Delivering the Goods - January/February - 2021

The Devil’s in the Details! A Machinist’s Look at the 6L80/6L90 Units

The 6L80 and 6L90 transmissions have become the most popular rear-wheel drive GM transmissions in most of our shops. We have seen several patterned failures, written numerous articles, and found countless fixes to keep these units on the road (and the list doesn’t end). With so many of them in need of repair, they have become a common unit to build. Unfortunately, with common units can come routine, repetition, and a lack of attention to detail. Here is where even the best rebuilders can fall prey to a no-go or a comeback. Let’s take a look at some specific areas where these units can beat you up if the details are not addressed.

The Repair Zone

To get an in-depth look at these units, I took a trip to my local machinist’s shop here in Phoenix, Arizona. Mike Tilley, from Arizona Transmission Machine Inc., walked me through the details of repairing the damage that commonly occurs in the pump area due to torque converter failure. As most of us know by now, torque converter failure is the number one issue that brings these units to our shops.

A total of .010” to .015” of material is removed from the stator support and bell housing to restore the surfaces on a typical pump assembly repair. The pump pocket depth is restored by removing material from the bell housing surface. This is a precise dimension that must be matched for proper rotor and slide clearance. Most machinists remove enough material to restore the pump pocket and then remove an equal amount from the pump surface to maintain OEM dimensions.

Some processes include removing material from the bell housing-to-case surface as well. Since most units being rebuilt have about 100k miles on them, unit clearance should be checked and adjusted anyway. We will discuss unit end play later in this article.

Checking the Fit

Just because you have parts that are new or reconditioned doesn’t mean that everything is okay. ALWAYS prove that your parts are going to work as designed. Check the clearance of your slide and rotor in your reconditioned pump pocket. Use a straight edge and feeler gauge to check for proper clearance. The clearance specification is .0008” – .0020” for the slide and rotor-to-stator surface. If it seems like you might not have enough clearance, use Plastigage® to provide an accurate measurement. Just because the slide and rotor move freely on the bench does not mean that they will move when heated up in the vehicle. Again, here is where detail is critical.

Other areas of concern are the high and low-pressure slide stop surfaces. These areas are designed to limit the movement of the slide. When the pump pocket is cut, machinists take extra care not to contact these stop surfaces. The high-pressure stop is the most critical since it allows the slide to come close to the rotor without contact (figure 1).

If this surface is altered, the rotor could hit the slide causing catastrophic failure. With this in mind, there is always a slight ledge at the base of both stops. When you place the slide into the pump pocket with the pivot pin, put the slide against each stop. The slide must contact these stops! If the slide does not contact the stops, either remove the material at the base of the stop or grind a chamfer on the slide to allow for full movement (figure 2). I prefer to use a file and make a small chamfer on the slide at the stops. Failure to address the ledge could cause the slide to stick.

Bell Housing Warpage

We discussed pump and stator support related concerns in previous seminars that can cause clutch failure and lock up issues. Another area of concern is the bell housing surface that mates to the pump; specifically, the area that feeds 1-2-3-4 and 3-5-reverse oil to the stator support (figure 3). Warpage in this area is common, to the order of .001”-.002”, which is more than enough to cause apply issues. Always use a straight edge and feeler gauges to check for flatness in this area.

Pump Alignment

Ensuring that the pump is aligned properly is important and noteworthy. Geartrain alignment is directly affected by pump alignment. As discussed in my article in GEARS magazine’s December 2020 issue, it is highly recommended to use the factory procedure to align the pump. Use the factory tool or an equivalent to center the stator support body’s outer circumference to the pump body. There are aftermarket options for this tool. Failure to use the proper alignment procedure can result in premature bushing wear and unit failure.

Unit Endplay

The 6L80’s and 90’s need to have minimal endplay for longevity. Given the fact that most of these units have torque converter failure and pump damage, the pump will most likely need to be repaired or replaced. With this in mind, you need to measure the endplay of the unit BEFORE you disassemble the unit. This will allow you to predict your endplay adjustment more easily, especially if you have your existing pump assembly repaired. These units usually come into the shop with .010”- .035” of endplay clearance.

As mentioned earlier, repairing the pump requires removing material. Here is where attention to detail produces a quality rebuild. You want to have an endplay clearance of .004”-.006”. Unfortunately, GM decided that this unit didn’t need an endplay adjustment! Thanks to the aftermarket industry, there are shims available to help get you within this range. Check with your soft parts supplier for availability.

Poor Ground

Poor grounding seems to be a thing of the past. However, with more and more electronic components added to vehicles comes a higher demand for good grounds. Here is where the R&R technician can wear the Superman cape!

During transmission removal, it is extremely important to keep track of all wiring harnesses and ground points, especially those that must be removed. Remember to ALWAYS put harnesses and grounds back where you found them; brackets and all. Engineers go to great lengths to ensure that ground paths are all routed safely back to the battery negative terminal. Always clean ground surfaces with a wire brush or a similar abrasive. Torque all ground bolts to manufacturer’s specifications.

The bell housing-to-engine block junction on these units happens to be an area where ground circuits get compromised. If you examine the mating surfaces on removal, you will often observe rust on the aluminum bell housing (figure 4). Be aware that the engine block and the bell housing mating surfaces must be cleaned to restore the vehicle’s proper ground. Otherwise, circuits will find ground elsewhere! This usually means that pump bushings and engine bearings can be damaged. If pump bushing damage is observed, add a ground strap directly from the unit bell housing to the battery negative terminal to ensure the ground will be directed away from bushings.

Clutch Clearance

Most builders are probably at the point where they ‘feel’ the clutch packs’ clearance and let it fly. Clutch clearance may almost seem trivial based on how many of these units we have built. But if this detail is overlooked, it will definitely create objectional shift quality.

All frictions are not created equal! Clutch manufacturers have nominal thicknesses that they try to achieve to ensure consistency with OEM standards. The closer they are to OEM standards, the more the clutch packs dial into the recommended clearance range without adjustment (figure 5). With the addition of high-performance clutch packs, things can change quickly.

For example, suppose the 3-5-Reverse clutch pack is stacked with not enough clearance. In that case, you can have a bind on the 2-3 shift that will only occur once per ignition cycle on the initial shift sequence (see Gears Magazine, March 2020 issue for more on this). Keeping the clutch packs within their specifications is critical in allowing computer programming to correctly control shift quality.

As common units age, repairs require greater attention to detail. When parts are replaced, always remember that all parts are not created equal. Be suspect and qualify your parts to ensure your repair has the best possible chance to stand the test of time. Remember to check the details to make sure the devil is not there!

A special thanks to Mike Tilley for providing technical information for this article!