John Deere 490D Excavator Background
The John Deere 490D hydraulic excavator was introduced in the late 1980s as part of Deere’s D-series lineup, which marked a shift toward more electronically integrated systems. Built in partnership with Hitachi, the 490D featured a 4-cylinder turbocharged diesel engine, electronically modulated hydraulic controls, and selectable operating modes—Economy and High Output. These modes adjust engine speed and hydraulic response to match jobsite demands, offering fuel savings or maximum performance as needed.
With an operating weight of approximately 27,000 pounds and a digging depth of over 19 feet, the 490D was widely used in utility trenching, site prep, and roadwork. Its blend of mechanical durability and early electronic control made it a transitional machine—reliable, but occasionally prone to electrical quirks.
Symptoms of Intermittent Idle and Mode Malfunction
Operators have reported that the machine’s idle speed fluctuates unpredictably, and the Economy and High Output mode switch behaves erratically. Sometimes the engine idles too high, other times it drops too low or surges unexpectedly. The mode switch may fail to respond or toggle modes without input.
Key symptoms include:

• Idle speed rising and falling without operator input
  
• Mode switch not engaging or switching randomly
  
• Engine stalling or racing at startup
  
• No consistent pattern to the behavior
  

• Loose or corroded ground wires: Especially those connected to the frame or control panel
  
• Damaged wiring harness: Chafed wires near the throttle actuator or under the cab
  
• Faulty mode switch: Internal wear or contamination can cause signal bounce
  
• Voltage fluctuations: Weak alternator or battery can cause control instability
  
• Moisture intrusion: Water in connectors or control boxes can short circuits
  

• Inspect all ground connections and clean to bare metal
  
• Use a multimeter to check voltage stability at the throttle actuator
  
• Wiggle test the wiring harness while monitoring idle behavior
  
• Replace the mode switch if resistance readings are inconsistent
  
• Check for stored fault codes if the machine has a diagnostic port
  

• Seal all connectors with dielectric grease
  
• Replace aging wiring harnesses with updated looms
  
• Install a surge protector or voltage regulator if alternator output is unstable
  
• Keep the control panel dry and free of debris
  
• Periodically test switch resistance and actuator response
  

A Short‑Radius Excavator With Big Ambitions
The Volvo ECR88 is part of Volvo’s short‑swing compact excavator lineup, designed to operate in confined job sites without sacrificing stability or digging power. With its tight tail swing, it’s particularly suited for urban construction, utility trenches, landscaping, and other applications where space is limited but performance still matters.
Technical Specs And Performance

• Operating weight: approximately 8,183 kg (≈ 18,040 lb) per third‑party data.
  
• Engine: Volvo D3.4, with a gross rating of ~59 hp (44 kW) at 2,100 rpm in older models.
  
• Hydraulic system: matched for both power and efficiency, with strong breakout and arm forces (based on Volvo’s design philosophy for compact machines).
  
• Dimensions: compact and maneuverable — overall width ~2.30m, boom arm reach and digging depths designed to give good coverage while minimizing collision risk.
  
• Other performance: decent swing speed (9.3 rpm on newer “D” variant) and two‑speed travel, making it both nimble and productive.
  

• Short Tail Swing: Great for tight spaces. The ECR88’s design avoids rear overhang, reducing the chance of hitting nearby obstacles.
  
• Operator Comfort: Spacious cab, slim pillars for good visibility, ergonomic controls, and proportional joysticks make extended use less fatiguing.
  
• Service Access: Key maintenance points—like filters and fluid checks—are grouped and accessible from ground level via a wide-opening engine hood.
  
• Hydraulics: The system uses a matched Volvo engine+pump design for smooth, responsive, and efficient performance.
  
• Diagnostics: For fleet users, Volvo offers its computer-based MATRIS tool and VCADS Pro software for analyzing performance data and fault codes.
  

• Utility work: Ideal for digging trenches in urban areas where larger machines can’t maneuver easily.
  
• Landscaping: The machine can work close to walls or fences without the tail swinging out and hitting nearby obstacles.
  
• Rental fleets: Because of its flexibility and ease of service, many rental companies include the ECR88 in their small‑to‑medium excavator offerings.
  
