Hydraulic Oil Temperature Above Upper Limit

Common Symptoms

• Platform LCD displays fault code 4291-15 and may scroll a temperature warning message
• Machine slows down or stops boom and drive functions mid-cycle
• Hydraulic pump runs but lift or drive response feels sluggish or spongy
• You notice a burnt oil smell coming from the hydraulic reservoir area
• Ground control panel shows a fault indicator light or audible alarm
• Machine may allow limited ground functions but restricts platform controls
• Hydraulic oil sight glass shows oil that looks dark or discolored compared to new fluid

Probable Causes (Ranked by Likelihood)

• Low hydraulic oil level causing the pump to cavitate and generate excess heat Very Likely
• Clogged or damaged hydraulic oil cooler or cooling fins restricting heat dissipation Very Likely
• Hydraulic oil cooler fan not running or running at reduced speed due to motor or wiring fault Likely
• Hydraulic oil that is degraded, contaminated, or wrong viscosity grade for operating conditions Likely
• Continuous heavy-duty cycle operation in high ambient temperature without cooldown intervals Possible
• Internal hydraulic system bypass or relief valve stuck open causing constant pump recirculation and heat buildup Possible
• Hydraulic oil temperature sensor or sender unit out of calibration or failing, giving a false high reading Less Likely

Step-by-Step Diagnostic Procedure

1. Start by shutting the machine down and letting it cool for at least 20 minutes. Check the hydraulic oil level at the reservoir sight glass or dipstick. Oil should be within the marked range. Low oil is the most common cause and is a free fix.

2. Inspect the hydraulic oil cooler core and fins for packed dirt, mud, or debris. On scissor lifts this is often under the chassis. On boom lifts check the engine bay area. Use compressed air to blow out the cooler from the clean side. A clogged cooler will not dissipate heat no matter how good the oil is.

3. With the machine at operating temperature and the engine running, check that the hydraulic cooler fan is spinning and moving air across the cooler. If the fan is not turning, check the fan motor fuse and wiring connector first before condemning the motor.

4. Pull a small oil sample from the reservoir drain or sample port and check the oil condition. New hydraulic oil is clear amber. Dark brown or black oil with a burnt smell needs to be changed. Confirm you are running the correct viscosity spec listed on the JLG decal on the reservoir, typically AW46 or equivalent.

5. Using a non-contact infrared thermometer, measure the hydraulic reservoir surface temperature after 15-20 minutes of normal operation. JLG specifies a normal operating range up to approximately 180 degrees F (82 degrees C). Readings consistently above 200 degrees F (93 degrees C) confirm a real overheat condition rather than a sensor fault.

6. Connect JLG Analyzer software via the data link at the ground controls to read the live temperature sensor value from the ADE controller. Compare that reading to your infrared thermometer reading. If the sensor reads 230 degrees F but your infrared shows 150 degrees F, the temperature sender itself is the likely fault.

7. Inspect the hydraulic system relief valve setting if the previous steps are all normal. A relief valve stuck open or set too low causes the pump to continuously bypass flow back to tank as heat. This step requires a hydraulic pressure gauge and is considered advanced work. Call a JLG-authorized service shop if you reach this point.

8. Clear the fault using JLG Analyzer software or EZcal after repairs are made. If the code returns within one work cycle, do not continue operating. Continued use with genuine overheat accelerates pump seal failure and can contaminate the entire hydraulic circuit.

Frequently Asked Questions