There are some things in life that have little meaning, while others stick with you till the grave. One of the many things my father taught me has stuck with me my whole life so far: “The more pieces of pipe you put in the plumbing, the greater chances you have for a leak.” And as more parts are added to today’s transmissions, the more places there are to look for a leak.
Most transmissions have very complex circuits to apply a set of clutches, starting from the pump, through multiple valves, solenoids, a valve body, sealed ports, the case, sometimes back through the pump, then into the drum to apply the piston that engages the clutch. And any leak in this plumbing can cause premature failure.
To trace a circuit and check where all the joints are in the plumbing usually requires a hydraulic diagram for the transmission you’re working on. Looking at most hydraulic diagrams for modern transmissions can be overwhelming at times. Isolating the circuit in question (figure 1) can make things clearer and diagnosis quicker. Let’s look at a common problem with a simple circuit.
One of the most common problems we keep seeing is the 68RFE overdrive clutches burning beyond recognition. Most of the overdrive clutch failures in these transmissions are due to valve body track erosion, which is caused by excessive torque from engines with tuners installed in the computer software. But there are several other areas in the plumbing where a leak or restriction can occur.
Pressure is created at the pump and is directed to the solenoids. Usually, any leak here will affect multiple components. But it’s a good idea to check line pressure to verify that it’s responding correctly.
Inside the solenoid pack, the overdrive solenoid seals, a restricted solenoid, or a solenoid mechanical failure can result in inadequate flow to the clutches and cause premature failure. Any crack in the solenoid housing is another potential leak. From the solenoid pack to the valve body, we have a screen plate with seals.
In the valve body, we want to check for severe warps or cracks; no real fix for this other than to replace. As I mentioned before, worm track erosion is one of the most common problems. Flat sanding the valve body and installing an aftermarket bonded separator plate (figure 2) does appear to be working well out in the field.
Another option is an aftermarket billet channel plate (figure 3), which also appears to be doing an effective job of keeping this transmission on the road. You can get these parts from most parts suppliers or online.
Another common area for leaks in the valve body is the plug bore located behind the switch valve (figure 4). Second gear oil moves the plug away from the valve toward the other plugs, and overdrive oil moves the plug toward the valve to keep the switch valve in the lockup position.
This plug is constantly moving, which creates wear in the bore. Vacuum testing (figure 5) will usually reveal an excessive leak between the plug and the bore. You can address the bore wear by reaming and installing oversized plugs. It’s always a good idea to repeat the vacuum test after the repair to verify you’ve corrected the problem.
Next in the valve body, the accumulator is another potential leak in the plumbing. Damaged seals or bore wear can lead to seeing this transmission again. If the bore is worn, Sonnax has a repair kit that can save you from having to buy another valve body. I’ve seen a few newer units with plastic accumulators. I personally don’t like the idea of plastic here: heavy springs, heat and fluid pressure can lead to cracks. I prefer installing aluminum pistons.
The case is another area for a potential leak. There have been a few cases that have had leaks between the case and the valve body. You may be able to reveal this using a depth micrometer across the feed hole surface (figure 6). Flat sanding the area and using a bonded separator plate should address this problem and stop the leak.
In the next issue, we’ll continue up through the case, pump, stator, and drum.