Back on the Bench - April - 2019

68RFE OD Clutch Failure: Part 2

In our last article about overdrive clutch failure on a 68RFE, we left off at surface damage where the valve body contacts the case. Your next step is to perform the same check at the case surface behind the pump. Though we haven’t seen or heard of any damage here yet, it’s a likely situation.

Into the pump, from the case to the stator support, check for worm track damage, warps, cracks, or modifications. Though there’s rarely pump track damage, other than builder-induced problems, it’s worth taking the time to inspect it with a straightedge.

We also recommend vacuum testing (figure 1). These units often show up on the bench with worn out pressure regulator valve bores. Next, have a look at the stator support. With most other transmissions, we’d only be concerned about the path through the stator support to the drum: stator support damage, worn sealing ring grooves, restrictions, and possibly bushings in most cases.

The RFE has something else to look out for: dribbler valves (figure 2). These valves are pressed into the stator support and have an orifice, checkball, and spring. These allow a small amount of fluid into the drum to keep the circuit pre-charged.

If the orifice is missing, too much fluid can get into the circuit and partially apply the clutches, causing them to drag and come on too early, possibly bind on the shift, and lead to premature failure. If the dribbler valves get restricted, it’ll take more time to apply the clutches and cause them to drag slightly, increasing friction and the likelihood of failure. Air should flow from pump to drum, but not drum to pump (figure 3).

From the stator support to the input drum, check the bushings that support the drum (figure 4). Excessive wear can cause damage inside the hub due to contact from the stator support, causing grooves and creating another area for a potential leak.

Inside the drum, damage to the surfaces on the input clutch retainer (figure 5) can lead to seal damage and premature clutch failure.

There’s a small orifice inside the overdrive/reverse piston (figure 6) that can lead to big problems with repeat overdrive clutch failure. This orifice is supposed to provide a controlled leak; if it gets restricted, the oil from the dribbler valves can build up pressure, partially apply the overdrive clutches, and make them look just like the ones that came out of the transmission originally.

Clutch clearance, though rarely considered, is more important than you might think. The manufacturers have reasons for setting specifications for clearances. The low end, or minimum, is to keep frictions from dragging, which would cause overheating and premature failure, as well as affect lubrication and apply and release times.

The maximum clearance is to prevent the piston from over-stroking and blowing out seals. Some may seem loose, like the 4T65E 2nd clutches and the overdrive clutches in most Chrysler units, but this is by design. Friction element apply-and-release timing is determined by fluid viscosity, orifices, pressure, solenoid or valve operation, piston fill time, and clearance.

Tightening clearances where the manufacturer seems to recommend an excessive amount will reduce piston travel and reduce the fill time of the piston chamber, which causes quicker apply of the friction elements. The computer is programmed to operate according to manufacturer’s specifications; adjusting clearance below specifications can result in a bindup during the shift. Even the slightest adjustment, one that only feels like a firm shift, can bring this transmission back quickly.

Though many other transmissions out there are more complicated than the overdrive circuit of the 68RFE, using this basic method of following the flow and using the hydraulic schematics as a road map will help you locate the cause of the problem and prevent you from seeing them back on the bench.