621K OEM Wheel Tractor-Scraper Steering System Caterpillar


Steering System
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1.1. Series 620K Steering System Overview
2.2. Main Steering System
3.2. Secondary Steering System
4.1. Steering System Troubleshooting
5.2. The Steering Metering Pump Makes Noise And The Steering Cylinder Rods Do Not Move Freely
6.2. More Than Normal Force Is Required To Turn The Steering Wheel
7.2. The Machine Does Not Turn When The Steering Wheel Is Turned
8.2. The Machine Turns Slowly In Both Directions
9.2. The Machine Turns Too Slowly In One Direction
10.2. The Steering Wheel Does Not Return To The Center Position Correctly
11.2. Oil Temperature Is Too Hot
12.2. The Steering Wheel Turns Without Operator Input
13.2. The Machine Will Not Stay In A Straight Line
14.2. Operation Of The Hand Metering Unit Is Erratic
15.2. The Steering Wheel Has Excessive Free Play
16.2. The Hand Metering Unit Locks Up
17.2. The Machine Turns In The Wrong Direction When The Operator Turns The Steering Wheel
18.2. The Steering Wheel Kicks At The Start Or At The End Of The Steering Stroke.
19.2. Severe Vibration Occurs When The Steering Is Activated
20.2. The Steering Wheel Or The Steered Wheels Move Unexpectedly

Series 620K Steering System Overview

The steering system for the Series 620K of Wheel Tractor OEM includes an electric drive pump to provide secondary steering. A pressure sensor has been added to the secondary steering pump discharge in addition to the primary steering pressure sensor. The additional pressure sensors allow for failure detection of both the primary and secondary steering pumps. Test ports have been added for the Hand Metering Unit crossover relief valve locations to improve serviceability. Service technicians no longer need to disconnect hoses to test and/or troubleshoot the crossover relief valve in the steering valve.

Main Steering System

The steering system for the Series 620K machines involves the following components:

  • High-pressure load sense pump.

  • Steering control valve.

  • Steering cylinders.

  • Servo follow-up cylinder.

  • Hand Metering unit (HMU).

  • Secondary steering electric pump.

  • Primary and secondary steering pressure sensors.

  • Primary and secondary steering check valves.

The oil for the steering system is sent from the steering section of the hydraulic pump through the screen group to the steering control valve. Part of the oil flows through the steering control valve and into the check and relief valve. The oil that flows into the check valve and relief valve charges the servo pilot system. This oil also supplies makeup oil for the servo pilot system.

When the hydraulic oil leaves the steering control valve, an orifice controls the oil flow that returns to the hydraulic tank and return filter. The orifice provides a return back pressure that allows oil to be directed to the hydraulic oil cooler. The oil that flows through the oil cooler is directed to a common pump suction mounted to the tank. The remainder of the oil flow returns to the hydraulic tank through a return filter.

Secondary Steering System

The secondary steering system for the Series 620H machines involves the following components:

  • Primary steering pump (standard).

  • Primary steering pressure sensor (standard).

  • Primary steering check valve (attachment).

  • Secondary steering pump (attachment).

  • Secondary steering pressure sensor (standard).

  • Secondary steering check valve (attachment).

The electric motor of the secondary steering system supplies hydraulic oil to the steering system when the engine stops or the primary steering pump fails. The secondary steering system is an optional attachment for the machine configuration.

The primary pump has a dedicated pressure sensor downstream of the pump discharge port. The pressure sensor sends a signal to the control module to indicate that the primary steering pump is detected. The primary and secondary steering check valves isolate the two systems so hydraulic oil cannot flow from one pump to the other pump.

If the engine stops or the primary steering pump fails, the primary steering pump will no longer provide hydraulic oil flow. The electric motor is activated for secondary steering pump flow. When the electric motor is activated, hydraulic oil flow is provided to the steering system providing the machine with steering functionality.

The secondary steering pressure sensor is used to detect a failure of the secondary steering pump. A switch is located in the cab that is used to perform a functional test of the secondary steering pump. When the secondary steering test switch in the cab is activated, a signal is sent to the electric motor that operates the secondary steering pump to provide hydraulic oil flow to the steering system. When the secondary steering pump provides hydraulic oil flow the secondary steering pressure sensor detects the secondary steering pressure. When the secondary steering pressure sensor detects hydraulic pressure, a signal is sent to the ECM that the secondary steering pump is providing hydraulic oil flow properly.

Steering System Troubleshooting

Note: Many of the following problems can create diagnostic codes or event codes in Cat® ET. These codes can help find the root cause of the problem. Cat ET should be used to retrieve the logged codes and active codes.

The Steering Metering Pump Makes Noise And The Steering Cylinder Rods Do Not Move Freely

  • There is a loose connection of the oil line on the inlet side of the pump.

  • The oil level in the hydraulic oil tank is low.

  • The viscosity of the oil is incorrect.

  • The pressure and flow compensator valve is adjusted too low.

  • There is air in the steering hydraulic circuit.

  • The pump has too much wear.

  • The suction screen in the tank is blocked.

More Than Normal Force Is Required To Turn The Steering Wheel

  • The oil level in the tank is low.

  • The oil is cold.

  • The steering cylinders have failed.

  • The suction screen in the hydraulic oil tank is blocked.

