Short answer first: the shift drum in a 42RLE is the mechanical “selector” that positions the transmission’s internal forks and collars so the right clutches/bands engage for each gear. When the drum or its forks/wear-surfaces wear or break you get missed gears, hard or late shifts, slipping, or the transmission stuck in a single gear. Repair means remove/inspect the drum and associated forks/shafts/bushings, replace worn parts and seals, reassemble carefully to factory specs, refill with the correct ATF, then test.

Below is a beginner-friendly, detailed-but-practical guide: what each part is and does, why failure happens, exactly what you’ll do to fix it, what to watch for, and common pitfalls. Read all of this before starting, have the factory service manual (FSM) for exact torque specs and clearances, and work safely.

A. Theory — what the shift drum does and why it matters
- Role: The shift drum is a precisely machined cylindrical cam with helically cut grooves/paths. As it rotates (or slides on some designs), the grooves guide shift forks/rings to move laterally. Those forks move clutch pistons, collars or bands that connect parts of the planetary gearset. The result: different gear ratios (1−4, R, N, P).
- Analogy: Think of the drum as a key and the forks as locks. Rotate the key a specific amount, the locks move to positions that let different “doors” (clutches) open.
- Why repairs are needed: wear from friction and load; forks wear where they contact the drum groove; drum grooves can develop scallops or burrs; bushings/journals wear causing play; a cracked fork or broken detent will cause lost or wrong gear engagement. Contamination (metal debris) accelerates wear.
- Symptoms of shift-drum/fork problems:
- Stuck in gear, or only one gear works
- Harsh, delayed, or misshifts
- Gear “hunting” (transmission searching)
- Unusual internal noises
- Metal particles in fluid/filter
- Shift lever detent issues (hard to move selector)

B. Main components you’ll see and what each does (detailed)
1. Transmission case/housing — outer shell that holds everything and provides mounting points.
2. Torque converter — fluid coupling between engine and gearbox; must be removed before separating the main case from the converter housing.
3. Input shaft / front pump — brings fluid pressure and mechanical input; pump supplies hydraulic pressure for shifting.
4. Valve body — hydraulic control center with valves/solenoids; tells which hydraulic circuits open to apply clutches. Not the same as mechanical selector but must often be removed to access internals.
5. Shift drum (selector drum) — cylindrical cam with grooves; rotates to move forks/rails. Key surfaces: grooves (track paths), journals/bushings where it rides, and any locator pins/keys.
6. Shift forks — 2–3 forks that ride in the drum grooves and move sliding collars or pistons. They have “shoes” or pads that contact collars; those pads wear.
7. Shift rails / selector shafts — shafts that guide forks and may carry collars/sync rings. They often have bushings.
8. Sliding collars / synchronizer-type collars / clutch hubs — move to connect different clutch packs or gearsets.
9. Planetary gearsets (sun/planet/ring) — form the gear ratios. Not usually removed unless major overhaul, but you’ll need to be aware.
10. Clutch packs / friction plates — engage to lock parts of the planetary sets. Wear here also causes problems.
11. Bushings, bearings, snap rings — small parts that locate and support shafts/drums.
12. Detent plate and pawl / selector linkage — provide tactile gear positions and hold the drum in position.
13. Seals/gaskets/filter/pan — fluid control and cleanliness.

C. Tools, consumables, and parts you’ll need
- Service manual for the 42RLE (critical for torque specs, sequences, clearances)
- Transmission jack (recommended) or sturdy floor jack + stands + engine support
- Basic hand tools: socket set (deep & shallow), combination wrenches, torque wrench, breaker bar
- Screwdrivers, pry bars, snap-ring pliers
- Special tools (may be needed or helpful): drum alignment tool, collar/shaft seals installer, bearing puller, press for bushings, dial indicator (endplay), feeler gauges
- Clean workspace, parts trays, marker/tagging for bolts
- New seals/gaskets, new shift forks (if worn/cracked), new shift drum (or refinished one), new bushings/bearings as needed, new filter and ATF (use Mopar ATF+4 or the fluid specified for 42RLE)
- Clean shop rags, solvent, RTV (if required), anti-seize assembly lube
- Safety gear: gloves, safety glasses

D. Safety first — before you start
- Working on a transmission is heavy and messy. Use a transmission jack and proper lifting points.
- Disconnect the battery before removing transmission.
- Drain ATF into a catch pan and dispose of fluid properly.
- Cleanliness is extremely important: contamination kills transmissions. Work on a clean table, bag parts if needed.

E. High-level step-by-step repair procedure (detailed but practical)
Note: This is a general guide. The 42RLE has specific fastener torques, shimming, and clearances; use your FSM for exact values.

1) Preparation and diagnosis
- Confirm symptoms and inspect ATF and filter. Metal flakes/strong metallic smell point to internal wear.
- Note gear selection behavior; check for TCM error codes (some shift problems are electrical). If codes indicate mechanical, proceed.

2) Remove transmission from vehicle (overview)
- Disconnect battery, remove driveline components (driveshafts, transfer case depending on model), unbolt transmission cooler lines (catch fluid), electrical connectors, shift linkage, speed sensors.
- Support transmission with a jack, unbolt torque converter from flexplate (or unbolt bellhousing and slide transmission back to disengage torque converter), then lower the unit. Keep transmission relatively level to avoid fluid spillage.

3) External disassembly to access the drum
- Remove transmission oil pan, filter, and valve body (label and organize bolts—valve body bolts often have lengths).
- Remove valve body carefully and place on clean surface. Inspect for obvious damage to valves/solenoids; but many valve-body problems mimic mechanical symptoms.
- Remove front pump/cover if required to access internals.

