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The Mack RD690S and Its Unique Transmission Setup
The Mack RD690S is a heavy-duty vocational truck built for rugged applications like dump hauling, lowboy towing, and site work. Powered by the E7 ETECH engine, often rated at 400 horsepower, this model is known for its durability and torque-rich performance. Mack Trucks, founded in 1900, has long emphasized integrated powertrains, meaning their engines, transmissions, and axles are designed to work together. This philosophy led to the development of proprietary transmissions like the Maxitorque series, including the 2070 7-speed.
Unlike Eaton’s 13-speed or Fuller’s 18-speed gearboxes, the 7-speed Maxitorque is a single-range transmission with wide gear spacing. It was designed for simplicity and reliability, especially in fleets where driver training and maintenance needed to be streamlined. However, its gear ratios can feel limiting to those accustomed to multi-range setups.
Driving Characteristics and RPM Behavior
Operators transitioning from 13-speed transmissions often find the 7-speed challenging. The key issue is the large gap between gears, which requires precise RPM management. Shifting smoothly demands letting the engine drop to around 800 RPM before engaging the next gear. However, Mack engines are not designed to pull effectively below 1,200 RPM. Their torque curve typically peaks between 1,200 and 1,900 RPM, and dropping below that range can cause lugging and poor acceleration.
This mismatch between shift timing and engine torque delivery leads to a learning curve. Drivers report that on uphill grades, the RPM falls too quickly during gear changes, making it difficult to catch the next gear before losing road speed. The result is often being forced into a lower gear than necessary, slowing the climb and increasing fuel consumption.
Comparing to Multi-Speed Transmissions
A 13-speed transmission allows for tighter gear spacing and split gears, enabling smoother transitions and better control under load. For example, a Ford LTL9000 with a 400 Big Cam Cummins and a 13-speed can pull a 160-class excavator with ease, maintaining momentum on hills and offering more flexibility in gear selection.
In contrast, the 7-speed Mack may struggle to match that performance, especially when towing heavy equipment. The lack of split gears means the driver must anticipate terrain changes and select the correct gear early, as mid-climb shifts are often impractical.
Solutions and Adaptations
To improve drivability:
Conclusion
Buying a Mack RD690S with a 7-speed transmission is not necessarily a mistake, but it does require a shift in driving technique and expectations. The truck itself is a reliable workhorse, and the E7 ETECH engine is capable of impressive pulling power when operated within its optimal RPM range. For those willing to learn its rhythm, the 7-speed can be a dependable partner. But for operators who prioritize gear flexibility and hill-climbing finesse, a multi-speed transmission may offer a more satisfying experience.
The Mack RD690S is a heavy-duty vocational truck built for rugged applications like dump hauling, lowboy towing, and site work. Powered by the E7 ETECH engine, often rated at 400 horsepower, this model is known for its durability and torque-rich performance. Mack Trucks, founded in 1900, has long emphasized integrated powertrains, meaning their engines, transmissions, and axles are designed to work together. This philosophy led to the development of proprietary transmissions like the Maxitorque series, including the 2070 7-speed.
Unlike Eaton’s 13-speed or Fuller’s 18-speed gearboxes, the 7-speed Maxitorque is a single-range transmission with wide gear spacing. It was designed for simplicity and reliability, especially in fleets where driver training and maintenance needed to be streamlined. However, its gear ratios can feel limiting to those accustomed to multi-range setups.
Driving Characteristics and RPM Behavior
Operators transitioning from 13-speed transmissions often find the 7-speed challenging. The key issue is the large gap between gears, which requires precise RPM management. Shifting smoothly demands letting the engine drop to around 800 RPM before engaging the next gear. However, Mack engines are not designed to pull effectively below 1,200 RPM. Their torque curve typically peaks between 1,200 and 1,900 RPM, and dropping below that range can cause lugging and poor acceleration.
This mismatch between shift timing and engine torque delivery leads to a learning curve. Drivers report that on uphill grades, the RPM falls too quickly during gear changes, making it difficult to catch the next gear before losing road speed. The result is often being forced into a lower gear than necessary, slowing the climb and increasing fuel consumption.
Comparing to Multi-Speed Transmissions
A 13-speed transmission allows for tighter gear spacing and split gears, enabling smoother transitions and better control under load. For example, a Ford LTL9000 with a 400 Big Cam Cummins and a 13-speed can pull a 160-class excavator with ease, maintaining momentum on hills and offering more flexibility in gear selection.
In contrast, the 7-speed Mack may struggle to match that performance, especially when towing heavy equipment. The lack of split gears means the driver must anticipate terrain changes and select the correct gear early, as mid-climb shifts are often impractical.
Solutions and Adaptations
To improve drivability:
- Use engine braking to help reduce RPM quickly during upshifts
- Avoid shifting on grades, and instead select the appropriate gear before the climb
- Consider reprogramming the ECM if the truck is equipped with electronic controls, allowing for better RPM matching
- Practice throttle modulation, as Mack engines respond differently than Cummins or CAT powerplants
Conclusion
Buying a Mack RD690S with a 7-speed transmission is not necessarily a mistake, but it does require a shift in driving technique and expectations. The truck itself is a reliable workhorse, and the E7 ETECH engine is capable of impressive pulling power when operated within its optimal RPM range. For those willing to learn its rhythm, the 7-speed can be a dependable partner. But for operators who prioritize gear flexibility and hill-climbing finesse, a multi-speed transmission may offer a more satisfying experience.
