2 hours ago
Overheating During Digging but Not Hammering
A CAT 325D excavator with approximately 4,700 operating hours began exhibiting overheating symptoms in both its engine and hydraulic systems—but only during digging, loading, and scraping operations. Interestingly, when using attachments like a hammer or pulverizer, the machine ran cool all day. This discrepancy pointed to a load-specific thermal issue rather than a general cooling failure.
Terminology Clarification
The radiator and hydraulic cooler were cleaned thoroughly using shop air and low-pressure water. Despite appearing immaculate, the overheating persisted. This led to deeper investigation into airflow and component separation. Technicians emphasized that true cleaning requires separating the radiator and cooler, as debris often lodges between them and restricts airflow.
Fan Drive and Belt Tension Issues
The CAT 325D typically uses a direct-drive fan via serpentine belt, but some units—especially those with high ambient cooling packages—feature an electrically controlled viscous coupling. These couplings are known to fail gradually, first by reducing fan speed and eventually by detaching entirely, causing catastrophic damage to the radiator cowling.
In this case, the fan hub was identified as faulty. A service letter (PS52059) from Caterpillar confirmed known issues with early viscous couplings. The manufacturer agreed to replace the fan hub, covering half the labor and parts cost due to machine age and hours.
Additionally, belt slippage was discovered. Though silent, it reduced fan speed under load. By manually pressing on the tensioner during operation, technicians observed increased fan RPM. The solution involved replacing the tensioner and belt, and wire-brushing the pulleys to restore grip.
Hydraulic Relief Pressure and Load Sensitivity
When using attachments like hammers, the machine operates below maximum hydraulic pressure. However, digging and raking push the system to its limits. If a relief valve is set too low or malfunctioning, it can cause oil to bypass prematurely, generating excess heat. Checking and adjusting relief pressures on all digging functions is essential to prevent hydraulic overheating.
Additional Checks and Recommendations
The CAT 325D’s overheating during digging was traced to a combination of restricted airflow between coolers, failing viscous fan hub, belt slippage, and possible relief valve miscalibration. While hammering operations remained cool due to lower hydraulic demand, digging exposed the system’s thermal limits. With proper cleaning, fan hub replacement, belt tension correction, and hydraulic pressure checks, the machine returned to stable operation—proving that overheating is often a layered problem requiring holistic diagnosis.
A CAT 325D excavator with approximately 4,700 operating hours began exhibiting overheating symptoms in both its engine and hydraulic systems—but only during digging, loading, and scraping operations. Interestingly, when using attachments like a hammer or pulverizer, the machine ran cool all day. This discrepancy pointed to a load-specific thermal issue rather than a general cooling failure.
Terminology Clarification
- Hydraulic Cooler: A heat exchanger that dissipates heat from hydraulic fluid, often mounted adjacent to the radiator.
- Viscous Fan Hub: A fan clutch that uses silicone fluid to regulate fan speed based on temperature, sometimes electronically controlled.
- Relief Valve: A hydraulic component that limits pressure by diverting excess flow, preventing system damage.
- High Ambient Cooling Package: An optional configuration with enhanced cooling components for machines operating in hot climates.
The radiator and hydraulic cooler were cleaned thoroughly using shop air and low-pressure water. Despite appearing immaculate, the overheating persisted. This led to deeper investigation into airflow and component separation. Technicians emphasized that true cleaning requires separating the radiator and cooler, as debris often lodges between them and restricts airflow.
Fan Drive and Belt Tension Issues
The CAT 325D typically uses a direct-drive fan via serpentine belt, but some units—especially those with high ambient cooling packages—feature an electrically controlled viscous coupling. These couplings are known to fail gradually, first by reducing fan speed and eventually by detaching entirely, causing catastrophic damage to the radiator cowling.
In this case, the fan hub was identified as faulty. A service letter (PS52059) from Caterpillar confirmed known issues with early viscous couplings. The manufacturer agreed to replace the fan hub, covering half the labor and parts cost due to machine age and hours.
Additionally, belt slippage was discovered. Though silent, it reduced fan speed under load. By manually pressing on the tensioner during operation, technicians observed increased fan RPM. The solution involved replacing the tensioner and belt, and wire-brushing the pulleys to restore grip.
Hydraulic Relief Pressure and Load Sensitivity
When using attachments like hammers, the machine operates below maximum hydraulic pressure. However, digging and raking push the system to its limits. If a relief valve is set too low or malfunctioning, it can cause oil to bypass prematurely, generating excess heat. Checking and adjusting relief pressures on all digging functions is essential to prevent hydraulic overheating.
Additional Checks and Recommendations
- Air Filter Condition: A clogged air filter restricts airflow, increasing engine load and heat. Replace or clean regularly.
- Infrared Temperature Gun: Use to compare inlet and outlet temperatures on the radiator. A small differential may indicate poor coolant flow or fan inefficiency.
- Serial Number Lookup: Confirm whether the machine has the high ambient cooling package. These units include a hydraulic-driven fan and rear-mounted cooler.
The CAT 325D’s overheating during digging was traced to a combination of restricted airflow between coolers, failing viscous fan hub, belt slippage, and possible relief valve miscalibration. While hammering operations remained cool due to lower hydraulic demand, digging exposed the system’s thermal limits. With proper cleaning, fan hub replacement, belt tension correction, and hydraulic pressure checks, the machine returned to stable operation—proving that overheating is often a layered problem requiring holistic diagnosis.
