Note: The NV3550 manual transmission does not use the vehicle radiator or radiator cap. Many people refer to adding a remote breather/vent or servicing the transmission fill/drain plugs when they say “radiator cap” for a gearbox. Below are clear, technician-style instructions for (A) servicing/draining & refilling the NV3550 gearbox on a Jeep Wrangler, and (B) installing/replacing a remote breather/vent (what some call a “radiator‑cap” style breather). No questions.

A — Service (drain & refill) NV3550 gearbox
Tools & supplies
- Floor jack and jack stands (or a 2-post lift). Wheel chocks.
- Safety glasses and gloves.
- Drain pan (capacity ≥ 6 qt).
- Socket set + ratchet and extensions (common sizes: 10–19 mm). Small hex/Allen set in case plug is hex.
- Breaker bar for stubborn plugs.
- Torque wrench.
- Fluid pump or squeeze bottle for filling (long thin spout or pump recommended).
- Scraper & cleaning rags.
- Replacement drain/fill plug washers (crush/copper washers) or O-rings if used.
- Manufacturer-specified gear oil (see note below). Typical: 75W-90 GL‑4 manual-trans gear oil is commonly used for NV3550, but confirm with Jeep service data.
- Thread locker/sealant only if service manual requires it (usually not required for plain steel/aluminum threaded plugs).
- New drain/fill plugs if threads are stripped or plugs are damaged.

Safety precautions
- Work on level ground. Chock wheels and support vehicle securely on jack stands — never rely on just a jack.
- Let the vehicle & transmission cool if recently driven — hot oil causes severe burns.
- Wear gloves and eye protection. Replace contaminated rags into a closed container.

Step-by-step — drain and refill
1. Warm the transmission: Drive the jeep a short distance (~5–10 minutes) to warm the fluid. Warm fluid drains faster and carries more contaminants.

2. Position vehicle safely: Park on level ground, set parking brake, chock front wheels if working at rear, raise vehicle and support on jack stands to give you safe access to the transmission pan/plug area.

3. Locate fill and drain plugs: On NV3550 the fill plug is on the passenger side of the case roughly mid‑height (about level with the transfer case flange on many Jeeps). The drain plug is usually the lowest plug on the case (rear or bottom). Consult a picture or manual for exact location on your model year.

4. Place drain pan under drain plug area.

5. Remove fill plug first: Loosen and remove the fill plug. This tells you it will accept fluid and prevents vacuum lock when draining. If nothing pours out when removing the fill plug, it may be low — but continue to drain first.

6. Remove drain plug: Break loose and remove the drain plug. Let fluid drain completely into pan. This can take several minutes. Inspect fluid color and smell — burnt or metallic bits indicate internal problems.

7. Inspect drain plug & washer: Clean magnet on plug (if equipped). Replace crush washer or O‑ring as required. Clean threads on case.

8. Reinstall drain plug: Thread it in by hand to avoid cross‑threading. Tighten snug and torque to spec if you have it. If you don’t have the exact spec, tighten until seated then an appropriate small torque — avoid over‑torquing aluminum case threads.

9. Fill transmission: Use a fluid pump or squeeze bottle to pump gear oil into the fill hole until oil reaches the bottom edge of the fill hole. The correct level is “until it drips out” or reaches the bottom of the fill hole — not to the top. Do it slowly and check level frequently.

10. Reinstall fill plug: Clean the plug and washer, then thread in by hand and torque to spec (or snug). Wipe away any spilled oil.

11. Lower vehicle and test: Start engine and shift through gears (with vehicle stationary as required) to circulate fluid. Re-check for leaks around plugs. Recheck fill level after a short drive and top as needed.

Common pitfalls (avoid these)
- Not removing the fill plug first — this can result in a vacuum that slows draining and can trap fluid.
- Overfilling: gearboxes should be filled to the fill plug level, not “full top to brim.”
- Using the wrong oil: automatic transmission fluid, gear oils with the wrong additive package (GL‑5 in place of GL‑4) can damage synchros. Use the manufacturer-recommended fluid.
- Cross‑threading or over‑torquing plugs — the NV3550 case is aluminum; stripped threads are costly to repair.
- Reusing old crush washers/O‑rings that are flattened — they leak.
- Not warming the transmission — cold fluid drains slowly and leaves more contaminants.

