Descrição
A gear pump in a fan circuit has one job and no intelligence. It takes engine rotation and delivers oil volume. When it wears, the volume drops, and the fan motor downstream simply cannot reach speed no matter how healthy it is. That is why a machine with a tired fan pump and a brand new fan motor still runs hot, and why the pair should be thought about together rather than one at a time.
What this part is
Caterpillar lists 200-3406 as a ten-tooth basic gear pump, a fixed displacement external gear unit. It is driven from the hydraulic pump train on the 3126B engined machines, and its output feeds the fan motor, catalogued as 179-9778 with the alternate reference 20R-0121. Both parts sit within the hydraulic cooler group on the machine, which is why a single heat complaint can point at either of them.
Fitment
CAT 322C, 322C FM, 322C L and LN, 325C, 325C FM, 325C L, 325C LN, 325C CR and LCR, and the M325C MH material handler. Caterpillar’s parts literature lists the machine serial prefixes for this family as EMR, MAR, AMH, BKW, JLC, CRB, DTF, MSG, PAN, BYL, M2H, C3R, HEK and DAA, with the 3126 and 3126B engines. Engine numbers matter more than model badges on machines of this age, because a 325C built late in production can carry a different pump group than an early one. Quote the prefix from your plate.
Pump or motor: how to tell which one failed
This is the question we get asked most, and there is a practical way to separate the two without special tools.
- Noise before heat. A worn gear pump whines or growls as it cavitates, and the sound changes with engine rpm. A fan motor usually fails quietly. If the operator heard something new before the temperature climbed, start with the pump.
- Fan speed at high idle versus low idle. A pump losing volume will often still turn the fan at low speed but never reach full speed under thermal demand. A motor with internal bypass tends to let the fan freewheel and heat the oil as it passes flow straight to tank.
- Feel the drain. Pull the case drain line from the motor and crank through. A large, gritty or continuous flow points at the motor. A thin, clean stream with the fan still slow points at the pump or its supply.
- Check the supply side first. A blocked suction strainer or a low tank level starves a gear pump and makes it look dead. Verify the obvious before buying parts.
- Look at the shaft seal. Fresh oil on the pump body around the shaft usually means the circuit ran hot or the drain was restricted, and the replacement will do the same thing unless the cause is removed.
The full test sequence, with gauge connections, is in our hydraulic fan pump diagnosis guide, and the motor side is covered in the fan motor diagnosis article.
Machine context
The 325C is a machine of about 26,900 kg standard undercarriage and 28,600 kg long undercarriage, with 140 kW (188 hp) net flywheel power to ISO 9249, main implement flow around 235 L/min and a 36 L/min pilot circuit at roughly 4,120 kPa. Its hydraulic system, including tank, holds about 30 litres of cooling capacity in the radiator circuit. Those figures are useful when you are judging whether a cooling complaint is plausible: a machine of this size with a clean core and a working fan drive should hold temperature in normal ambient conditions.
Fitting notes
Match the shaft form, the pilot diameter, the mounting flange and the port thread against the old pump rather than against a photograph. Prime the pump and the suction line with clean oil before cranking; a dry gear pump will cavitate and score its own end covers in seconds. Fit a new return filter element, and check the suction strainer while the tank is open. Confirm rotation direction before the guard goes back on.
On cleanliness, the practical target for piston components in the same circuit is around ISO 4406 18/16/13 on Bosch Rexroth’s mobile hydraulics guidance. If the old pump was shedding material, flush before you fit, and read the seal kit rebuild scope checklist on what a proper hydraulic repair includes.
The mating motor is on our 179-9778 fan motor page, and the rest of the CAT cooling drive range sits under Caterpillar hydraulic fan motor.
Terms
Minimum order one piece. New aftermarket units carry a three to six month warranty as stated on your proforma invoice. Each pump is inspected, photographed against your order and packed in a wooden case, with delivery 5 to 8 working days from payment subject to stock. Shipping by sea or air for volume, or DHL, FedEx, TNT and EMS for single units. Payment by L/C, T/T, PayPal or Western Union.
