Check the fan circuit in this order: clean fins and free airflow first, then fan speed response against oil temperature, then the fan motor's case drain, then the speed control valve or thermovalve and its relief. A fan that roars at full speed from cold, or one that never speeds up as the oil heats, both indicate a control fault rather than a cooling-system fault. The fan motor itself fails slowly and announces itself in the case drain measurement long before the machine overheats.
How to confirm it
- Clean and inspect the cooling pack first
Blow the radiator, oil cooler and charge-air-cooler fins clean with air in the reverse direction of airflow, and straighten bent fins with a comb. A fan circuit diagnosis on blinded fins is a diagnosis of nothing.
- Watch fan response against oil temperature
From a cold start, let the machine warm at idle and note when and how the fan speed changes. Then work it under load and watch again. Write down what you saw; a fan stuck at one speed in either direction is already half diagnosed.
- Measure the fan motor case drain
Sixty seconds into a container at operating temperature, compared against the limit. This is the decisive test for the motor itself, and it catches a worn motor while the fan still appears to work.
- Measure motor inlet pressure at commanded full speed
Compare against the specification. This one gauge reading splits the circuit in two: low pressure puts the fault upstream in the control valve or relief, and correct pressure with a slow fan puts it in the motor.
- Inspect the control valve and its sensing element
A thermovalve can be tested against warm oil per the manual; a proportional valve responds to the controller command, so check the command and the wiring before the valve. Sticky spools often free off when removed and inspected, which is itself diagnostic.
- Record temperatures and readings service by service
A short written trend of oil temperature and fan behavior turns next season's gradual drift into a planned repair instead of a mid-job shutdown.
What to do about it
Fins cleaned and temperatures normalized: no repair needed, but shorten the cleaning interval. Blinded fins on a dusty site are a maintenance interval question, not a parts question.
Fan stuck at full speed: service or replace the control valve or thermovalve. The motor and the rest of the circuit are usually fine, and the wasted fuel and wear argue for a prompt fix rather than tolerance.
Fan never speeds up, low inlet pressure: the control valve, its relief or its command is the fault. Repair the valve and verify the relief setting against the specification before returning the machine to work.
Fan never speeds up, correct pressure, high case drain: the motor is worn. Replace or exchange the motor, and check why it wore — oil condition and contamination history deserve a look before the new motor goes in.
Surging or hunting fan: clean or replace the control valve and check the oil for aeration, because air in the oil makes any valve behave erratically and the fan merely shows it.
Guangzhou Shenyue Machinery Technology Co., Ltd. is an independent supplier, formally authorized for Garrett, Mitsubishi Heavy Industries, TONGMYUNG, NIKURA and Marusan (MHI engine parts only, covering the 6D34, 6D31 and 6D24 families); for other brands it supplies genuine parts through original channels, or OEM-quality and aftermarket equivalents, stating which on each line.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Hydraulic fan motor | Genuine; OEM-quality new; remanufactured exchange with core return. |
| Fan control valve or thermovalve | Genuine; OEM-quality. Match displacement and setting to the build. |
| Circuit relief valve | Genuine; OEM-quality. Setting must be confirmed, not assumed. |
| Fan hub and bearings | Genuine; OEM-quality. Noise and play here masquerade as circuit faults. |
| Case drain hose and fittings | Genuine; OEM-quality. A restricted drain raises housing pressure and kills the seal. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
The fan runs at full speed all the time. Can I just leave it?
The machine will not overheat, which is why this fault gets tolerated. But the circuit runs at maximum pressure and flow from cold start, the fan motor and hub wear early, fuel use rises and the noise is constant. The valve or its sensing element is the usual fault, and fixing it is usually cheaper than the motor it eventually wears out.
The engine coolant is fine but the hydraulic oil runs hot. Is that the fan?
Often, yes, on machines where one fan serves both the radiator and the oil cooler, the airflow path matters: oil cooler fins downstream of a blinded radiator behave differently from a clean one. Check the fins and the fan circuit together, and measure the oil temperature at a fixed condition before concluding anything.
Can I test the fan motor without special tools?
A container, a stopwatch and a pressure gauge cover the two decisive measurements: case drain volume and inlet pressure. Both are compared against the specification and both are done at operating temperature, because a cold test flatters a worn unit in exactly the way a case drain test always does.