Behind most diesel engines in pickup trucks is an automatic transmission that will likely give up the ghost long before the engine shows signs of wear. The 68RFE falls squarely into this category. Paired with the 6.7L Cummins Turbo Diesel engine from 2007 1/2 and up applications, it has remained fundamentally the same for most of its life. While the torque handling capacity of this unit is sufficient to handle some of the most extreme engine modifications, issues of wear and pressure management cause the best rebuilds to fail unless you thoroughly understand what causes these units to fail. Let’s look into areas of concern you must address to keep this transmission on the road after a rebuild.
THE INITIAL DIAGNOSIS
Looking at the entire vehicle is imperative to fixing the root cause of failure for any transmission-related concern. When diagnosing issues with the 68RFE, the same holds with the addition of knowing as much as possible about the entire history of vehicle repairs and modifications. External control issues cause numerous driveability and durability concerns. Most repeat failures of these units are related to low-pressure or poor-pressure management. First, let’s look at identifying issues using a diagnostic test drive.
Nothing beats a good old-fashioned drive with a scan tool! Looking at commanded pressure versus actual pressure and the Line Pressure Duty Cycle % will tell you a lot about the general health of the unit. While observing these values in graphing mode, you have a pressure-related issue if the Line Pressure Duty Cycle % remains at 5% consistently and the actual pressure is lower than the desired pressure (Figure 1). However, the pressure issue may have a root cause outside or inside the unit. Vehicle modifications such as oversized tires, aftermarket programming, or internal valve bore wear can be the source of pressure issues. Let’s take a closer look at how we can narrow the possibilities.
OVERSIZED TIRES
One of the most popular aftermarket modifications on diesel trucks is getting oversized tires. The aggressive appearance of large tires gives an otherwise typical truck a sought-after look. Unfortunately, the modification requires adjusting the overall gearing of the vehicle to match the relative torque loss inherent in changing the tire diameter. Keeping the stock gearing results in a truck that feels underpowered. The engine works too hard to get the vehicle rolling and keep it up to speed.
Observing scan data at cruise can give you an accurate picture of what is happening to the transmission relative to the oversized tires. While maintaining a steady speed on a flat stretch of highway, observe the Line Pressure Duty Cycle %, with the commanded and actual line pressures in graphing mode. If the Line Pressure Duty Cycle % is tending toward or remaining at 5%, the unit is working too hard to keep up with the pressure required to keep this unit from slipping. Applications that have this issue also often will have downshift sensitivity.
With diesel tunes tipping the scales in excess of 700 foot-pounds of torque, it is easy to make up for this low-power ‘feeling’; however, the transmission pays the price. The transmission operates at higher pressures for extended periods based on engine load. The byproducts are excessive heat build-up in the transmission, clutch trauma, and lubrication failures.
None of these can be compensated by modifying the transmission build. The result is repeat failure unless the vehicle is regeared (see September 2023 GEARS article, Where Did My Power Go?).
UNDERSTANDING THE HYDRAULIC SYSTEM
While external issues may be obvious, internal hydraulic faults are some of the more common and easily missed items. When it comes to solving the root-cause issues with the 68RFE, you must understand how the hydraulics respond to the computer commands from the TCM. Knowing the duty of the critical valves in this unit is imperative, so let’s look at some of the most critical valves in this unit.
THE PRESSURE REGULATOR VALVE
The Pressure Regulator valve generates mainline pressure. This valve also handles converter charge and TCC Release oil as soon as you start the engine. Once the torque converter is full, the valve begins to generate pressure, and the TCM/ PCM signals the EPC solenoid to adjust the duty cycle according to the commanded line pressure.
Once the commanded duty cycle stabilizes, you can observe the actual versus commanded line pressure compared to the Line Pressure Duty Cycle % on a scan tool. You have healthy line pressure control if your idle commanded duty cycle is 18% or higher. If you witness a 5% duty cycle command, the computer is maxing out, and, most likely, the desired line pressure is lower than the commanded.
As a rule of thumb, anything under 10% Line Pressure Duty Cycle is not suitable for this transmission. An aggressive tune or oversized tires may cause a commanded 225 PSI line pressure in 6th gear. Low engine rpms and high commanded pressure almost always cause the Line Pressure Duty Cycle to drop below normal, causing the pressure regulator valve to be out of balance and restricting cooler oil volume.