• Road construction: Particularly useful for curbside or tight road widening tasks.
  

• Fuel Consumption: While efficient for its class, smaller engines like those in the ECR88D may still run inefficiently under very heavy load, especially if not used optimally.
  
• Digging Depth: As with many compact machines, there’s a trade‑off between reach and stability; very deep or heavy digging may be slower than with a full‑size excavator.
  
• Counterweight: The heavy counterweight that gives the machine its balance also adds to transport weight, potentially increasing costs to move between sites.
  

• Follow Volvo's regular maintenance schedule for hydraulic filters and oil, especially under heavy use.
  
• Use genuine Volvo parts to maintain performance and reliability—especially for the hydraulic system.
  
• Monitor usage via diagnostic tools like MATRIS to catch performance drops or fault trends before they become serious.
  
• Inspect undercarriage roller and track wear regularly, particularly for machines working in tight or abrasive environments.
  

• Volvo ECR88 Swing Bearing – critical for boom rotation; wear here can affect smoothness or alignment.
  
• Hydraulic Pump Assembly for ECR88D – essential for maintaining proper hydraulic flow; failure here hits both digging and travel.
  
• Volvo ECR88 Left‑Hand Arm‑Rest Kit – improves operator ergonomics and comfort during long digging tasks.
  

Kobelco ED195 Blade Runner Overview
The Kobelco ED195 Blade Runner is a hybrid machine that combines the digging capabilities of a full-size excavator with the grading precision of a dozer blade. Designed for versatility in site preparation, road building, and forestry work, the ED195 features a six-way dozer blade mounted to a center pivot assembly beneath the cab. This pivot allows the blade to tilt and angle hydraulically, giving operators the ability to shape terrain with precision.
Kobelco Construction Machinery, founded in Japan in 1930, has built a reputation for hydraulic efficiency and structural durability. The ED195 was introduced in the early 2000s and quickly gained popularity in North America for its dual-function design. With an operating weight of approximately 43,000 pounds and powered by a 143-horsepower engine, the ED195 delivers strong breakout force and smooth blade control.
Center Pivot Seal Function and Failure Symptoms
The center pivot seal is a critical component that prevents hydraulic fluid from leaking out of the swivel joint that controls blade movement. Over time, this seal can degrade due to:

• Abrasive contamination from dust and debris
  
• Hydraulic pressure cycling during blade articulation
  
• Thermal expansion and contraction from operating in extreme temperatures
  

• Hydraulic fluid leaking from the pivot housing
  
• Reduced blade responsiveness or drift
  
• Visible wetness or staining around the pivot joint
  
• Increased need to top off hydraulic fluid
  

• Secure the blade in a neutral position using blocks or stands to prevent movement
  
• Disconnect hydraulic lines feeding the blade tilt and angle cylinders
  
• Remove the pivot housing bolts and carefully lift the upper blade mount
  
• Inspect the swivel joint for scoring or wear before removing the seal
  
• Extract the old seal using a seal puller or hook tool, taking care not to damage the bore
  
• Clean the housing thoroughly and install the new seal with hydraulic grease
  
• Reassemble the joint and torque bolts to factory specification
  
• Bleed the hydraulic system to remove air and restore full blade control
  

• Flush hydraulic fluid annually to remove contaminants
  
• Inspect the pivot joint monthly for signs of wear or leakage
  
• Use high-quality seals rated for dynamic hydraulic applications
  
• Avoid overloading the blade during grading, which increases pressure on the pivot
  
