Ordered, theory-focused procedure for replacing the transmission (front/hydraulic) pump on a Mitsubishi 4D56 automatic — how each step relates to hydraulic theory and how the repair cures the fault. No extra chatter.

1) Symptoms / theory of failure (why you suspect the pump)
- Symptoms: severe gear slippage, no/weak engagement in drive or reverse, long/delayed engagement, whining from front of transmission, very low or falling ATF pressure, metal contamination in fluid.
- Theory: the transmission pump (front cover/rotor pump) is a positive-displacement gear/rotor pump driven by the torque converter or input shaft. It generates hydraulic pressure and flow for clutches, bands, valve body and torque converter lock-up. Wear of rotors, bushings, seals, or scoring in the pump housing increases internal leakage (pump volumetric inefficiency) so pressure and flow fall. Low pressure = insufficient apply pressure on clutch packs → slippage, no engagement. Replacing the pump restores internal clearances/seals and the ability to build pressure.

2) Confirm diagnosis (pressure test)
- Theory: measure hydraulic line (main) pressure under cranking/idle and under load with a calibrated gauge at the test port. Compare to factory specs. If pressure is below specification and not caused by low fluid/externals, the pump is primary cause.
- Quick checks to rule out other causes: confirm ATF level & condition, check external leaks, inspect filter and screen (if accessible) for debris that could block flow.

3) Preparation and safety (how preparation affects repair quality)
- Disconnect battery (prevents starter/ECU issues while working).
- Raise vehicle securely and support on stands: transmission removal requires space. Use an engine support or transmission jack.
- Drain ATF: prevents spills and reduces weight.
- Theory note: clean workspace prevents contamination of new pump/valves; particulate contamination causes rapid hydraulic wear.

4) Required parts and tools (why each matters)
- New pump assembly or rebuild kit including rotors, bushings, thrust plate, seals, gasket(s).
- New front cover gasket, torque converter seal, oil seals, filter, ATF (correct spec), pan gasket.
- Tools: pressure gauge, transmission jack, engine support, torque wrench, pullers, seal drivers, cleaning solvent, safety gear.
- Theory: replacing only the pump but leaving contaminated fluid or damaged valve body/clutches will not fully fix symptoms; also check torque converter and valve body for damage.

5) Removal sequence (ordered mechanical steps, with theory why)
- Remove driveline: disconnect battery, remove starter (to access bellhousing bolts), disconnect driveshafts/prop, speed sensors and wiring from transmission.
- Support engine if needed; support transmission with jack.
- Drain ATF and remove pan and filter (inspect for debris: metal particles indicate internal wear).
- Unbolt torque converter cover/inspection cover if present.
- Unbolt transmission-to-engine bellhousing bolts; separate transmission from engine. (On many models you must separate and lower the transmission to access pump.)
- Remove torque converter from transmission input shaft (torque converter must be removed to access front pump). Mark orientation and inspect torque converter for damage/wear.
- Remove transmission front cover (pump cover) by unbolting; remove pump rotor/inner/outer rotors and thrusts, bushings and seals.
- Theory at each step: removing torque converter and cover exposes pump assembly; you must preserve cleanliness and orientations to maintain correct hydraulic clearances and pump timing.

6) Inspect components and measure (diagnostic theory applied during repair)
- Inspect pump housing and rotors for scoring, galling, worn lobes, high clearance, or damage.
- Check bushings and thrust plate wear; measure end-play/clearances with feeler gauges; compare to service limits.
- Inspect torque converter input splines and hub, and trans front bushing/bearing.
- Theory: pump replacement only cures volumetric inefficiency. If housing is scored or bearings are worn beyond limits, replacing only rotors may not restore pressure. Contaminants can ruin new pump quickly.

7) Replacement / reassembly steps (ordered, and why done that way)
- If using rebuild kit: replace rotor set, bushings, thrust plate, and seals. If using a new pump assembly, inspect mating face and clean.
- Fit new front cover gasket/seal surfaces, install pump assembly to correct orientation, torque cover bolts to factory specs in a cross pattern to avoid distortion.
- Replace transmission filter and pan gasket. Replace any external seals (output shaft, front pump seal).
- Reinstall torque converter carefully onto input shaft to correct seating depth; ensure torque converter engages pump drive correctly (improper seating leads to pump starve/shaft damage).
- Mate transmission to engine ensuring bellhousing alignment; torque bolts to spec.
- Reconnect lines, sensors, driveshafts, linkage, starter, etc.
- Fill with specified amount and type of ATF initially per service manual (some fill after first run and then top to specification).
- Theory: proper seating of torque converter and sealing, correct gasketing and torquing maintain clearances and prevent external/internal leak paths that would reduce pressure or starve the pump.

8) Initial priming and checks (why these steps restore function)
- Prime pump: rotate starter with engine off (or follow manufacturer priming method) to build initial pressure and circulate fluid, but don’t start for extended cranking unless specified.
- Check for external leaks at pump seal and pan.
- Attach pressure gauge and verify main line pressure at idle and during gear changes; confirm within spec.
- Theory: priming prevents dry-run damage to the new pump; pressure verification confirms volumetric output restored and that valve body/clutches respond.

9) Controlled road test and recheck
- With vehicle on jack stands, run engine and cycle through all gears to confirm engagement and check for leaks and correct hydraulic pressure.
- Road test under controlled conditions, checking shift quality, torque converter lock-up, absence of whining/slippage.
- Recheck fluid level hot per manual and re-torque accessible fasteners if required after thermal cycles.
- Theory: under load is where insufficient pressure becomes evident; testing ensures the pump now provides required pressure/flow under real conditions.

10) How the repair fixes the fault (summary in system-theory terms)
- Replacing the pump restores the pump’s volumetric efficiency (reduced internal leakage and restored rotor clearances), allowing required hydraulic pressure and flow to be generated.
- Proper new seals and gaskets prevent internal bypass and external leaks; correct seating of torque converter ensures pump drive engagement and avoids cavitation/starvation.
- With restored pressure, clutches and bands receive correct apply force, eliminating slippage and restoring gear engagement and shift quality. If contaminants or secondary damage (valve body, worn clutches, torque converter) remain, symptoms can persist — that’s why inspection & pressure verification are essential.

11) Common pitfalls and how theory prevents them (fast list)
- Installing pump without cleaning contamination → new pump failure (contaminant-induced scoring).
- Incorrect torque converter seating → pump starvation/cavitation → low pressure.
- Reusing severely scored housing or worn bushings → still low pressure.
- Incorrect fluid type/level → wrong viscosity reduces pressure and cooling.

12) Final verification checklist
- Main line pressure within spec at idle and under load.
- No external leaks at pump seal/pan.
- Shift quality normal, no slipping, no whining.
- ATF level correct hot and fluid clean.
- No metal in pan/filter on first drain after short break-in.

Use factory service manual for model-specific torque specs, clearances, pump orientation and test ports. This procedure explains theory behind each action and how replacing the pump resolves hydraulic pressure/flow failures that cause the observed transmission faults.
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