An exhaust brake turns the diesel engine into a big air compressor and uses the drivetrain to absorb power from the rear wheels. The problem is that braking force is being driven backward through the transmission while the throttle is closed, so the transmission sees idle and line pressure drops. That is exactly how clutches start slipping, building heat, and burning up clutch faces. Our answer is SmartLock, built into the Banks Brake. It locks the torque converter clutch where equipped so the converter is not slipping while power is being pushed backward through it, and it raises transmission line pressure so the clutches stay applied. That matters because downhill braking can be substantial—we measured about 205 horsepower being dissipated. If you do not control the transmission, that load can damage it. If you do, towing gets safer and the transmission has a fighting chance.
Applying an exhaust brake to a diesel drivetrain sends power backward through the transmission. In effect, the engine becomes a large air compressor, and the resistance it creates absorbs power from the rear wheels through the driveline. That braking load is substantial, but it occurs while the driver is at closed or near-closed throttle.
At drop throttle, the transmission sees an idle condition, so line pressure falls accordingly. That creates a mismatch: the transmission is being asked to carry significant reverse torque from the exhaust-braking event while its hydraulic control system is behaving as though the vehicle is simply idling. If that condition is left uncorrected, the transmission is not properly prepared for the braking load being forced back through it.
To solve that problem, Banks developed a system called SmartLock. Its job is to make the transmission compatible with exhaust-brake operation. The system does two things that are central to durability: it locks the torque converter clutch, when a torque converter clutch is present, and it raises transmission line pressure.
Locking the torque converter clutch is important because a torque converter is fundamentally a somewhat one-way device and is not really intended to transmit power backward through fluid coupling alone. It can do it, but not ideally. When the clutch is locked, there is no converter slip, and the reverse power flow created by exhaust braking is transmitted much more effectively and with far less loss.
Some earlier diesel applications did not have torque converter clutches. Even so, the SmartLock strategy still addressed the transmission side of the problem by increasing line pressure so the unit could better withstand the braking load. The key point is that exhaust braking cannot be treated as an engine-only feature; the transmission must also be managed for the operating condition.
During brake development more than 20 years ago, Banks used an onboard dynamometer invention called the Dynafact. The system was sold to various universities, GM bought one, and Chrysler's wind tunnel had one as well for work such as aerodynamic drag measurement. It could also measure braking horsepower, which made it useful for understanding what the drivetrain experienced during exhaust-brake operation.
Using that instrumentation, Banks measured roughly 205 horsepower being dissipated on a downhill grade during braking. That means about 205 horsepower was being driven backward through the transmission. This was not a trivial load or a theoretical concern; it was a measured condition that showed why transmission protection had to be part of the exhaust-brake system design.
If the transmission is not addressed under those conditions, the internal clutches can slide, overheat, and damage the clutch friction faces. That risk exists regardless of which gear the transmission is in, because the reverse torque load is still being carried through the unit. For that reason, SmartLock was made part of the brake product itself, ensuring that exhaust braking included both torque converter clutch control and increased line pressure to protect the transmission while delivering effective braking.