Lockup in automatic transmissions has been with us for better than a half-century. However, lockup strategies have significantly evolved. With government-imposed regulations demanding vehicle manufacturers to produce more and more efficient powertrains, transmission shift scheduling and lockup management have evolved into a science in itself. Unfortunately, as these units age, inevitable pattern failures come to light and challenge us to determine the root cause of concerns. Here, we will look into the popular NAG1 transmission to see the various torque converter clutch-related complaints, causes, and fixes for this unit!
A LITTLE HISTORY
The NAG1 has been used by Chrysler since 2006 and has logged millions of miles in numerous applications. Its design comes directly from the Mercedes 722.6. NAG1 stands for New Automatic Gearbox, generation one. Another designation used is W5Axxx, indicating the following:
- W = Hydraulic Controlled with Torque Converter
- 5 = Number of forward ranges
- A = Automatic shift
- xxx = Used to denote maximum input torque in Newtom-Meters (580, 380, or 300).
The most popular application that uses the NAG1 (W5A580) is the Dodge Sprinter diesel. However, it is also behind the 5.7L and 6.1L V8 engines in the Charger, Challengers, and some Jeep models equipped with the 3.6L motor.
While these units are very durable, they tend to share a common complaint as they age; torque converter clutch-related shudder issues. Let’s look at the technology behind the TCC operation to understand why these units share this common concern.
THE TRANSMISSION STRATEGY
On a good day, these transmissions work wonderfully. The shift transitions and lockup-apply strategy work seamlessly and almost unnoticed from the driver’s seat. Acceleration is smooth, and the engine power is not interrupted by the shifts. When you lift your foot from the throttle, the tachometer drops politely to idle as you come to a complete stop. All of this is controlled through TCM and PCM programming.
The powertrain programming is engineered to save fuel whenever possible and provide power on demand. With that in mind, the torque converter clutch usually applies between 35 and 50 miles an hour under moderate acceleration. Depending on throttle opening and engine load, it can either be fully applied or partially applied. Under light throttle, the torque converter clutch can be commanded “on” as early as 2nd gear.
On deceleration, the torque converter is partially applied in order to bring the engine RPM down to about idle speed. The TCC command and injector shut-off mode commanded by the PCM allows the vehicle to save fuel when coasting or coming to a stop. The fuel injectors are commanded on, and the torque converter clutch is commanded off before the vehicle comes to a complete stop.
In short, the torque converter clutch cycles on as often as the computer can turn it on to save fuel. It is by far the busiest clutch in this unit. Therefore, patterned failures tend to occur that are directly related to TCC control.
THE PROBLEMS
The NAG1 transmission has endured a history of torque converter clutch-related concerns. As the vehicle and the unit ages, the issues become more prominent and easier to duplicate. The most common complaints are:
- Torque converter clutch shudder between 35 and 50mph.
- Powertrain vibration upon TCC apply and release events.
- Rough engine idle (cold) due to TCC release issues.
Often, a vehicle will come into a shop with seemingly mild symptoms, with over 100k miles on the odometer, and needing an overhaul based on poor shift quality, codes, and debris in the fluid. At this point, most builders would call for a basic overhaul, replacing all rubber, paper, and friction material. However, if you don’t look deeper, you may rebuild the unit only to have the above complaints amplified! So let’s take a look at the areas that are causing these issues with the NAG1.
THE TORQUE CONVERTER
An obvious component to blame is the torque converter. In many cases, the torque converter needs replacement, but let’s look closer at what is happening inside. First, this converter is different than conventional designs. Most converters use the converter charge oil to control the converter clutch release and apply by controlling the direction of the feed oil into the converter. Conventional converters can be referred to as two-pass or two-circuit designs, using a converter charge and return circuit.
The NAG1 uses a conventional charge and return oil for the torque converter function but adds a dedicated oil circuit to control the apply and release of a multi-disc converter clutch. The converter clutch resembles a clutch drum that could be found inside a transmission. It uses a clutch piston to apply a clutch pack, usually two or three frictions and steels. Converter charge oil acts on the release side of the piston to release the piston when the converter clutch is commanded off. Since there are converter charge, return, and TCC control oil circuits, this torque converter can be referred to as a three-pass or three-circuit design.
The converter works quite well when it is new. However, age causes concerns with wear. In addition, the earliest units suffer clutch deterioration (flaking), resulting in numerous chatter and slip-related concerns. The clutches used in rebuilt units are updated to address this problem.
The pilot bushing inside the torque converter can wear and cause oil control issues. In some cases, the pilot can crack (figure 1). It supports the input shaft and keeps TCC apply oil separated from the torque converter charge oil. A D-ring seal also positively seals the circuit inside the torque converter pilot. When the bushing wears and the D-ring seal is compromised, TCC shudder and driveline vibrations are related complaints.
Rough engine idle on a cold start is a common complaint traced directly to the torque converter. Since the TCC apply piston relies on converter charge oil to keep the piston in the released position when commanded off, residual oil on the apply side can partially apply the piston due to the centrifugal force of the engine turning the torque converter. As a result, the cold transmission fluid remains trapped, applying the TCC and causing the engine to stumble.
The OEM updated the torque converter to help remedy these concerns. Also, the aftermarket has numerous fixes available to eliminate these issues. Investing in a quality rebuilt torque converter will likely ensure you will not have problems here.
THE VALVE BODY
As mentioned, the torque converter clutch shifts more than any clutch pack inside the transmission. That means the valves that control the torque converter clutch stroke more than others in the valve body. Five valves are associated with lockup apply and release in the NAG1 transmission (figure 2). Always inspect the bores for wear and look for broken or collapsed return springs. Most issues related to lockup apply and release concerns are addressed by fixing problems in the valve body. There are numerous aftermarket repair options available to address these valves. Repair and replace as needed.
TCC apply and release issues may be caused by mechanical or sealing issues with the Torque Converter Clutch solenoid. Remove and inspect carefully (figure 3). The O-ring seals become brittle and leak with time. Test, repair, or replace the solenoid as needed.
OUTSIDE THE BOX!
With the lockup strategy in mind, the customer complaint might be a normal operating condition for the transmission. Complaints of a shudder while coming to a stop can be an issue outside the transmission but amplified due to lockup being applied. Shudder complaints of this nature often occur on cars with V8 applications. To verify, manually downshift the transmission while coming to a stop. Manually shifting the transmission disables the TCC coast-down strategy, keeping the TCC released. If the shudder goes away, the issue is most likely isolation (mounts) or engine related.
Also, check for engine codes and verify engine idle quality and cylinder balance. Noise, vibration, and harshness (NVH) complaints can be related to the engine mounts. Always use OEM engine mounts when servicing. For certain models, there are updated engine mounts from the OEM to address this issue.
A THOROUGH TEST DRIVE
A thorough test drive is necessary so you can focus on the lockup apply and release quality from a cold start to operating temperature as it relates to the customer’s complaint. Use your scan tool in graphing mode when data is available to monitor the TCC command versus the TCC slip rate, especially between 35 and 50 mph. You can also use the N2 and N3 speed sensor data versus the engine RPM PIDs to look for speed variations if diagnosing a powertrain vibration.
Keeping the TCC strategy in mind while focusing on the customer’s complaint is extremely important. For example, shudders and vibration complaints that occur while coasting or coming to a stop MAY indicate an engine misfire, cylinder imbalance, or other mechanical, driveline, and isolation issues!
Patterned failure analysis can help us better understand why certain failures occur in transmissions. It also gives us a better focus to create a diagnostic path that will guide us to the root cause of issues more efficiently. In the end, it will help you deliver the goods and get rid of NAGging problems!
