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The Akerman H12BLC and Its Hydraulic Steering System
The Akerman H12BLC is a Swedish-built tracked excavator known for its robust mechanical design and straightforward hydraulic systems. Manufactured during the late 1980s and early 1990s by Akerman, later acquired by Volvo Construction Equipment, the H12BLC was engineered for reliability in forestry, roadwork, and general excavation. Its travel system relies on dual track motors controlled by a combination of foot pedals and a hand-operated steering lever, allowing for forward, reverse, and pivot-in-place maneuvers.
Unlike modern excavators with electronically modulated travel controls, the H12BLC uses servo pressure routed through mechanical valves to control track speed and direction. Steering is achieved by selectively reducing or halting flow to one track motor, allowing the other to drive the machine into a turn.
Symptoms of Steering Failure Despite Normal Travel
A puzzling issue arises when the machine moves forward and backward with full power but refuses to turn left or right. The steering lever slows the machine as expected, indicating partial engagement, but no directional change occurs. Pressure tests show correct control pressure reaching the travel valves, and the machine responds to all pedal inputs. Yet, when attempting a pivot turn or lifting one track to isolate movement, both tracks continue to drive in sync.
This behavior suggests that the machine is locked in a “series” travel mode—where both track motors receive flow in tandem—preventing differential movement required for turning.
The Role of the Series/Parallel Valve and Solenoid Control
At the heart of the issue is a hydraulic valve known as the series/parallel selector (identified as valve 6a in Akerman schematics). This valve determines whether the track motors operate in series (high-speed travel) or in parallel (low-speed with steering capability). In series mode, oil flows through one motor and into the other, forcing both tracks to move identically. In parallel mode, each motor receives independent flow, enabling steering.
The transition between modes is controlled by a solenoid valve (14a), which receives an electrical signal from the steering lever switch. When the lever is moved, the solenoid should activate, shifting valve 6a into parallel mode. If the solenoid fails, or the signal is interrupted by a blown fuse, broken wire, or faulty switch, the valve remains in series mode—locking the tracks together and disabling steering.
Diagnostic Steps and Pressure Testing
To confirm the fault:
• Inspect the fuse panel for blown fuses related to travel or steering circuits
• Test the steering lever switch for continuity and voltage output
• Locate solenoid 14a on the main valve chest and verify power delivery when the lever is engaged
• Check for manual override switches or travel speed selectors that may affect valve 6a
• Observe track behavior during pivot attempts and raised-track tests
In one case, replacing a blown fuse temporarily restored steering, confirming the electrical nature of the fault. However, the issue recurred, suggesting an intermittent short or failing solenoid coil.
Understanding the Hydraulic Logic and Brake Interaction
The H12BLC uses spring-applied, pressure-released track brakes. These brakes disengage when servo pressure exceeds 65 bar, typically triggered by pedal movement. Since forward and reverse travel remain unaffected, the brakes are likely functioning correctly. The steering issue is isolated to the flow control logic between the steering lever, solenoid valve, and series/parallel selector.
Valve 6a is mounted beneath the central swivel joint, with a small activation port visible from the cab side. If the valve spool is stuck due to contamination or wear, it may fail to shift even when electrically commanded. Cleaning or rebuilding the valve may be necessary if electrical tests confirm proper solenoid function.
Manufacturer History and System Philosophy
Akerman’s design philosophy emphasized mechanical simplicity and hydraulic efficiency. The series travel mode was intended to prevent high-speed turning, which could damage tracks and undercarriage components. The system automatically shifted to parallel mode during steering to reduce speed and allow differential track movement.
Volvo’s acquisition of Akerman in the mid-1990s led to the integration of more electronic controls and diagnostic systems in later models. However, legacy machines like the H12BLC remain popular in forestry and rural excavation due to their durability and field-serviceable components.
Solutions and Recommendations
To restore steering:
• Replace damaged fuses and test for recurring shorts
• Clean or replace the steering lever micro switch
• Verify voltage at solenoid 14a during lever engagement
• Manually test solenoid coil resistance and function
• Inspect valve 6a for spool movement and contamination
• Confirm that the travel speed selector is set to low-speed mode
If electrical diagnostics are inconclusive, consider bypassing the steering lever switch with a direct toggle to test solenoid response. Always document wiring changes and restore factory configurations after testing.
Conclusion
When an Akerman H12BLC excavator refuses to turn despite normal forward and reverse travel, the issue often lies in the series/parallel valve control circuit. A failed solenoid, broken wire, or stuck spool can lock the machine in high-speed mode, disabling steering. By understanding the hydraulic logic and electrical triggers, operators can methodically diagnose and resolve the fault—restoring full maneuverability to a machine built for rugged terrain and precise control.
