pump provides the manufacturer with the freedom to locate the pump in such a way so as to save on the length of the transmission. With the 8L90, this pump is an integral part of the valve body inside the main case. When the pan is removed, the main filter inserts directly into this pump (figure 2). The pump is chain driven (figure 3) by the torque converter which lugs to the pump drive gear called a Drive Sprocket (figure 4). When the valve body is lifted off of the transmission, the pump can be unbolted and removed from the housing (figure 5). Immediately you will discover three critical seals; a beaded pump gasket (pump cover to the valve body pump housing), a fluid passage sleeve (sits in the pump suction [bypass] circuit), and the pump seal (between the pump pressure plate and valve body pump housing).

Figure 3
Figure 4
Figure 5

Removing the first of two retaining rings will allow the pressure plate to slide off of the pump’s drive shaft (figure 6).  Two locating pins are then removed (figure 7) from the pump cover. Using a pencil magnet or equivalent, each of the pump vanes can be removed followed by the pump cam (figures 8 and 9).

Figure 6
Figure 7

When the second retaining ring is removed (figure 10), the pump drive gear and rotor can be separated from the pump cover (figure 11). What will come into view is yet another critical seal located between the pump drive shaft bearing and the pump’s driven sprocket (figure 12).

The pump drive shaft is locked to the driven sprocket by a sliding tension clip. Locate the clip’s tab seen below the seal in figure 12. This tab needs to be lifted up and out of the slot in the driven gear and pulled forward (figure 13).  This will unlock the drive shaft from the driven gear to gain access to the pump cover seal for servicing (figure 14).

This is the heart of the transmission. The filter and pump assembly with all of its seals need to be right. They are critical components and if compromised, can result in costly repairs. Using sub-standard parts here might save some pennies upfront but could cost many dollars later.

Figure 8
Figure 9
Figure 10

Besides having increased flexibility in packaging locations for this style pump, it also aids in fuel economy. This has certainly become a major issue with manufacturers due to Federal Regulations concerning Corporate Average Fuel Economy (CAFE) ratings they have to meet. The mandatory overall fleet in miles per gallon to be met for 2016 is 34.1. The Obama administration is now pushing the auto industry to be at 54.5 mpg by 2025.

With this news, every aspect of the car is being looked at for fuel economy. And this style of pump is just one such example. The design of the pump provides two discharge ports that it can alternately output through.  Or, it can output through only one discharge port. When it alternately outputs through both discharge ports, it is in the full displacement/volumetric output of the pump. When it only outputs through one discharge port, it is in a partial displacement/volumetric output of the pump.

This means the pump can provide full or partial volumetric output on demand. Unlike the on-axis variable displacement vane pump, this binary displacement vane pump can respond faster to partial or full volumetric output. During non-peak demands and steady state operations, the pump can be in the partial mode at which time there is less load on the engine which equates to fuel economy. The pump can quickly adjust to full volumetric output when torque increase demands it so.

Figure 11
Figure 12

The variable displacement vane pump in comparison was not as fast to adjust as it had a slide to move. Additionally, the very nature of the slide also allowed for internal leakage affecting its efficiency. And as previously mentioned, the binary displacement vane pump provides a much faster response between partial and full volumetric output as its control is not within the pump itself.

The binary displacement vane pump’s two discharge ports in the 8L90 are referred to as “Line” and “Secondary Line”.  Both are routed to their own dedicated locations at the pressure regulator valve. On their way to the pressure regulator valve, these two circuits connect together through a shuttle ball (figure 15). When the pressure in the secondary line circuit is low, the line circuit pushes the ball against its seat closing this connection. But when the pressure in the secondary line becomes greater than the line circuit, the ball unseats and the two circuits are joined together. This is probably why GM calls this ball the number 8 ball as it is in an unfavorable position. Talk about being pushed around!

So how does the line and secondary line circuit go from being two separate circuits to the secondary line supplying pressure to the main line circuit? Glad you asked as this equates to the partial and full volumetric output operation of the pump.

Figure 13
Figure 14

The “Line” circuit is regulated at the pressure regulator valve in a typical fashion and is the main pressure supply to the transmission. It also supplies pressure to the converter feed circuit. A line pressure blow off ball can be found in the main line pressure circuit as well.

The “Secondary Line” is connected to a pump suction circuit at the pressure regulator valve called the Bypass circuit. As torque increases, variable force signal oil from the pressure control solenoid moves the pressure regulator valve to close off the bypass circuit. As the bypass circuit (pump suction) is being closed off from the secondary line circuit, the pressure in the secondary line circuit increases. Eventually, this pressure exceeds main line pressure pushing the number 8 ball open joining the two circuits together.

Understanding the operational efficiency of this style of pump, stepping into the throttle is not the only way for it to go from a partial to a full volumetric output. Placing the transmission into gear is one such example. When a gear selection is made, a significant demand is placed on the pump to supply the necessary volume of fluid to load a clutch assembly. It will be the transmission range sensor’s job to inform the computer of the gear engagement to initiate the pump’s full volumetric output. As quickly as a command can be made for full pump output, is as quickly as a command can be made to produce a partial output. Throughout a drive cycle, this pump is constantly running between full and partial output. It meets the needs of the transmission while increasing fuel economy.

Figure 15

With the pressure regulator valve controlling the pump’s volumetric output, this adds to the heart of the matter. Due to the continuous demands placed on the pump to alternate between partial and full volume output, no doubt that pressure regulator valve bore wear can be a concern; especially if the wear takes place in the bypass and secondary line circuit area. When full volumetric output is called for, it will not be there. If the number 8 plastic check ball becomes compromised, it could allow main line to dump into the secondary line circuit and get sucked up by the pump. This would not be the added fuel economy the engineers had in mind.

Thinking in terms of being a transmission cardiologist, it would be nice to schedule a stress test to ensure that everything from the filter to the pressure regulator valve is functioning well. The difficulty in scheduling such a test is that a line pressure tap is not made available with this transmission. The best that can be performed is a scan-o-cardiogram to see what the pressure control amps are doing in comparison with tap and steady state tap cells.

This reminds me of the cardiologist who owned a 2015 Corvette that needed to have his 8L90 transmission rebuilt. The transmission was completely disassembled and cleaned with all the parts spread out. The doctor had just stopped by to see how things were progressing with his Corvette. The talented young builder brought him out to the shop to show him his transmission and said, “Look at all these parts I have to put together. I have to do it just right for it to be able to work flawlessly. In many ways we are similar. But I sure would like to get paid the same way you do”. To which the cardiologist replied, “Yes you are right, in many ways you can draw a comparison. But, if you can rebuild that transmission while it is still running in the car then you can get paid the same way I do.”

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