• Keep the joint clean by washing down after muddy or dusty operations
  

Background On The John Deere 310C
The John Deere 310C is a classic backhoe loader featuring a 65 hp (48.5 kW) 4239D four‑cylinder diesel engine and a full‑power‑shift reverser with multiple wet‑disk clutches.  It’s widely used in small construction, utility, and agricultural work. Over its production run, many operators have relied on its ruggedness, but its reverser transmission is also known to require careful maintenance and rebuilds.
The Problem: No Forward After Rebuild
A typical scenario: an owner acquires a 310C that hasn’t run in a while, then notices weak reverse and almost no forward drive. After removing the reverser, they order a full rebuild kit including the pump, and when disassembling they find the forward clutches are severely worn. They install all new friction plates, steels, seals, and bearings, flush all hydraulic lines and the cooler, then reassemble. However, once back together, the machine spins the tires in reverse but still refuses to pull forward. Disconnecting the clutch solenoid wiring doesn’t fix it.
Likely Culprit: Forward Clutch Pack Clearance
One experienced technician immediately suggests checking the forward pack clearance, and this is not guesswork: improper clearance within a clutch pack can prevent the forward clutch from engaging correctly, especially after a rebuild. If the clearance is too tight, the new friction plates may bind; if too loose, hydraulic pressure may not be adequate to clamp the pack under load.
Other Possible Causes
Beyond clutch clearance, several common issues on the 310C’s reverser system can lead to a “no forward” condition:

• Worn or damaged control valve: Even though the owner cleaned and inspected the control valve during rebuild, residual debris or internal wear may prevent correct hydraulic routing.
  
• Improper pump calibration: If the charge or pressure pump is not properly set, the system may build enough pressure for reverse (which often requires less) but fail under forward load.
  
• Incorrect fluid type or level: Using the wrong hydraulic/transmission fluid grade or neglecting correct fill procedure can lead to poor clutch apply pressure or air entrainment.
  
• Solenoid or wiring issue: Though the owner tried disconnecting the solenoid (which didn’t help), a weak solenoid, poor wiring, or intermittent electrical problem can still prevent full engagement.
  
• Torque converter or pump wear: Even with a rebuilt reverser, if the torque converter is worn or the pump support bushing (or pump drive) is out of spec, forward drive force may be compromised.
  

1. Re‑check forward clutch pack clearance
   • Remove the reverser cover if necessary.
     
   • Measure the clearance between the friction plates and steels according to Deere’s service specs. Adjust if out of tolerance.
     
2. Conduct pressure testing
   • Install a hydraulic pressure gauge on the test port for clutch apply pressure.
     
   • Compare measured pressure during forward engagement to manufacturer specification. If pressure is low, the cause may be internal leakage, worn pump, or incorrect pump preload.
     
3. Inspect the control valve again
   • Disassemble and inspect all spools for scoring, sticking, or wear.
     
   • Clean all passages and replace any o-rings or seals that look damaged or old.
     
4. Verify solenoid operation
   • Check voltage and continuity on the clutch solenoid coil.
     
   • Activate the solenoid manually (using a jumper) to confirm it functions and moves the valve.
     
5. Examine the torque converter and pump drive
   • If pressure testing is inconclusive, remove the reverser and inspect the torque converter.
     
   • Check the front bushing on the pump or support housing for wear; excessive clearance there can starve the system.
     
6. Bleed the system properly
   • After rebuild, ensure all air is removed from hydraulic and transmission lines. Trapped air can compress under load and reduce effective clutch force.
     

• When rebuilding a reverser, don’t neglect the finer details like clutch clearances — even small mistakes can render a rebuild ineffective.
  
• Use the correct hydraulic fluid. Old or incorrect fluid can damage new clutch components quickly.
  
• Plan to pressure-test after reassembly, not just rely on function check.
  
• Keep logs of each build: record clutch clearances, pressure test numbers, solenoid resistance, etc., so future diagnostics are easier.
  

Hitachi EX235 Excavator Background and System Overview
The Hitachi EX235 is a mid-sized hydraulic excavator developed in the late 1990s as part of Hitachi’s EX series, which gained global recognition for their durability, smooth hydraulic control, and operator-friendly design. Hitachi Construction Machinery, a division of the Japanese conglomerate Hitachi Ltd., has been a leader in hydraulic excavator technology since the 1960s. The EX235 was designed to compete with models like the Caterpillar 320 and Komatsu PC220, offering a balance of power, reach, and fuel efficiency.
With an operating weight of approximately 52,000 pounds and powered by a six-cylinder Isuzu diesel engine, the EX235 features a load-sensing hydraulic system, electronically modulated pump control, and a pilot-operated joystick interface. These systems work together to deliver precise implement control and efficient power distribution.
Symptoms Following Engine Overhaul
A common issue reported after an engine overhaul is that all hydraulic implements—boom, arm, bucket, and swing—become noticeably slower. This occurs even though the engine starts and runs normally. The problem is not isolated to a single function, which suggests a systemic issue rather than a localized hydraulic failure.
Key symptoms include:

