Electrical Faults and Oil-Cooled Engine Maintenance on the JLG 400S with Deutz F3M2011

[MikePhua]

The JLG 400S and Its Role in Mid-Range Aerial Access
The JLG 400S telescopic boom lift is a mid-sized aerial platform designed for construction, maintenance, and industrial applications requiring elevated reach and mobility. With a working height of 46 feet and a horizontal outreach of over 33 feet, it balances compact transport dimensions with jobsite versatility. Since its introduction in the early 2000s, the 400S has been widely adopted across North America and Europe, often powered by the Deutz F3M2011 diesel engine—a compact, oil-cooled three-cylinder unit known for reliability and low maintenance.
JLG Industries, founded in 1969 and now part of Oshkosh Corporation, has long emphasized modular design and serviceability. However, as with any machine, electrical and fluid system faults can emerge over time, especially in older units.
Terminology Annotation

• Start Relay: An electromechanical switch that controls current flow to the starter motor
  
• Starter Control Wire: A low-voltage wire that signals the relay to engage the starter
  
• Oil-Cooled Engine: An engine that uses circulating oil as both lubricant and coolant, eliminating the need for a separate water-based cooling system
  
• Drain Cock: A valve used to release fluid from a reservoir, often located in hard-to-access areas
  

Unexpected Starter Engagement and Relay Failure
One operator reported a critical fault in a 2005 JLG 400S: after pressing the stop button from the platform, the engine appeared to shut down, but the starter motor continued to engage, eventually burning out. Even after removing the key and powering down the system, 12V remained present on the starter control wire, indicating a stuck relay or shorted circuit.
The root cause was traced to the start relay, located beneath the oil filters and covered in grime. After cleaning the relay, it reset to its default open position, restoring normal function. This highlights a common vulnerability in older machines—electrical components exposed to oil mist, dust, and vibration can fail in unpredictable ways.
Recommendations for prevention:

• Inspect and clean relays during scheduled service intervals
  
• Use dielectric grease on connectors to prevent corrosion
  
• Replace relays showing signs of heat damage or sticking
  
• Monitor starter engagement duration during shutdowns
  

Oil Drain Challenges and Misleading Dipstick Readings
Another technician encountered difficulty during an oil change on the Deutz F3M2011 engine. The drain plug, located on the side of the pan, caused oil to spill across the mounting plate, making cleanup difficult. After draining for 20 minutes and refilling with 5 quarts of synthetic oil, the dipstick showed full—but the oil turned black after only a few minutes of operation.
This behavior is typical of oil-cooled engines, where residual oil remains in the cooler and lines unless fully drained. The Deutz F3M2011 requires draining both the pan and the oil cooler to achieve a complete oil change.
Clarified oil capacities:

• Oil pan only: 10.5 quarts
  
• With filter: 11 quarts
  
• Full system including cooler: 16 quarts
  

To access the drain cock and facilitate cleaner draining, the engine module can be swung out from the machine’s center by removing a single 15/16" bolt at the rear underside. This design allows easier access to the internal drain hose, which should protrude through the lower plate.
Dipstick Confusion and Fill Point Identification
Operators have noted discrepancies between dipstick markings. Some dipsticks show only one set of hash marks, while manuals depict two—one for cold readings and one for hot. This inconsistency may stem from aftermarket replacements or production variations.
To verify oil level:

• Check dipstick after engine has run and cooled for 10 minutes
  
• Use both fill caps (top and rear) to access oil fill points
  
• Confirm dipstick part number against engine serial number
  
• Avoid mixing unknown oil types—drain and refill if uncertain
  

Synthetic 5W-40 oil is acceptable for the Deutz engine, especially in variable climates. However, mixing with unknown residual oil may compromise additive performance. If contamination is suspected, a full drain and refill is recommended.
Timing Belt and Oil Cooler Access
During timing belt replacement, the oil cooler must be removed, offering an ideal opportunity to perform a complete oil change. Technicians should:

• Replace cooler seals during reinstallation
  
• Flush cooler lines with clean oil before refilling
  
• Use a funnel to fill the cooler from upper lines to prevent air pockets
  

One field mechanic shared that skipping cooler drainage during oil changes led to premature oil degradation and increased wear on hydraulic lifters. After adopting full-system drainage, oil clarity and engine response improved noticeably.
Conclusion
The JLG 400S with Deutz F3M2011 engine presents unique challenges in electrical and fluid maintenance. From stuck start relays to incomplete oil changes, understanding the machine’s design and service logic is essential for long-term reliability. By inspecting relays, accessing hidden drain points, and verifying dipstick accuracy, operators can avoid costly failures and keep their boom lifts running smoothly. In the world of aerial access, precision matters—and so does knowing where the oil really hides.