JLG 40H Throttle Control Troubleshooting

[MikePhua]

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
  

Yet the electric speed control fights any attempt to move away from idle, and whenever the threaded shaft is adjusted away from the idle limit switch, the control drives it right back to idle.
Layout Of The Addco Throttle Control System
The Addco throttle actuator on the JLG 40H contains:

• 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
  

Outside the Addco box, JLG added:

• 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
  

This forms an electric polarity-reversal circuit. The DC motor always turns one way or the other depending on how battery positive and negative are applied to its two posts. The relays, together with the limit switches, decide when and in which direction it should move:

• Toward SW1 for idle
  
• Toward SW2 for high speed
  

As a safety measure, high engine speed is only allowed when the boom is in the stowed position and the horizontal limit switch is made.
Funky But Logical Relay Circuit
The throttle circuit can look confusing at first, but its logic is straightforward if you break it down:

• 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
      

This arrangement means:

• 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
  

In that condition, the motor spins in a direction that pulls the threaded block toward SW1, the idle side:

• 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
  

Thus, without any relay activation, the system always “returns to idle.”
When the machine is ready to allow high speed:

• 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
    

Both relays energize at the same time. When that happens:

• 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
  

The motor reverses direction and the threaded block travels toward SW2, the high-speed side:

• 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 two pegs on the threaded block act as mechanical calibration points, defining the physical positions for low and high RPM.
Symptom Analysis Engine Will Not Rev Beyond Idle
On the problematic machine:

• 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.
  

This behavior points directly at the relay and limit-switch circuit, not the motor or cable:

• 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.
    

In effect, the system stays in its default state, which is “go to idle and stay there.”
Detailed Relay And Limit Switch Operation
To understand where to test with a voltmeter, it helps to trace the logic step by step.

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.”
       

This is why, in the real case, the motor would only behave correctly when the technician forcibly swapped the motor wires.
Systematic Troubleshooting Strategy
For a technician facing a JLG 40H throttle that won’t rev, a structured method will save a lot of time:

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.
     

Role Of The Boom Stowed Limit Switch
One key point in this system is the horizontal limit switch at the boom pivot. This switch does more than just tell the machine the boom is down:

• 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.
  

If the switch fails open or its wiring is damaged:

• 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.
  

Because JLG and other manufacturers care about stability and safety, they prevent high-speed operation when the boom is raised, so diagnosing the throttle system always involves verifying the status of that limit switch.
Why The System Is Designed This Way
At first glance, using two relays and multiple limit switches to control a small DC motor might seem overly complex. However, the design reflects several goals:

• 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.
    

Similar concepts are used in many aerial lifts, telehandlers, and industrial machines: a motorized throttle or pump swashplate positioner driven by relays and limit switches.
Preventive Maintenance For Throttle Controls On Older Booms
Although the throttle circuit itself is simple, several age-related issues can lead to symptoms like the 40H’s stuck idle:

• 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.
    

A simple maintenance checklist can avoid many failures:

• 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.
  

Conclusion
The case of the 2000 JLG 40H with a Deutz engine and Addco speed control that would not rev above idle illustrates how a seemingly mysterious electronic problem often comes down to a few basic elements:

• 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
  

By following the wiring logic, testing relay power and grounds at terminals 85 and 86, confirming the boom stowed limit switch, and observing voltage at the motor terminals, a technician can quickly isolate whether the issue is:

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

Once the true cause is corrected, the Addco throttle resumes its proper behavior, smoothly shifting between idle and high speed at the flick of a switch, and the JLG 40H returns to being a reliable workhorse on the jobsite rather than a puzzling idle-only machine.