Shop Talk - July - 2018

It Worked on the Road Test! The 6T40 4-5-6 Drum Interchange Quandary

For years, we rebuilders have made changes to transmission functionality by either interchanging parts, installing upgrades, or making valve body modifications. It’s just part of the business. A little tweak here, a slight modification there, and presto! it’s ready for the road test.

A quick drive on our “road test course” and we’d either deliver the car or work on it some more. The road test was the great qualifier and we’d know just how to use it to check our work.

That method worked pretty well 40 years ago when I was a rebuilder and it works pretty well today, too… for the most part. But looking back, I can remember modifications and “upgrades” that I thought were the right way to go, but, knowing what I know today, I’d either do something different or not make the modification at all.

You see, the seat-of-the-pants test always doesn’t give us the complete story; sometimes we may not realize the true effects of our work until months later. We saw this with ideas like removing the 3-4 clutch release springs on the 4L60E. Great idea… until we learned that it made things worse. We can go through a list of great ideas that didn’t work; this one deals with the 6T40.

Here, we’ll cover a common interchange scenario with the General Motors 6T40 4-5-6 clutch drum assembly. The 6T40 was introduced in 2008 for the Chevy Malibu. In 2009, it was added for the Saturn Aura and then in 2010 it became standard on a range of GM vehicles.

The 6T40 has undergone several changes over the years, the most significant of which began in 2012. So much so that the early and late transaxles are referred to as GEN I and GEN II. During this upgrade, several things changed:

  1. Valve body
  2. Valve body solenoids
  3. TECHM
  4. Pump
  5. Case
  6. Friction material for all clutch packs
  7. Piston return springs for all clutch packs
  8. Wave plates removed
  9. 4-5-6 clutch drum assembly

There were additional parts changes, but let’s stop here with the 4-5-6 clutch drum assembly. The issue at hand is ring tower and 4-5-6 clutch wave plate failure for GEN I transaxles. The GEN II version didn’t have a wave plate and the ring tower seemed to hold up much better than the one in the earlier version.

So you get a GEN I unit with a wiped out clutch due to a broken wave plate and a ring tower that’s worn out. One solution is to upgrade to the GEN II ring tower and 4-5-6 clutch drum assembly. You make the swap and it drives pretty well on the road test. “Wow! That was a great idea! I think I’ll make that upgrade on all my early units!” you think.

Not so fast. Let’s take a closer look at these parts and see what really happens with this “upgrade.” First we’ll revisit the changes and examine the differences. An obvious difference is the ring tower. GEN I uses four sealing rings while GEN II only uses three (figure 1). The 4-5-6 clutch pack for GEN I uses four frictions and a wave plate, while the GEN II uses five frictions and no wave plate (figure 2).

The 4-5-6 clutch drum assemblies look similar at first glance, but a striking difference is that the fluid dam on GEN I models has an inner and outer seal, while the GEN II version is missing the inner seal (figures 3 and 4). This is significant and enough of a difference that we need to know more.

So what is this fluid dam and why is it used? It’s a pretty simple concept: When you release a clutch drum, there’s still oil trapped behind the piston. There’s no pressure preventing the piston from fully releasing; it’s just… present. It’s just there.

If you spin up the drum, like during a kickdown maneuver, centrifugal force throws the oil toward the piston edge with enough force that it can actually move the piston and drag the clutch. You can use a heavier piston return spring, but now you have to overcome applying the piston, especially during low-pressure conditions.

One answer is the fluid dam. This isn’t a new idea. The first unit I recall using this concept was the AX4N. They called it a return piston but it worked the same way. The idea is to have fluid present on both sides of the piston: the fluid that’s causing our problem and fluid on the return-spring side as well.

Now, when you spin up the drum during full-throttle, centrifugal force is working on both sides of the piston. And since it’s only in effect during high RPM, there’s no downside during normal shifting. What a great idea!

Now let’s get back to our 6T40. The return piston on the GEN II versions is like the piston in every other transmission that uses this concept: no inner seal. But the GEN I version has an inner seal. Why might that be?

To answer that, we’ll need to look at the oil schematics. Let’s start with the GEN II version (figure 5). Like other transmissions that use this concept (even going back to the AX4N), the valve body simply dribbles oil into the fluid dam circuit. It’s never pressurized because there’s no inner seal. For it to work, it just needs a tiny bit of oil.

On the 6T40 you can see that it gets the oil from many sources, so, as the transmission is working and shifting, there are several valves just squirting a bit of oil into the fluid dam circuit.

But the GEN I system is different. It uses a regulator valve to feed the fluid dam circuit (figure 6). This valve (the compensator feed regulator valve) delivers low pressure to the fluid dam. The principle is the same: Balance the centrifugal force of the oil to both sides of the piston, but for reasons that escape me, they decided to use a regulator.

Now you see the need for the inner seal and the 4th sealing ring. For the regulator to work properly, you have to contain the oil so it can actually reach the regulator’s pressure setting.

Let’s spoil that whole idea now by using a GEN II ring tower and drum assembly. With the 4th ring and inner seal of the fluid dam missing, the regulator no longer regulates; it just sits there wide open (figure 7). Now you have a leak that’s only contained by any restrictions in the system, through the channeling and fluid passages, but for all intents and purposes, you’ve got a hefty leak on your hands.

“But it worked okay on the road test,” you say. Providing the pump can overcome the leak, you may not notice much of a problem. But, over time, you’re likely to have it fall out of gear at a stop or exhibit some other low-line-pressure related complaint.

For me, the swap is a non-starter (and we haven’t even gotten into the frictions and wave plate changes). I’ve seen a lot of upgrades, modifications, and innovations over the years, and this industry always finds a way around nagging problems, but this one needs to go back on the shelf for review.

And besides, I’m sure a better solution is right around the corner.


Thanks to Dalyn Hester at Whatever it Takes Transmission Parts, Inc. for supplying the parts for the research of this article.