The main pump on a CAT 336 is a tandem variable-displacement axial piston pump: two pumping sections in one housing, driven off the engine flywheel through a coupling, with a gear-type pilot pump on the back. It is the most expensive hydraulic component on the machine, it weighs several hundred pounds, and a slow or weak excavator is more often a regulator, relief, or contamination problem than a worn pump. Here are the tests that prove the pump is at fault, then the replacement step by step.
Before You Order a Pump: Prove It
Slow cycles, weak digging, a whining pump, and hot oil are shared between the pump, the main reliefs, the pump regulator and its solenoids, a bypassing cylinder or swing motor, and a plugged suction. Run these tests with the machine at operating temperature.
| Test | What a healthy 336 shows | What a bad result points to |
|---|---|---|
| Main relief pressure at the pump test ports, functions stalled | Both sections reach the relief setting | Low on both: relief or regulator first. Low on one: that section or its control |
| Pilot pressure | Steady at spec under operation | Low pilot pressure slows every function: pilot pump, pilot relief, or pilot filter |
| Flow at rated RPM with a loading valve | Flow holds up as pressure rises to the relief setting | Flow falls off sharply under load: internal leakage in the pump |
| Case drain flow | A small, steady flow, not much hotter than the tank | High or hot case drain: worn barrel, valve plate, pistons, or shoes |
| Regulator and solenoid response | Displacement follows the command; no pump control codes | A stuck regulator spool imitates a bad pump |
| Oil sample and filter cut-open | ISO 4406 near 18/16/13, no bright metal | Metal in the filter: the pump or something upstream is shedding |
Fails flow or case drain with wear metal in the sample: it is the pump. Fails pressure with good flow: relief or regulator. Dirty sample: find where the metal came from first, because a new pump installed into a contaminated system fails in weeks.
Tools and Parts
Line wrenches, torque wrenches from ORFS sizes through the main mounting bolts, a crane rated well above the pump's weight with slings and a spreader, and caps and plugs for every line and port. Parts: the pump (new, CAT Reman, or rebuilt exchange), a mounting O-ring plus O-rings for every fitting disturbed, new return, case drain, and pilot filters, and Cat HYDO Advanced 10 (HYDO Advanced 30 where the manual specifies it for high ambient).
Step 1: Position the Machine and Relieve Pressure
- Firm, level ground. Boom down with the bucket flat on the ground and the stick and bucket curled in, so no cylinder is holding weight.
- Shut the engine down. Key ON, hydraulic lockout lever unlocked, and move both joysticks and the travel pedals through every position several times to bleed pilot pressure and the accumulator. Lock the lever.
- Release the tank pressure at the hydraulic tank cap; the tank is pressurized and will spray if opened hot.
- Battery disconnect off, locked, and tagged. Let the machine cool; hot hydraulic oil burns.
Step 2: Deal with the Oil
The tank on a 336 holds on the order of 45 gallons and the full system well over twice that. If the pump wore out cleanly and the sample was clean, close the suction shutoff valve at the tank, drain only the suction line and pump, and keep the oil. If the pump shed metal, the tank comes down and the oil is scrap. Containment goes down before the first fitting comes loose.
Step 3: Open the Pump Compartment
The pump sits on the flywheel housing under the pump compartment cover on the upper structure. Remove the cover and anything in the lift path, then clean the whole compartment before opening a single line; dirt that falls into an open port is the next failure.
Step 4: Disconnect and Cap Every Line
- Tag each line: the two delivery lines, the suction line, the case drain, the pilot pump delivery, and the regulator control and sensor lines.
- Unplug the pump control solenoids and pressure sensors and tie the harness back.
- Break the split-flange delivery lines and the suction line and catch the oil; line wrenches on the ORFS control lines.
- Cap or plug every open line and port immediately. Not after the next one; immediately.
Step 5: Rig and Remove the Pump
- Sling the pump by the housing, never by the pilot pump or a fitting, and take the weight on the crane until the mounting bolts are unloaded.
- Remove the mounting bolts at the flywheel housing. Slide the pump straight back off the coupling; do not let it hang on the splines.
- Set it on the tarp and inspect the coupling for cracks and the spline for fretting; a dry, fretted spline is a common cause of a pump that "failed" at the shaft.
Step 6: Prepare the New Pump
- Match the part number to the serial number; the electro-hydraulic control on later machines is not interchangeable with the earlier pump.
- Transfer the pilot pump, brackets, and sensors if the replacement came bare, using new O-rings.
- Fill the pump case with clean oil through the uppermost case port before installation; a piston pump started dry scores the barrel in seconds.
- Grease the coupling splines with the grease the manual calls for and install a new mounting O-ring.
Step 7: Install and Torque
- Lift the pump into place and align the splines to the coupling by hand; never pull the pump onto the coupling with the bolts.
- Start all mounting bolts by hand, snug in a cross pattern, then torque in stages to the value in the service manual for that bolt size.
Step 8: Reconnect the Lines with Torque Discipline
- SAE split flanges: new O-ring, lightly oiled, seated in the groove. Snug the four bolts in a cross pattern so the halves stay parallel, then torque to the chart value for the bolt size in a second cross pass. A cocked flange pinches the O-ring and leaks at full pressure.
- O-ring face seal (ORFS) fittings: new O-ring in the face groove, hand-tight, then torque to the value for the dash size. Over-tightening splits the O-ring; the leak looks under-tightened and gets tightened again.
- Reconnect the suction line, case drain, pilot lines, and harness, and open the suction shutoff valve; a closed suction valve on the first start destroys the pump.
Step 9: Refill, Filters, and Power
Install new return, case drain, and pilot filters. Fill with Cat HYDO Advanced 10 to the sight glass with the boom in the checking position the manual specifies. Reinstall the guards and cover and turn the disconnect back on.
Step 10: Prime, Bleed, Break In, and Re-Check
- Start and hold low idle. Listen for cavitation; if the pump growls, shut down and check the suction valve and line.
- Low idle for 5–10 minutes with no load, then cycle each function slowly to the ends of its stroke to purge air. Check the sight glass and top off.
- Walk the compartment for leaks at every flange and fitting with the oil warm.
- Re-check main relief and pilot pressures, confirm cycle times, and feel the case drain line for heat.
- Change the return filter after the first 50 hours and pull an oil sample at 250 hours.
When the System Has to Be Flushed
If the old pump shed metal, the return filter caught a fraction of it; the rest is in the tank, main control valve, cylinders, swing and travel motors, cooler, and hoses. Clean the tank by hand, flush the lines, replace hoses that cannot be flushed, open the cooler, and check the motor case drains for metal. It is a full day on top of the pump swap, which is why the oil sample before the job matters.
A 336 working red clay at 100°F ambient with a packed cooler holds its oil above 180°F all afternoon, and hot, thin oil is what shortens the new pump's life. Clean the cooler as part of the job.
FAQ
Can this be done at the jobsite?
Yes. Testing, removal, installation, and flushing are field work with a crane-equipped service truck; see excavator repair and CAT repair.
Need This Done On-Site in Tulsa?
Tulsa Fleet Repair is a mobile operation — the service truck comes to your yard, jobsite, or breakdown location anywhere within 120 miles of Tulsa, with 24/7 emergency response when a unit can't wait. Call (918) 925-9356 or request service online.