May 07, 2024

Torque Converter Can of Worms: Lockup & Aftermarket Programming

Randall Schroeder

Lockup torque converters have been around now for some time. They pretty much came into production when there were fuel mileage demands put into effect by the government, and the auto manufacturers needed to do something to better connect the fluid coupling (torque converter) of the automatic transmission to the motor. By doing this, OEMs found that there were big benefits to eliminating the slip that the earlier non-lockup torque converters had, plus noticeable gains of up to 65% to 70% in fuel efficiency. Awesome! This changed the industry.

To achieve this lockup, the OEMs added a disc clutch (normally held off by release oil and converter charge) to be forced against the cover. In most cases, this is achieved by controlling the exhaust (Figure 1).

Figure 1 – GM 4L60-E, 4L70-E, 4L75-E Lockup 300mm Performance Converter Kit
GM 4L60-E, 4L70-E, 4L775-E Lockup Performance Converter Kit
Illustrated is an aftermarket multi-plate lockup kit.

By releasing the oil holding the clutch off, it then allows the charge oil to force the disc against the cover. This is the most common type of lockup torque converter. In some applications though, the OE installed an actual clutch drum inside of the torque converter to be used for lockup (Figure 2).

Figure 2 – Mercedes 722.6 Captive Internal Clutch Pack
Mercedes 722.6 Captive Internal Clutch Pack

In this design, instead of releasing oil, it sends apply oil back into the converter to force the clutch pack to apply, thereby locking the input shaft to the engine rotation. The auto manufacturers, by adding either of these styles of lockup clutch setups, could maintain a lock directly to engine rotation.

The early lockup systems were no different than a light switch mounted against the wall. They were either ON (locked) or OFF (multiplying torque). This exchange typically only happened in high gear when either in 3rd or 4th Gear. Ratios changed as technology evolved, though, and now we are dealing with 6-, 8- and 10-speed transmissions. These changes, like everything else in the automotive industry, produced great gains over time in fuel savings. Manufacturers found that they could bring the converter clutch on to a particular lock condition during lower speeds, essentially a dimmer switch, to meet government fuel economy mandates.

Today, OEMs are bringing on the converter clutch as early as 2nd Gear and continuously allowing for a slip of the clutch disc or drum to fine-tune the fuel management and maintain engine balance to the transmission. This brings a whole new problem in the transmission shop when dealing with converter clutch damage and engagement feel, shudders, bangs and vibrations. Often, the technicians are faced with tackling the car’s computer programs, and vehicles need updates in programming to fix issues. Transmission fluids have also become very specific and critical to maintain converter life. Incorrect fluids can cause low-speed chatters (since they are bringing the clutch on as early as 2nd Gear, and it doesn’t grab due to the lack of modifiers), creating more heat than can be cooled and, ultimately, clutch failures. Converter failures are filling the transmission with metal debris and causing valves in the control bodies to stick, solenoids to restrict and hard parts to fail. Expensive repairs!

What we know:

  • The Positive: Gains in fuel mileage are achieved by bringing the torque converter clutch on early, such as when driving in and around town.
  • The Negative: Because the converter is so active, transmission issues now are commonly caused by the converter and failures surrounding it.

Repair shops and customers are aware of issues caused by early lockup and often resort to what’s often called “aftermarket programming” to change the timing or the gear when lockup comes on (off-road use only) to the higher gear ranges. It is a whole new can of worms when this happens, especially if the vehicle still has a manufacturer warranty. Always investigate the legal issues before resorting to the aftermarket tuning. Just know that these changes will in many instances fix all the issues with torque converter timing, but it may void other warranties. Be wise when making changes.

Randall Schroeder is a Sonnax technical sales and training specialist. He is a member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Transmission Company technicians.

While Sonnax makes every effort to ensure the accuracy of technical articles at time of publication, we assume no liability for inaccuracies or for information which may become outdated or obsolete over time.