• All hydraulic functions operate but at reduced speed
  
• No unusual engine noise or warning lights
  
• Engine RPM stable under load
  
• No visible hydraulic leaks or overheating
  

• Pump control solenoid wiring: Look for pinched, broken, or disconnected wires near the engine and pump
  
• Ground connections: Ensure all engine and frame grounds are clean and tight
  
• ECM connectors: Verify that all plugs are fully seated and free of corrosion
  
• Fuse and relay panels: Check for blown fuses or relays related to the hydraulic control system
  

• Label all connectors before disassembly
  
• Use protective loom and routing clips to prevent abrasion
  
• Perform a continuity test on critical circuits before startup
  
• Keep a wiring diagram on hand for reference during reassembly
  
• After reassembly, test all functions under no-load conditions before returning to service
  

What’s The Debate About Nylock Nuts On Excavator Teeth
On heavy‑equipment forums, operators often debate whether nylon‑insert locknuts (Nylock nuts) are appropriate for securing bucket teeth—especially when digging in rock. The core concern is that Nylock nuts depend on a nylon ring for locking. That ring can degrade under vibration and heat, and may not hold up to the shock loads and temperature extremes common in heavy excavation. Some users argue it's safer to use more robust locking solutions.
Why Nylock Nuts Are Popular
Nylock nuts feature a nylon (or plastic) insert that creates "prevailing torque"—meaning, as the nut is tightened, the bolt threads cut slightly into the nylon, producing resistance that helps prevent loosening.  They're widely used where vibration is an issue because of that self‑locking feature. On bucket teeth, vibration is definitely a major factor, which is probably why some people tried Nylock nuts in the first place.
Limitations And Failure Points In Heavy-Duty Use
Despite their advantages, Nylock nuts come with serious trade‑offs in rock or very abrasive applications:

• Temperature Sensitivity
  Nylon loses its locking strength at elevated temperatures. One user noted that Nylock nuts retain their locking ability only up to about 250 °F (~121 °C).  In rock digging, especially when the bucket is rubbing or grinding inside a tight trench, component surfaces may reach high temperatures (though 120 °C is likely only in very extreme or sustained conditions).
  
• Strength Grade
  According to seasoned mechanics, typical Nylock nuts used in GET (Ground Engaging Tools) hardware are often only equivalent to Grade 5 hardware.  Grade 5 means moderate strength, but in high-impact or high-side-load scenarios, they may deform or fail.
  
• Vibration and Impact
  When heavy side or impact loads hit a bucket tooth hard, a Nylock nut’s nylon insert may compress, tear, or degrade over time, particularly if the bolt is repeatedly stressed. If the fastener loosens, you risk losing a tooth.
  

• Grade-8 Locking Nuts or Stover Nuts
  These nuts are made of higher-strength steel and handle better under shock loads. As one mechanic put it, “use grade 8 crown locks … that’ll last a lot longer than Nylocks under impact.”
  
• All‑Metal Lock Nuts or Lock Washers
  Some prefer prevailing torque nuts that rely on all-metal deformation, or use lock washers, rather than nylon, for more durability in rugged environments.
  
• Proper Torque and Maintenance
  Even with the right nut, the rest of the fastening system has to be in top shape:
  • Clean mating surfaces—dirt, rust, or paint can prevent a proper torque.
    
  • Re‑torque bolts after initial use. Many pros recommend "work it a little and re‑torque."
    
  • Inspect regularly — check for side cutters or edge misalignment, which can put undue stress on the fasteners.
    

• Loose nuts usually result from improper torque, worn or mismatched GET parts, or badly prepared mating surfaces—not necessarily from wrong nut style.
  
• Cleanup of rust, dirt, and wash‑out is critical before applying locking hardware.
  

• If you’re working in rock or high-impact excavation, avoid Nylock nuts for securing teeth. Use higher-strength locknuts or locking systems designed for shock and vibration.
  
