A boom begins moving more slowly than usual. A dump body hesitates before lifting. An outrigger responds normally one moment and unpredictably the next.
Since the movement is hydraulic, the first assumption is usually that the hydraulic system is failing. The pump may be blamed for inadequate flow, the relief valve may be suspected of opening too early, or a cylinder, motor, or directional valve may need to be replaced. However, then the replacement part doesn’t solve the problem.
A hydraulic actuator may produce the symptom, but the actual fault can originate much earlier in the control chain from a joystick, sensor, connector, wiring harness, controller, safety interlock, or software setting.
Misdiagnosis can lead to unnecessary parts replacement, repeated service visits, longer downtime, and technicians chasing a problem from one side of the truck to the other. The solution is not to assume that every problem is electrical; it’s to troubleshoot the truck as an integrated electrohydraulic system.
ONE FUNCTION, SEVERAL SYSTEMS
On a traditional hydraulic system, an operator may move a mechanical lever that directly shifts a valve spool. In this case, the relationship between input and movement is relatively simple.
On an electronically controlled system, moving a joystick may begin a much longer sequence:
- The joystick generates an electrical command.
- A controller receives and interprets that command.
- The controller verifies that operating conditions and safety interlocks have been satisfied.
- The controller sends an output to a solenoid or proportional valve.
- The valve regulates hydraulic flow or pressure.
- A cylinder or motor moves the load.
- Sensors may report position, pressure, speed, or other conditions back to the controller.
A fault anywhere in that sequence can cause the same visible symptom: the hydraulic function does not move correctly. That is why the location of the symptom should not automatically determine the location of the failure.
BEGIN WITH THE SYMPTOM, NOT THE SUSPECTED PART
Effective troubleshooting starts with a precise description of what the truck is doing. “Boom is slow” is not enough. A technician needs to know:
- Is the function slow in both directions or only one?
- Does the problem affect one function or the entire machine?
- Does it occur only under load?
- Does it begin when the truck is cold or after it reaches operating temperature?
- Is it consistent or intermittent?
- Does engine speed change the behavior?
- Does the problem occur from every operator control station?
- Did it begin after a repair, software update, battery replacement, attachment change, or pressure adjustment?
These details immediately narrow the possibilities. If every hydraulic function is slow, the technician may need to investigate pump output, engine or PTO speed, fluid level, filtration, system pressure, supply voltage, or a system-wide control limitation. If only one function is affected, the problem is more likely to be in that function’s command input, wiring, valve section, plumbing, actuator, or mechanical load.
Intermittent failures that appear after vibration, moisture exposure, or chassis movement often justify close inspection of connectors, grounds, and harness routing. A fault that develops as the system heats up could be hydraulic leakage or viscosity-related, but it could also result from changing electrical resistance in a proportional valve coil.
The symptom provides evidence. Treating it as merely a reason to replace a component wastes that evidence.
LOOK FOR THE BASIC FAILURES FIRST
Before connecting diagnostic software or disassembling a valve, conduct a basic visual inspection. On the hydraulic side, check:
- Fluid level and condition;
- External leakage;
- Damaged or collapsed hoses;
- Loose fittings;
- Restricted filters;
- Unusual pump noise;
- Excessive fluid temperature;
- Bent or mechanically obstructed equipment;
- Signs that a load is binding.
On the electrical side, inspect:
- Battery condition;
- Power and ground connections;
- Blown fuses or tripped circuit protection;
- Loose, damaged, or corroded connectors;
- Bent or backed-out connector pins;
- Harnesses rubbing against brackets or sharp edges;
- Wiring exposed to exhaust heat;
- Moisture intrusion;
- Improperly repaired or spliced wiring.
Work trucks create a punishing environment for electrical components. Vibration, temperature changes, road spray, deicing chemicals, dirt, and repeated articulation can produce failures that are invisible until the harness or connector is moved. A connector can also appear secure while a terminal inside it has loosened. A wire may look intact, while several conductor strands have broken beneath the insulation.
DO NOT CONFUSE VOLTAGE WITH USABLE POWER
One of the most common electrical diagnostic mistakes is checking voltage at an unloaded connector and concluding that the circuit is healthy. A digital multimeter may show normal system voltage when the solenoid is disconnected. Once the circuit is asked to carry current, corrosion, a weak ground, a damaged conductor, or excessive resistance may cause the voltage to collapse.
Voltage-drop testing underload can reveal resistance that a basic continuity check misses. Technicians should follow the equipment and component manufacturers’ procedures and compare measurements with the appropriate electrical schematics and specifications.
Ground circuits deserve equal attention. A poor ground can create erratic behavior across multiple functions, particularly when components share a grounding point. It may also cause signals to fluctuate as other electrical loads are switched on.
UNDERSTAND HOW PROPORTIONAL VALVES BEHAVE
The electrical side of an on/off solenoid is relatively straightforward: the valve is either commanded, or it isn’t. Proportional valves are more nuanced. The controller varies current to the coil to control spool movement and, consequently, hydraulic flow or pressure.
Coil resistance increases as temperature rises. Without proper current control, which increase in resistance can reduce coil current and change valve output. This is one reason a function can behave normally when cold but become sluggish or inconsistent after extended operation.
Pulse-width-modulated outputs create another diagnostic trap. A conventional meter may display an averaged value that does not fully describe what the controller is sending. Depending on the system, technicians may need a current measurement, duty-cycle reading, oscilloscope, manufacturer service tool, or live controller data.
The correct test depends on the control architecture. Guessing based on voltage alone is unreliable.