One recommended modification for high-performance applications is to ensure you have a line-to-lube circuit, either by drilling an orifice or using a modified pressure regulator valve. Do not use a line-to-lube valve if the casting has already been drilled for it. The orifice size should be 0.055 to 0.062” (Figure 2). The added circuit will prevent converter charge and lubrication starvation while running high pressures. The modification will not prevent overheating, but it at least aids in preventing a total unit meltdown in extreme operating conditions.
THE TORQUE CONVERTER REGULATOR VALVE
The torque converter regulator valve works with the Low Reverse/ TCC solenoid in the solenoid pack assembly. This valve regulates the applied pressure to the converter clutch to get a smooth or short lockup depending on engine load relative to line pressure. It is common for this valve bore to be worn and cause torque converter apply and release concerns such as:
- Engine stall coming to a stop.
- Harsh TCC engagement and/or release.
- Excessive TCC slip (can be mistaken for a flare on the 4-5 shift).
- There are aftermarket solutions available to fix this problem.
- The TCC Accumulator Valve/Plunger
The accumulator works with the Low Reverse/TCC solenoid and TC Regulator valve (Figure 3). It is designed to smooth the ‘bumps’ produced through pulse width modulation in the hydraulic circuit from the solenoid operation. It is common for this bore to have egg-shaped wear. If you use vacuum testing to check for wear in this bore, you must locate the valve in the area of greatest wear while testing. Since it is a plunger, it can co*ck in the bore and give a false good reading. Use a wiggle test and take the lowest reading.
The same torque converter scheduling and shift-related issues with TC Regulator Valve bore wear will occur with TCC Accumulator bore wear. There are also aftermarket repair solutions for this valve bore.
THE SOLENOID SWITCH VALVE AND BORE PLUGS
By now, builders are familiar with the fact that the Solenoid Switch Valve and Bore Plugs play a critical role in the life of this transmission (Figure 4). The valve assembly is responsible for directing oil to different areas of the unit. When the bore is compromised, oil can leak into adjacent circuits, causing pressure switches to trigger and clutches to burn. Some of the most common issues observed are:
- DTC P0871 (OD Pressure Switch Rationality).
- Burnt OD Clutches.
- 2nd gear starts.
- Binding on the shifts.
If the unit has any or all of these issues, the Solenoid Switch Valve and Bore Plugs must be inspected for wear. Valve body warpage can also cause these issues. Flat sanding both halves or using gaskets resolves concerns in this area. There are several aftermarket fixes.
STOCK VS. AFTERMARKET PROGRAMMING
One powerful piece of upfront information you hope to receive from the customer is that the 68RFE-equipped vehicle you are working on has been tuned (or not)! Unfortunately, not all customers will give you the luxury of this knowledge. Since tuning means changing parameters in the transmission control program, the results, good or bad, lie in the hands of the selected tune! If you feel that you might be getting less than the truth, you can look at scan tool data to help determine whether or not you are dealing with a tune.
Using parameters in the transmission data section, display the Actual Pressure, Commanded Pressure, and Line Pressure Duty Cycle %. At idle, the typical pressure is about 60 to 70 PSI commanded and actual, with the Line Pressure Duty Cycle % at 22 to 24 percent (figure 5). Tuned vehicles can command pressure beyond the pump’s capacity, causing a maximum command of 5% for the Line Pressure Duty Cycle % and an actual line pressure lower than commanded. At this point, it would be a good idea to find out as much as possible about the tune on the vehicle before proceeding with a repair you plan to warranty. Some tunes will cause repeat failures (see Sean Boyle’s GEARS article, ‘Listening to Tunes,’ January/February 2023).
Ensuring you address hydraulic, mechanical, and electrical concerns with vehicles coming to your shop for repair is imperative to prevent comebacks and no-gos. Understanding how these systems interact to produce a smoothly operating and reliable powertrain is equally important. ATRA is here to keep you in the know at all levels, from diagnosis to delivery, to ensure you deliver the goods to your customers with confidence that you did the job right the first time!
A special thanks to Pepe Torres of Aus Tex Transmissions for providing this article’s technical information and pictures!
