JohnnyO
08-20-2004, 07:10 PM
The Allison LCT 1000 automatic transmission is offered in 3/4- and 1-ton Silverado and Sierra trucks with the 8.1L gasoline and 6.6 Duramax diesel engines (fig. 1). This robust transmission offers five forward speeds and one reverse. The forward and reverse gear ratios are accomplished by five clutches.
TIP: The Allison is unique in having no bands, no sprags, and no roller clutches.
Transmission performance is controlled by six solenoids operated by the Transmission Control Module (TCM), which is linked by a Class 2 data line to the Powertrain Control Module (PCM).
Operating Characteristics
As these unique features suggest, the Allison transmission has some unique operating characteristics. A customer who is familiar with only a lighter-duty drivetrain may interpret perfectly normal operation as a cause for concern.
You can avoid unnecessary diagnosis if you can understand the reasons behind these characteristics. Here are some highlights.
Grade Braking – The primary purpose of the grade-braking feature is to utilize engine braking to slow a heavy vehicle on steep grades, in order to reduce wear on the traditional braking system. The method used to slow the vehicle is by overriding the PRNDL position, effectively pre-selecting the next lower gear range automatically.
TIP: The engine is designed to operate in the dashed red zone (fig. 2) on the tachometer but not in the solid red zone. The engine can operate in the dashed red range but cannot produce any power in this range. Do not operate the engine in the solid red zone.
When driving downhill on grades with heavy loads, the vehicle’s inertia can be great enough to increase vehicle speed and consequently engine speed without applying the throttle. To increase the availability of grade braking and provide for additional engine braking, the engine has been designed to allow operation in the dashed red zone region. Operation in this region is normal and will not damage the engine.
Refer to Service Bulletin 02-07-30-004B.
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TIP: The Allison is unique in having no bands, no sprags, and no roller clutches.
Transmission performance is controlled by six solenoids operated by the Transmission Control Module (TCM), which is linked by a Class 2 data line to the Powertrain Control Module (PCM).
Operating Characteristics
As these unique features suggest, the Allison transmission has some unique operating characteristics. A customer who is familiar with only a lighter-duty drivetrain may interpret perfectly normal operation as a cause for concern.
You can avoid unnecessary diagnosis if you can understand the reasons behind these characteristics. Here are some highlights.
Grade Braking – The primary purpose of the grade-braking feature is to utilize engine braking to slow a heavy vehicle on steep grades, in order to reduce wear on the traditional braking system. The method used to slow the vehicle is by overriding the PRNDL position, effectively pre-selecting the next lower gear range automatically.
TIP: The engine is designed to operate in the dashed red zone (fig. 2) on the tachometer but not in the solid red zone. The engine can operate in the dashed red range but cannot produce any power in this range. Do not operate the engine in the solid red zone.
When driving downhill on grades with heavy loads, the vehicle’s inertia can be great enough to increase vehicle speed and consequently engine speed without applying the throttle. To increase the availability of grade braking and provide for additional engine braking, the engine has been designed to allow operation in the dashed red zone region. Operation in this region is normal and will not damage the engine.
Refer to Service Bulletin 02-07-30-004B.
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