4) Split case / main internals access
- With valve body off and fluid drained, separate the case halves following FSM sequence (usually removing bellhousing bolts and dowels). Be careful of internal snap rings and springs that may pop out.
- You’ll see planetary sets, clutch packs, and the shift drum region.

5) Remove shift drum and shift forks
- Identify the drum and its retaining snap rings/bolts. Remove any retaining plates and slide out the drum. Note its orientation and indexing relative to gear selector (marking with marker is helpful).
- Slide the shift forks off their rails; inspect all rails for scoring and bushings for wear.
- Keep small parts in organized trays, mark them so reassembly order is clear.

6) Thorough inspection (most critical step)
- Shift drum: examine grooves—look for scalloping, wear grooves, burrs, or cracked sections. Measure journal diameters and compare to FSM limits. If grooves are worn such that forks can move sideways, drum must be replaced or reground by a specialist.
- Shift forks: check fork contact surfaces/shoes. Look for wear gouges, concave areas, or cracks. If fork shoe thickness is below spec or broken, replace fork (or install fork liner if available).
- Fork pins/rails/bushings: check play. Worn bushings cause misalignment; replace bushings or rails.
- Detent plate and pawl: ensure detent engages positions cleanly, inspect for wear.
- Clutch packs and plates: inspect for glazing or burnt plates—if heavy wear found, consider clutch pack overhaul.
- Bearing and snap rings: replace any that show damage.

7) Decide repairs
- Replace drum if groove wear, cracks, or out-of-round beyond spec. Drum repair beyond simple polishing is not a reliable DIY job.
- Replace forks that show wear or cracks. Replace bushings and bearings as needed.
- Replace any seals/gaskets and filter.
- If debris present, consider cleaning and further inspection of clutches/gearsets; metal in the pan often means other components are damaged.

8) Reassembly — key points
- Clean everything thoroughly with solvent; dry and inspect again.
- Install new bushings/bearings pressed to spec. Lightly lubricate moving surfaces with assembly lube.
- Slide shift drum back into place oriented exactly as removed. Engage detent/selector linkage, ensure drum rotates smoothly and seats into each gear detent with no binding.
- Reinstall shift forks aligned in their grooves; check that forks move smoothly along rails and that collars slide freely.
- Reassemble planetary/clutch components as FSM directs, replacing any worn parts.
- Reinstall valve body (gaskets and filter), torque bolts to FSM values. Misplaced bolts can crack housings or cause leaks.
- Reinstall case halves, front pump, torque converter (clean and inspect torque converter; ensure it seats properly on input shaft), and all external components. Use new pan gasket and filter.

9) Torque, clearances, and checks
- Use FSM torque values for all fasteners. Incorrect torque can cause leaks or internal distortion.
- Check endplay and gear selector indexing per FSM with dial indicator or feeler gauges. If endplay is excessive, you may need to replace shims/bearings.
- Verify valve body solenoid connectors are reattached.

10) Fluid, bleed, and test
- Reinstall transmission, torque mounts, reconnect everything.
- Fill with specified ATF (42RLE uses Mopar ATF+4 in most applications — check FSM). Fill to the correct level with engine idle and transmission in park or per FSM procedure.
- Warm up the vehicle to operating temp, cycle through gears with brakes on, and check for leaks.
- Road test with cautious driving: verify shifting behavior in all gears, check for overheating or slipping.
- Recheck fluid level after settling.

F. What can go wrong during repair and how to avoid it
- Contamination: dirt or metal shavings cause catastrophic re-wear. Keep parts clean; plug openings.
- Incorrect reassembly order or missing snap rings: can leave internal parts loose. Organize fasteners and mark parts as you disassemble.
- Wrong torque: under-torqued bolts leak, over-torqued bolts can warp housings. Use torque wrench and FSM values.
- Not replacing wear items: reusing worn forks or bushings will cause repeat failure.
- Misaligning the drum/detent: leads to wrong gear indexing. Verify engagement points and feel through selector.
- Using wrong ATF: can change friction characteristics and damage clutches. Use correct fluid.
- Improper seating of torque converter: can damage front pump and cause immediate fluid loss. Ensure the converter fully engages input shaft before bolting to flexplate.
- Ignoring valve body or solenoid faults: sometimes electrical/hydraulic issues mimic mechanical; check codes and solenoids before major teardown if possible.

G. Troubleshooting after repair
- If still shifting poorly:
- Re-scan for codes; check solenoid operation.
- Verify drum/fork assembly alignment and free movement on bench before full reassembly.
- Check fluid level and fluid condition (overheating turns ATF brown/black).
- Verify torque converter seating and pump engagement.
- If the transmission slips under load, suspect clutch pack wear even if drum/forks looked fine.

H. Final tips and checklist
- Buy new shift forks and drum if worn — these are wear items.
- Replace filter and pan gasket every time you open the valve body.
- Label everything during disassembly. Take photos to help reassembly.
- Have the FSM on hand — it contains sequences, shim specs, and torque values you can’t improvise safely.
- If any journal surfaces or grooves need machining, consider professional help. Some jobs (drum reground or bushing replacement pressed to precise tolerance) are best left to a machine shop or transmission shop.
- Expect the job to take many hours for a first-timer. Don’t rush.

Closing concise reminder: this repair is a precision job — cleanliness, correct parts, and following the factory manual are essential. If you find the drum or forks badly worn or bushes scored, replace them with OEM or good-quality aftermarket parts. If you prefer, a transmission shop can bench-rebuild to factory specs with the right presses/tools.

No questions asked — follow the FSM and safety practices, and you’ll have the best chance of a successful 42RLE shift drum repair.
rteeqp73

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