  • The margin pressure setting on the steering pump is too low.

  • The pressure cutoff valve is adjusted too low.

  • Oil pressure from the steering metering pump is low. The pump is worn. Refer to "The Machine Turns Slowly In Both Directions".

  • The steering metering pump is damaged.

  • The operation of the relief valves is not correct.

The Machine Does Not Turn When The Steering Wheel Is Turned

  • The steering frame lock is installed.

  • The oil level in the tank is low.

  • The hydraulic system is contaminated.

  • Leaks can occur at these locations: seals in the steering cylinder pistons, the accessory valve between the cylinder lines and the ports.

  • The operation of the steering metering valve is not correct.

  • The metering section of the hand metering unit has a lack of oil.

  • No oil flow going to the hand metering unit.

  • Air is in the steering system. Refer to "Operation Of The Hand Metering Unit Is Erratic".

  • The steering metering pump operation is not correct. Refer to "The Machine Will Not Stay In A Straight Line".

  • There is a restriction in the steering linkage. Refer to "The Machine Turns Slowly In Both Directions".

  • The operation of the relief valves in the steering metering pump or the anti-cavitation valves in the steering metering pump are not correct. Refer to "The Machine Will Not Stay In A Straight Line".

  • The steering column upper shaft is loose or damaged.

  • The lower splines on the steering column may be disengaged or broken.

  • The axle differential lock is engaged.

The Machine Turns Slowly In Both Directions

  • The operation of the steering metering pump is not correct.

  • The oil flow from the piston pump is low.

  • The margin pressure setting on the steering pump is too low.

  • The seals in the steering cylinders leak.

  • The operation of the relief valves in the steering metering pump is not correct. Refer to "The Machine Will Not Stay In A Straight Line".

The Machine Turns Too Slowly In One Direction

  • The steering linkage is worn or damaged.

  • A seal in the steering cylinder leaks.

  • There is a restriction in the steering metering valve. Refer to "The Machine Will Not Stay In A Straight Line".

  • The operation of the crossover relief valve in the steering metering pump is not correct. Refer to "The Machine Will Not Stay In A Straight Line".

The Steering Wheel Does Not Return To The Center Position Correctly

  • The pressure in the tank line is high.

  • The valve spool in the steering metering valve has a restriction.

  • The centering springs in the steering metering pump are worn.

  • Contamination is in the steering metering pump. Refer to "The Steering Wheel Turns Without Operator Input".

  • The steering column is not in the correct alignment with the steering metering pump.

Oil Temperature Is Too Hot

  • The viscosity of the oil is incorrect.

  • There is a restriction in the hydraulic lines.

  • Air is mixed with the oil. Refer to "Operation Of The Hand Metering Unit Is Erratic".

The Steering Wheel Turns Without Operator Input

  • The drive timing in the steering metering pump is not correct. The pin must align with the valley of the internal gear in the steering metering pump.

  • The centering spring in the steering metering pump is broken.

  • Due to foreign material, the sleeve valves in the steering metering pump stick open.

  • The steering column is not in the correct alignment with the steering control unit.

The Machine Will Not Stay In A Straight Line

  • The cylinder rod is bent.

  • The steering cylinder piston is loose.

  • The cylinder relief valves or the anti-cavitation valves are leaking.

  • The hand metering unit is worn.

  • Air is in the hydraulic system. Refer to "Operation Of The Hand Metering Unit Is Erratic".

  • The steering linkage is worn or damaged.

  • The steering cylinder pins are worn or broken.

  • The axle differential lock is engaged.

Operation Of The Hand Metering Unit Is Erratic

  • Air is in the hydraulic system.

  • A steering cylinder piston is loose.

  • The flow control spool is sticking.

The Steering Wheel Has Excessive Free Play

  • The steering wheel nut is loose.

  • The shaft of the steering column is worn or damaged.

The Hand Metering Unit Locks Up

  • Insufficient hydraulic power is being received from the steering piston pump.

  • Large contamination particles are in the metering section of the hand metering unit.

  • The pin between the spool and the sleeve is severely worn or broken.

The Machine Turns In The Wrong Direction When The Operator Turns The Steering Wheel

  • The hydraulic lines to the steering cylinders are connected to the wrong ports on the hand metering unit.

The Steering Wheel Kicks At The Start Or At The End Of The Steering Stroke.

  • The inlet check valve on the steering control unit is missing or damaged.

Severe Vibration Occurs When The Steering Is Activated

  • Air is in the hydraulic system.

  • The compensator valve in the pump is not working correctly.

  • Standby pressure is too low.

The Steering Wheel Or The Steered Wheels Move Unexpectedly

  • The steering column is not correctly aligned.

  • The back pressure from the hydraulic tank is excessive. Back pressure should not exceed 1050 kPa (152 psi).

  • Hoses to the hand metering unit are not connected correctly.

Information System:

621K OEM Wheel Tractor-Scraper Event Codes
621K OEM Wheel Tractor-Scraper Determining Diagnostic Trouble Codes
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D5R2 and D6R2 Track-Type Tractors Fuel Tank Overfills (Fast Fill Fuel)
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631K and 637K Wheel Tractor-Scraper Systems General Information (Steering System)
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D5 Electrical & Hydraulic Troubleshooting ECM Software - Install
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