• If you choose to use Nylock nuts, make sure mating surfaces are clean, torque them correctly, and inspect often.
  
• Always keep spare locking hardware on hand—retightening or replacing fasteners during preventive maintenance is far safer than losing a bucket tooth on a job.
  
• For very high‑risk applications, consider advanced locking solutions from specialist fastener suppliers designed for mining or heavy industrial use.
  

• 3/8-16 Nylock Nut (Backhoe Bucket) – commonly used in smaller bucket assemblies
  
• 150‑Piece Zinc Nylock Nut Assortment – good for shops with multiple sizes
  
• Steel‑Core Nylock Nut Assortment – stronger construction for heavier use
  

Bobcat T200 and Deutz Engine Integration
The Bobcat T200 was introduced in the late 1990s as part of Bobcat’s push into the compact track loader market. Designed for versatility in grading, excavation, and material handling, the T200 featured a vertical lift path and a robust undercarriage suited for soft terrain. It was powered by a Deutz BF4M1011F diesel engine, a four-cylinder, air-cooled unit known for its fuel efficiency and mechanical simplicity.
Deutz AG, founded in 1864 in Cologne, Germany, has long been a leader in diesel engine technology. The BF4M1011F was widely used in compact equipment due to its compact dimensions and ability to operate without a liquid cooling system. However, its fuel system—particularly the shutoff solenoid—can be a source of confusion during troubleshooting.
Understanding the Fuel Shutoff System
The fuel shutoff solenoid is an electrically actuated valve that controls the flow of diesel to the injection pump. When energized, it opens to allow fuel delivery; when de-energized, it closes to stop the engine. On the Deutz BF4M1011F, the solenoid is typically mounted directly on the injection pump, often beneath or behind the intake manifold, making it difficult to access without removing surrounding components.
Symptoms of a faulty or disconnected solenoid include:

• Engine cranks but does not start
  
• No fuel reaching injectors
  
• Audible click absent during key-on cycle
  
• Solenoid visibly disconnected or corroded
  

• Follow the fuel line from the tank to the injection pump
  
• Look for a cylindrical component with two wires connected to a plug or terminal
  
• The solenoid may be partially obscured by intake plumbing or wiring harnesses
  

• Turn the key to the ON position and listen for a click
  
• Use a multimeter to check for 12V at the solenoid terminal
  
• If voltage is present but no click, the solenoid may be seized
  
• If no voltage is present, trace the wiring back to the ignition switch or relay
  

• Remove the solenoid from the pump
  
• Use a small tool to depress the internal plunger
  
• Reinstall the solenoid with the plunger held open
  
• Start the engine and monitor closely—this disables automatic shutdown
  

• Inspect wiring harnesses quarterly for abrasion or rodent damage
  
• Apply dielectric grease to solenoid terminals to prevent corrosion
  
• Replace solenoids every 2,000 hours or at signs of wear
  
• Use OEM parts to ensure compatibility with the Deutz injection system
  

Identifying The Issue
On the Genie / TerexLift 2506 telehandler, some operators report hydraulic oil leaking from the “negative pressure brake switch,” which is referred to in Genie’s parts book as “proximity switch #05.4329.002.” Despite the name, this component functions as a hydraulic switch that detects a certain brake circuit pressure. In the field discussion, the user wonders whether this switch is truly essential: “Can I remove it and simply plug the hole?”—especially since the machine is mostly used on flat terrain.
Role Of The Negative Pressure Brake Switch
This switch is part of the machine’s braking safety system. When the hydraulic brake pressure circuit builds up (for example, to release spring‑applied brakes), the switch detects that certain pressure threshold. According to Genie’s GTH‑2506 service manual, this brake pressure switch is a critical component to the parking brake and service brake circuits. Should it fail or leak, the switch may compromise proper functioning of the brakes or brake‑related warnings. The operator manual even lists this switch among the “safety devices,” indicating its role in system integrity.
Possible Consequences Of A Leaking Switch
A hydraulic leak at the switch can lead to several potential risks:

• Loss of brake‑release pressure, which may prevent brakes from fully disengaging.
  
• Air ingress into the braking circuit if the leak is internal or around seal areas.
  