Replacement parts required
- Gear oil (quantity depending on vehicle/transmission — typically 3–4 quarts; check service manual).
- New crush washers or O‑rings for drain/fill plugs.
- New drain/fill plugs only if damaged or threads stripped.

B — Installing/replacing a remote breather/vent (what some call a “radiator_cap” for gearbox)
Purpose: prevent water ingress during deep water crossings or minimize contamination by routing the vent to a higher location. Many people run a short hose from the factory vent to up high near the firewall or use a screw‑in breather cap.

Tools & supplies
- Appropriate aftermarket transmission breather kit or a small automotive vent cap (breather) with correct thread or a 1/4" hose fitting.
- Small diameter hose (3/8" or 1/4") rated for oil/temperature, clamps or cable ties.
- Drill/tap if you plan to install a screw-in breather where no vent exists (not recommended unless you know what you’re doing).
- Basic hand tools.

Procedure (safe method without case modification)
1. Find factory vent: Many manual transmissions have a small vent/port on the top of the case or tail housing. If present, it will be a small raised boss with a tiny hole or a rubber grommet. If you don’t find one, do NOT drill the case unless you have the service manual guidance.

2. Attach hose to existing vent: If the vent is a rubber grommet, pull it out carefully and push a 1/4" hose onto the nipple or over a small barbed breather. Secure with a small clamp or zip tie.

3. Route hose high: Run the hose up to a secure, dry location — near the cowl or inside the engine bay above splash/waterline. Secure hose with zip ties; do not kink it.

4. Fit breather cap at high end: Install a small vent cap (filtered breather cap) onto the hose end to keep dirt out. Some people run the hose out of the engine bay behind the fender; keep it protected from heat sources and snagging.

Alternative (if no factory vent and you want a fill-port option)
- Some install an externally mounted breather that uses the fill plug location — e.g., remove fill plug, install a screw-in breather adapter that seals into the fill hole. Do NOT do this if it compromises the ability to check/fill fluid; use a purpose-built adapter that includes a seal and can be removed for filling. If you are unsure, use an external vent hose attached to an existing vent or have a professional do a tapped port installation.

Breather pitfalls to avoid
- Drilling/tapping case incorrectly — cross‑threads or water ingress.
- Routing hose too low or into areas where it will scoop water.
- Using a hose material that will collapse or kink — that blocks ventilation.
- Sealing the vent completely — transmission MUST vent; do not cap it watertight.

How the tools are used (quick notes)
- Fluid pump: insert intake tube into the gear oil container, place pump tube into fill hole, and pump until oil appears at the bottom of the fill hole. Pumps let you avoid removing driveshafts for filling.
- Socket/breaker bar: break loose seized plugs; always start threading by hand to avoid cross‑threading.
- Torque wrench: tighten plugs to specified torque (or manufacturer guidance) to avoid leaks or stripped threads.
- Drain pan: position under drain plug, ensure capacity larger than expected oil volume to avoid spillage.

Final notes (short)
- Confirm exact fluid spec, fill capacity, plug locations, and torque values with the Jeep factory service manual or trusted repair data for your Wrangler model year.
- If internal metal debris is present in drained oil, or if you suspect internal damage (grinding, difficulty shifting), stop and inspect — get a professional diagnosis.
- If you want a remote breather (for off‑road water crossing), follow the non-invasive hose routing method above or buy a purpose-built kit.

Done.
rteeqp73

NV3550 After 190k...What's inside?? This is the NV3550 transmission out of our 'LS TJ'. We decided to swap it due to the amount of play during shifting and the ...

B&M SuperCooler 70268 Jeep Wrangler TJ Mounting 2001 Jeep Wrangler TJ Wrangler B&M SuperCooler 70268.

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