Other Articles - March - 2022

Rise and Fall – 6L80/6L90 Pressure Issues

The 6L series of transmission was introduced starting in the 2006 model year. Since then, it has become one of the most commonly repaired transmissions in shops across the country. Just the sheer number of vehicles built with these transmissions is mind-boggling, as millions and millions are on the road today.

If one darkens your door, it will typically have an issue related to the pump and/or the torque converter. The 6L series units all share the same design and layout whether it is a 6L45,6L50, 6L80, or 6L90.

A variable displacement vane-style oil pump is integral to the bellhousing. Anytime the converter decides to “take a permeant vacation,” the metal particulate from the converter will likely migrate back into the pump and, of course, into the cooler. As you are aware, this severely damages the pump cover and its mating surface, the bellhousing. The amount of metal generated varies depending on how long the vehicle is operated with a compromised pump and converter. Usually, it is pretty extensive as most customers do not pay close attention to issues that are developing regarding how the transmission is operating.

The metal contamination can lead to valve and solenoid issues which must be addressed when you are repairing the root cause of the issue. Simply putting in a converter and pump may fix the root cause of the issue, but the other issues the transmission may be experiencing will still need to be addressed.

Pressure in the 6L series of units is controlled by a pressure regulator valve just like other transmissions. The regulator valve is located in the pump cover on all applications. The regulator valve position is controlled by a pressure control solenoid located in the TEHCM (Transmission Electrical Hydraulic Control Module). Like many other GM transmissions, the pressure regulator controls the position of the pump slide, which ultimately controls the pump volume. As you are aware, pressure is created by a resistance to flow, so as the pump volume changes, so does the pressure. This interconnected system works great when all of the moving parts are doing what they are supposed to, but when metal contamination is present all bets are off.

Pressure regulator valve wear is very common even on applications that do not have pump and converter issues. Wear or damage to the pump and cover are generally due to internal converter issues such as extensive friction disc wear leading to metal-to-metal contact between the TCC pressure plate and the converter cover or spline damage to the stator one way clutch outer race bore.

The aftermarket has several kits available to address the pressure regulator valve wear issues, but when it comes to the pump and bellhousing, new parts are available through GM or someone reselling the OEM parts. Buying a new bellhousing or pump cover is a challenge today as they are very hard to find in some parts of the country.

Many shops are choosing the have their bellhousing and pump cover machined or they are purchasing the re-machined parts from their supplier or online. Buying re-machined parts can be cost-effective, but one must understand that simply installing the parts and hoping for the best may not be the best course of action. When the components have been machined, you need to be aware of a few things that need to be checked:

  • The pump slide and rotor clearance should be checked as these components are available in selective sizes which are matched to the bellhousing pump bore depth. The clearances will have an effect on pump efficiency as well as pressure. If the clearances are too tight, the slide can stick or the rotor can damage the bellhousing bore and pump cover. The simplest way to check the slide and rotor end clearance is to use plastigage. Place green plastigage across the slide/rotor in multiple positions. Bolt the pump cover to the bellhousing and torque the bolts to specifications. Remove the bolts and separate the parts. Measure the plastigage. The correct rotor and slide selection should provide 0.0008-0.002” (0.02032- 0.0508mm) end clearance between the flat area of the bellhousing machined surface and the slide and rotor (Figure 1).
  • Input endplay should be checked as machining the components may have an impact on the unit endplay. The 6L applications do not utilize an endplay measurement or a way to adjust the endplay as they are “net build” units. Endplay should typically be in the 0.004-0.006” range. Aftermarket companies such as Superior sell products to address the endplay issue you may be trying to address.
  • When the bellhousing is machined, the distance between the bottom of the slide spring bore and the top of the bore changes. Very few machinists address this during the machining process. The slide spring end clearance needs to be checked as it is critical to keep the slide spring from wedging between the cover and bellhousing. If the spring sticks, the slide will not react, which can lead to erratic pressures and shift feel issues. This process is accomplished by placing a straight edge across the slide. A feeler gauge is then placed between the slide spring and the straight edge. Typically, one would like to see at least 0.020” or more between the spring and the straight edge (Figure 2). In addition, always examine the spring bore. Many times, you will see issues where the spring has worn into the bore, which can lead to slide issues (Figure 3). In either case, take a die grinder and touch up the bore so you do not have any issues that can jump up and bite you.

As you can see, simply buying some parts and bolting them together can create some issues that can ruin your day. Comebacks and upset customers are things that none of us need. Now that you know the little things to look for when using remachined components, you and your customers will be a lot happier. Until next time remember, “The secret to success is to do common things uncommonly well.”