• Gradual degradation of hydraulic fluid if the leak draws in contaminants.
  
• Hose damage or safety hazards from external leakage.
  

• This switch provides important feedback to the machine’s control logic; bypassing it may disable warning functions.
  
• If the switch fails off-line and is removed, the hydraulic circuit may no longer be sealed correctly or safety functions might be compromised.
  
• The service manual describes procedures (including bleed points and test ports) assuming the switch is present and functioning.
  
• Removing the switch but not bleeding the circuit properly could trap air, affecting brake performance.
  

1. Confirm the Leak Source
   • Clean the area around the switch thoroughly.
     
   • Run the machine through brake cycles and watch for hydraulic seepage.
     
   • Determine whether the leak is external (around the switch body) or internal (through the switch).
     
2. Check System Pressure
   • Use a hydraulic gauge on a test port to verify that the expected operating pressure is present.
     
   • Compare the pressure readings to Genie’s specified range for brake activation.
     
3. Inspect Or Replace The Switch
   • Remove the switch carefully, relieving system pressure first.
     
   • Check for damaged threads, worn o‑rings, or cracked housing.
     
   • Replace with a correct OEM part (or high‑quality equivalent) of the same pressure rating.
     
4. Bleed The Brake Circuit
   • According to Genie’s service manual, the 2506 has bleed valves designed for both the service‑brake circuit and the parking‑brake circuit.
     
   • Use a compressor and open the appropriate bleeder valve (called “valve A” in the manual) to purge air.
     
5. Verify After Repair
   • Refill fluid to correct level.
     
   • Cycle brakes repeatedly, observing for leaks and proper switch operation.
     
   • Monitor for any brake drag or warning lights during operation.
     

• Verifying switch integrity during hydraulic fluid changes.
  
• Inspecting for external leaks every few hundred hours of operation.
  
• Consulting authorized service centers if repeated leaks or failures occur.
  

Hitachi’s ZX200 Line and Its Engineering Legacy
The Hitachi ZX200 excavator, part of the Zaxis series launched in the early 2000s, represents a significant advancement in hydraulic efficiency and electronic control integration. Hitachi Construction Machinery, a division of Hitachi Ltd. founded in 1970, has long been known for its precision engineering and durable machines. The ZX200, introduced in 2006 and refined through subsequent years, became a popular choice for contractors in earthmoving, demolition, and utility trenching due to its balance of power, fuel efficiency, and operator comfort.
With an operating weight of approximately 44,000 pounds and powered by a 6-cylinder Isuzu engine producing around 150 horsepower, the ZX200 features electronically controlled fuel injection, a stepping motor for throttle modulation, and a multi-function LCD panel for diagnostics and performance monitoring.
Symptoms of Sudden Stalling After Startup
A recurring issue reported by operators involves the machine starting normally, idling slightly higher than usual, and then stalling within five seconds. The stepping motor can be heard clicking during this time, and repeated attempts to restart yield the same result. This behavior suggests a failure in either the fuel delivery system or the electronic throttle control.
Key symptoms include:

• Immediate startup followed by rapid stall
  
• Audible stepping motor activity post-stall
  
• No visible wiring damage or rodent interference
  
• No error codes displayed on the control panel
  

• Drain and inspect the fuel tank for debris
  
• Pressure wash the interior if contamination is found
  
• Replace primary and secondary fuel filters
  
• Flush fuel lines and inspect for soft or collapsed hoses
  
• Bleed the system to remove air pockets
  

• Inspect the throttle control panel for loose connectors or moisture intrusion
  
• Test voltage at the stepping motor during startup
  
• Check for software faults or calibration errors using a diagnostic tool
  
• Replace the throttle panel if internal failure is suspected
  

• Coolant level and sensor function
  
• Oil pressure at startup using a mechanical gauge
  
• Safety shutdown relay behavior
  

Background Of The JLG 40H And Its Engine Control
The JLG 40H is a self-propelled telescopic boom lift with about 40 feet of platform height, commonly used for maintenance, construction, and industrial work. In the early 2000s, many units were offered with a Deutz diesel engine, chosen for reliability, fuel efficiency, and long service life in rental fleets. Tens of thousands of JLG booms of all models have been sold worldwide, and the 40H sits in the mid-range size category that is popular for general jobsite use.
Unlike older purely mechanical throttle setups, this 40H uses an electric throttle actuator produced by Addco (sometimes written Addco/Atco) and a two-speed engine control from the platform: idle and high RPM. The operator selects engine speed with a toggle switch on the control panel, while the Addco unit physically pulls the throttle cable by means of a small DC motor driving a threaded block. Limit switches inside the Addco box define the idle and high-speed positions.
A typical symptom in this case is simple to describe but tricky to diagnose:

