HD4070 is the Allison's heavy-duty automatic transmission.

The military and heavy-duty commercial truck markets require transmissions that are not only capable of heavy loads, rigorous duty cycles, and proven reliability, but must also provide value for the end-user and maintain compatibility with leading-edge technological improvements of engines and other vehicle systems. The HD 4070 transmission is one of the latest offerings from Allison Transmission to meet these next-generation needs, according to Senior Project Engineer Timothy Cooney. It is designed to be compatible with the fast-changing electronics capabilities of modern vehicle systems.

In some applications, the HD 4070 will replace the Allison HT 700 and CLT 700 transmission series. These units received electronic controls in the 1980s but can no longer be uprgraded with the control systems necessary for use with future engines and vehicles. The new unit is based on the company's heavier-duty HD 4000 World Transmission but has been adapted to the needs of military and commercial markets. Based on customer needs and company mandates for the new product, requirements included more ratio coverage; additional gear ratios with single transition shifts, without loss of power during shifting; a common transmission sump; accommodation of current rear output retarder and rear cover, the latter to simplify assembly and installations; and provision of mounting pads. In addition, the new unit had to use WTEC III controls with SAE J1587 communications for diagnostics, be capable of SAE J1939 communications, have adaptive shifting, and provide similar I/Os as World Transmission applications.

These requirements resulted in a decision to modify the HD 4000 with an additional gear range and a high mechanical ratio. The design of the seven-speed HD 4070 is substantially similar to Allison's HD 4060, except for the added gear range. The seventh speed is obtained by adding a fourth planetary gear set and a sixth clutch. This combination provides for a mechanical ratio of 7.63:1 in the low range—about 1.5 times the low range in the CLT 700 transmission. In many cases, this allows for the elimination of the low auxiliary range needed in the transfer case.

A seventh speed in the HD 4070 is obtained by the addition of a fourth planetary gearset and sixth clutch.

Vehicle interface procedures and input/output functions are the same as all Allison World Transmissions with WTEC III Version 8 (and later) software.

The new transmission features second-gear starting because it is anticipated that first-range capability will be needed only for special conditions. The addition of solenoids for the C6 clutch resulted in the need to remove a solenoid used in some configurations for retarder accumulators. Due to this limitation, HD4070 units equipped with output retarders will not be able to use accumulators. The retarder accumulator is designed to "precharge" the retarder cavity to improve response time for start/stop vocations, in which tenths of a second in the apply time can lead to significant savings in service brakes.

The HD 4070 was released for military and commercial use in early 1999, with first use in the U.S. Marine Corp.'s Medium Tactical Vehicle Replacement (MTVR) program. Production is scheduled to ramp up slowly for the first 18 months, with many other military applications expected to be added by the end of 2000. The transmission's first commercial application was in fire crash trucks due to their similarity with the MTVR program. Other commercial applications are expected to include crane carriers, to be added in mid-2000 after ramp-up.

The HD 4070 has the same rating as the HD 4060. For general-truck applications such as rear dumpers, transit mixers, and on/off-highway tractors, gross input power is 347 kW (465 hp) and gross input torque is 1986 N•m (1465 lb•ft). Typical specialty applications, which include airport and fire crash trucks and heavy equipment transporters, can have a maximum gross input of 388 kW (520 hp) and 2100 N•m (1550 lb•ft).