The Akerman H12BLC is a Swedish-built tracked excavator known for its robust mechanical design and straightforward hydraulic systems. Manufactured during the late 1980s and early 1990s by Akerman, later acquired by Volvo Construction Equipment, the H12BLC was engineered for reliability in forestry, roadwork, and general excavation. Its travel system relies on dual track motors controlled by a combination of foot pedals and a hand-operated steering lever, allowing for forward, reverse, and pivot-in-place maneuvers.
Unlike modern excavators with electronically modulated travel controls, the H12BLC uses servo pressure routed through mechanical valves to control track speed and direction. Steering is achieved by selectively reducing or halting flow to one track motor, allowing the other to drive the machine into a turn.
Symptoms of Steering Failure Despite Normal Travel
A puzzling issue arises when the machine moves forward and backward with full power but refuses to turn left or right. The steering lever slows the machine as expected, indicating partial engagement, but no directional change occurs. Pressure tests show correct control pressure reaching the travel valves, and the machine responds to all pedal inputs. Yet, when attempting a pivot turn or lifting one track to isolate movement, both tracks continue to drive in sync.
This behavior suggests that the machine is locked in a “series” travel mode—where both track motors receive flow in tandem—preventing differential movement required for turning.
The Role of the Series/Parallel Valve and Solenoid Control
At the heart of the issue is a hydraulic valve known as the series/parallel selector (identified as valve 6a in Akerman schematics). This valve determines whether the track motors operate in series (high-speed travel) or in parallel (low-speed with steering capability). In series mode, oil flows through one motor and into the other, forcing both tracks to move identically. In parallel mode, each motor receives independent flow, enabling steering.
The transition between modes is controlled by a solenoid valve (14a), which receives an electrical signal from the steering lever switch. When the lever is moved, the solenoid should activate, shifting valve 6a into parallel mode. If the solenoid fails, or the signal is interrupted by a blown fuse, broken wire, or faulty switch, the valve remains in series mode—locking the tracks together and disabling steering.
Diagnostic Steps and Pressure Testing
To confirm the fault:
• Inspect the fuse panel for blown fuses related to travel or steering circuits
• Test the steering lever switch for continuity and voltage output
• Locate solenoid 14a on the main valve chest and verify power delivery when the lever is engaged
• Check for manual override switches or travel speed selectors that may affect valve 6a
• Observe track behavior during pivot attempts and raised-track tests
In one case, replacing a blown fuse temporarily restored steering, confirming the electrical nature of the fault. However, the issue recurred, suggesting an intermittent short or failing solenoid coil.
Understanding the Hydraulic Logic and Brake Interaction
The H12BLC uses spring-applied, pressure-released track brakes. These brakes disengage when servo pressure exceeds 65 bar, typically triggered by pedal movement. Since forward and reverse travel remain unaffected, the brakes are likely functioning correctly. The steering issue is isolated to the flow control logic between the steering lever, solenoid valve, and series/parallel selector.
Valve 6a is mounted beneath the central swivel joint, with a small activation port visible from the cab side. If the valve spool is stuck due to contamination or wear, it may fail to shift even when electrically commanded. Cleaning or rebuilding the valve may be necessary if electrical tests confirm proper solenoid function.
Manufacturer History and System Philosophy
Akerman’s design philosophy emphasized mechanical simplicity and hydraulic efficiency. The series travel mode was intended to prevent high-speed turning, which could damage tracks and undercarriage components. The system automatically shifted to parallel mode during steering to reduce speed and allow differential track movement.
Volvo’s acquisition of Akerman in the mid-1990s led to the integration of more electronic controls and diagnostic systems in later models. However, legacy machines like the H12BLC remain popular in forestry and rural excavation due to their durability and field-serviceable components.
Solutions and Recommendations
To restore steering:
• Replace damaged fuses and test for recurring shorts
• Clean or replace the steering lever micro switch
• Verify voltage at solenoid 14a during lever engagement
• Manually test solenoid coil resistance and function
• Inspect valve 6a for spool movement and contamination
• Confirm that the travel speed selector is set to low-speed mode
If electrical diagnostics are inconclusive, consider bypassing the steering lever switch with a direct toggle to test solenoid response. Always document wiring changes and restore factory configurations after testing.
Conclusion
When an Akerman H12BLC excavator refuses to turn despite normal forward and reverse travel, the issue often lies in the series/parallel valve control circuit. A failed solenoid, broken wire, or stuck spool can lock the machine in high-speed mode, disabling steering. By understanding the hydraulic logic and electrical triggers, operators can methodically diagnose and resolve the fault—restoring full maneuverability to a machine built for rugged terrain and precise control.