• The boom lift will not rev up for any function
  
• The engine remains at idle regardless of high/low speed toggle position
  
• Mechanically, the throttle cable and linkage move freely when operated by hand
  

• A DC motor with two terminals (often labeled 1 and 2)
  
• A threaded shaft and block connected to the engine throttle cable
  
• Two limit switches:
  • SW1 for low idle
    
  • SW2 for high engine speed
    
• Adjustment pegs on the threaded block that mechanically contact each switch
  

• Two relays labeled “Mid RPM” and “High RPM”
  
• A boom stowed position limit switch, often called the horizontal or pivot limit switch
  
• A high RPM toggle switch at the control station
  

• Toward SW1 for idle
  
• Toward SW2 for high speed
  

• With the relays not energized:
  • Battery positive (B+) is connected to:
    • Relay 1 terminal 87A
      
    • Relay 2 terminal 87
      
  • Battery negative (B−) is connected to:
    • Relay 1 terminal 87
      
    • Relay 2 terminal 87A
      

• One motor terminal will see positive through SW1’s normally closed (NC) contact and its common (COM) terminal
  
• The other motor terminal will see negative through SW2’s NC contact and its COM terminal
  

• As the block moves, the idle peg eventually presses SW1
  
• When SW1 is pressed, its NC contact opens and its normally open (NO) contact closes
  
• That switches the motor terminal to negative on both sides, stopping the motor precisely at idle
  

• The boom is fully lowered, closing the horizontal limit switch
  
• The high RPM toggle is turned ON
  
• The relays both receive:
  • Positive at their 86 terminals from the high RPM switch
    
  • Ground at their 85 terminals from the horizontal limit switch
    

• Each relay internally switches its common terminal (30) from 87A to 87
  
• This reverses the polarity sent to the motor through the SW1 and SW2 path
  
• Now one motor terminal becomes negative through SW1 and the other becomes positive through SW2
  

• When the high-speed peg presses SW2, its contact arrangement changes so that both motor terminals again see negative
  
• The motor stops at the defined high-throttle position
  

• The operator checked the mechanical linkage and confirmed no binding.
  
• By reversing the motor wires manually, they confirmed the DC motor could spin in both directions and pull the engine up to high RPM.
  
• However, in normal wiring configuration, the system only returned the actuator to idle whenever it was moved away from that position.
  

• The “return to idle” behavior proves the idle side of the circuit (SW1 and its path) is working.
  
• The lack of response to the high RPM command suggests:
  • The relays are not being energized together, or
    
  • The horizontal (boom stowed) limit switch is not providing ground, or
    
  • The high RPM toggle signal is not reaching relay terminal 86 on both relays.
    

1. Idle Condition
   • High RPM switch is OFF.
     
   • Boom may be anywhere, but the relays are not energized.
     
   • Positive is present at:
     • Relay 1 terminal 87A
       
     • Relay 2 terminal 87
       
   • Negative is present at:
     • Relay 1 terminal 87
       
     • Relay 2 terminal 87A
       
   • Through the NC posts of SW1 and SW2, this sends:
     • Positive to motor terminal 1
       
     • Negative to motor terminal 2
       
   • Motor runs toward SW1 until the idle peg activates it, then stops with both terminals negative.
     
2. High-Speed Command With Boom Stowed
   • Boom is fully lowered and horizontal limit switch makes.
     
   • High RPM switch is turned ON.
     
   • 12 V positive is delivered to terminal 86 on both relays.
     
   • Ground (negative) from the limit switch is present at terminal 85 of both relays.
     