FOLLOW THE COMMAND THROUGH THE SYSTEM
A disciplined electrohydraulic diagnosis tracks the operator’s request from input to movement.
1. Confirm the input
When the joystick or switch is moved, does the controller receive the expected command? Diagnostic software may show the joystick’s live output, including neutral position and full-range travel. On an analog system, a technician may need to measure supply, ground, and signal circuits directly.
A joystick that does not return cleanly to neutral may cause drift. A damaged sensor may produce an erratic signal. A poor reference-voltage or ground circuit can affect several inputs at once. Additionally, if a joystick has been replaced or the controller has lost its calibration, the physical movement of the control may no longer correspond correctly to the expected command.
2. Check permissives and interlocks
A valid joystick command doesn’t guarantee the controller will permit movement. Common work truck interlocks may include:
- Parking brake status;
- Transmission position;
- PTO engagement;
- Engine speed;
- Seat or operator-presence switches;
- Outrigger position;
- Body position;
- Emergency-stop circuits;
- Overload or stability systems;
- Remote-control selection;
- Fault conditions from other controllers.
A failed or misadjusted sensor may tell the controller that an unsafe condition exists. The controller may then disable the function, limit its speed, or behave as though the operator never requested movement. Before condemning the hydraulic system, verify the machine believes it’s allowed to operate.
3. Verify the controller output
If the input and permissives are correct, determine whether the controller is commanding the valve.
Check live output data, coil current, duty cycle, or voltage as appropriate. If the controller sees the operator input but does not produce an output, the problem may involve control logic, configuration, an active fault, or a failed output channel.
If the controller produces the correct command but it does not reach the coil, the problem is likely in the connector or wiring between them. If the correct command reaches the coil but the valve does not respond, attention can shift toward the coil, valve, hydraulic circuit, or mechanical load.
4. Confirm the hydraulic response
Once the electrical command has been verified, pressure and flow testing can determine whether the hydraulic system is responding appropriately.
Does pressure build when the function is commanded? Is flow reaching the valve and actuator? Is the system going over relief? Is the valve spool shifting? Is there excessive internal leakage? Is the load mechanically binding?
This is the point where pressure gauges, flow meters, temperature measurements, and hydraulic schematics become essential. The objective is to find the boundary where a correct action stops producing the expected result.
CLUES THAT POINT TOWARD AN ELECTRICAL CAUSE
No single symptom proves that a problem is electrical, but several patterns should raise suspicion:
- The failure is intermittent.
- Movement changes when the harness or connector is disturbed.
- The problem arises after rain, washing, or salt exposure.
- Multiple unrelated functions fail simultaneously.
- A function operates from one control station but not another.
- The controller displays an input but not an output.
- The system works after cycling electrical power.
- Performance changes when other electrical loads are activated.
- The valve works through an approved manual or service-tool test but not through the normal operator control.
- The malfunction began after a controller, joystick, sensor, or harness was replaced.
- Conversely, normal command signals combined with incorrect pressure or flow will direct the investigation back toward the hydraulic system.
THE MOST DIFFICULT PROBLEMS INVOLVE BOTH
Technicians should avoid treating electrical and hydraulic failures as mutually exclusive. An electrical fault can create a hydraulic problem. Low current to a proportional valve may prevent the spool from opening fully, restricting flow, and generating heat.
A hydraulic fault can also create an electrical symptom. A contaminated or sticking spool may fail to follow the controller’s command, causing a position-monitoring system to register an error. A mechanically overloaded function may trigger pressure or current limit that looks like controller intervention.
Temperature-related problems are especially deceptive. As hydraulic oil warms, its viscosity changes and internal leakage may become more apparent. At the same time, electrical resistance in coils and wiring can increase. Both sides of the system must be tested under the conditions in which the failure occurs.
AVOID THE PARTS-SWAPPING TRAP
Replacing parts until the machine begins working is not diagnosis. It is an expensive experiment, especially when a replacement temporarily disturbs a connector or harness and makes the problem disappear.
That temporary improvement can lead technicians to believe the replaced hydraulic component solved the issue when the real cause was a poor electrical connection nearby. The truck returns to service, vibration reintroduces the fault, and the entire process begins again.
Record the evidence before replacing anything:
- Fault codes;
- Live input and output values;
- Supply voltage;
- Voltage-drop measurements;
- Coil current;
- Pressure and flow readings;
- Fluid and component temperatures;
- Conditions under which the symptom occurs.
Good documentation also helps identify repeat failures across a fleet and gives engineering or technical support teams something more useful than “it still doesn’t work.”
SAFETY MUST CONTROL THE DIAGNOSTIC PROCESS
Electrohydraulic systems can move unexpectedly and may retain significantly stored energy even after the engine has been shut down. Before disconnecting components, checking resistance, manually actuating a valve, or working near raised equipment, technicians must follow the manufacturer’s service procedures and the employer’s hazardous-energy-control practices. Loads should be lowered or mechanically supported, hydraulic pressure relieved, and electrical energy isolated as required.
Manual overrides and forced outputs are diagnostic tools, but they can bypass normal interlocks. They should only be used by qualified personnel, in a controlled area, and in accordance with the equipment manufacturer’s instructions.
The bottom line is that modern work trucks are integrated machines. Their troubleshooting process must be integrated as well. When a persistent performance problem crosses the boundary between hydraulics and electronics, working with a partner, like Bailey International, that understands both sides of the system can help shorten the diagnostic process and prevent unnecessary component replacement.
about the author
Robin Cooper is the sales application engineer at Bailey International.