   • Both relays energize and switch their commons.
     
   • The wiring effectively swaps which side of the motor sees positive and which sees negative.
     
   • Motor runs in the opposite direction, toward SW2, until the high-speed peg activates it and power is removed.
     
3. High-Speed Command Fails
   • If either 12 V at 86 or ground at 85 is missing for one or both relays:
     • The relays do not change state.
       
     • Polarity never reverses.
       
     • The motor can only move in the idle direction when the system tries to “correct.”
       

1. Confirm The Motor And Mechanicals
   • Disconnect the motor leads and briefly apply 12 V directly:
     • Positive to terminal 1, negative to terminal 2, then reverse.
       
   • Ensure the motor runs smoothly in both directions.
     
   • Confirm the threaded block moves the cable and that the cable fully returns by spring force.
     
2. Check Limit Switch Behavior
   • With a continuity tester, verify:
     • SW1 and SW2 change from NC to NO when pressed.
       
   • Inspect the adjustment pegs and ensure they physically contact each switch at the right points.
     
   • Misadjusted pegs can cause the motor to hunt or never stop.
     
3. Verify Relay Inputs
   • Locate the mid and high RPM relays, often labeled R12 and R13 in the wiring diagram.
     
   • With the key ON and high RPM switch ON and boom fully stowed:
     • Check for 12 V at terminal 86 of both relays.
       
     • Check for a good ground at terminal 85 of both relays.
       
   • If 12 V is missing:
     • Inspect the high RPM switch, wiring, and any interlocks.
       
   • If ground is missing:
     • Inspect the horizontal limit switch at the pivot end of the boom and its wiring.
       
4. Check Relay Outputs And Polarity At The Motor
   • With the system wired normally and high RPM requested:
     • Measure voltage and polarity at the motor terminals.
       
   • You should see roughly 12 V with one terminal positive and the other negative.
     
   • If you only see the same polarity as in idle mode, the relays are not changing over.
     
5. Inspect For Corrosion Or Loose Connections
   • Many throttle problems in aerial lifts are caused by:
     • Corroded relay sockets
       
     • Broken crimp connectors
       
     • Water intrusion in control boxes
       
   • Physically remove and inspect each relay and its terminals, cleaning or replacing as needed.
     

• It enables high engine speed by providing ground to the relay coils.
  
• It also influences high drive speed and high pump volume modes, acting as a safety interlock.
  

• The machine may still function at low speed, but it will never allow high RPM.
  
• Operators may misinterpret this as an engine problem while it is actually a simple interlock failure.
  

• Fail-safe behavior
  • Without relay activation, the system always drives the throttle back to idle.
    
  • Loss of power, loss of ground, or relay failure defaults the engine to low speed.
    
• Polarity reversal without special components
  • Instead of a dedicated H-bridge module, JLG used common automotive-style relays to swap polarity.
    
  • This makes field replacement easy and keeps parts costs down.
    
• Precise, adjustable stop points
  • Mechanical pegs and limit switches provide precise, repeatable idle and high-speed positions.
    
  • Adjustments can be made simply by shifting the pegs, without reprogramming any electronics.
    

• Corroded contacts
  • Relays, limit switches, and connectors exposed to years of moisture often develop high resistance or intermittent faults.
    
• Damaged wiring
  • Repeated boom articulation and vibration can break wires, especially near pivot points and control box entries.
    
• Sticking mechanical components
  • The threaded block in the Addco controller can accumulate dirt and dried grease, causing sluggish or uneven movement.
    
• Misadjusted pegs
  • After cable replacement or engine work, mechanics may not set the pegs correctly, leading to inconsistent RPM.
    

• Inspect and exercise the throttle control several times per year.
  
• Clean and protect relay contacts and connectors.
  
• Verify that the boom stowed limit switch is secure, dry, and operating reliably.
  
• Lubricate the threaded shaft and check the cable for fraying or excessive friction.
  

• A DC motor that must see reversed polarity to move in both directions
  
• Two relays that provide that polarity reversal
  
• Limit switches and a horizontal interlock that decide when high RPM is safe
  

• A bad relay
  
• A failed or misadjusted limit switch
  
• Or damaged wiring