What you’re dealing with: the “radiator” for a Jeep NSG370 6‑speed is the transmission/gearbox cooler (either a separate air cooler or a fluid‑to‑coolant core built into the vehicle radiator). It’s there to remove heat from the transmission fluid so the gearbox stays within safe operating temperatures. Below is a beginner‑level, step‑by‑step explanation of every component, how the system works, why repairs are needed, what can go wrong, and how to replace/repair the cooler and lines.

Quick high‑level analogy
- Think of the gearbox as a small engine that gets hot from friction. The transmission fluid is the coolant/oil that carries heat away. The cooler is like a radiator for that oil: it dumps heat into the engine coolant or the passing air, keeping the oil and gears at a safe temperature.

Important safety notes before you start
- Work on a cool vehicle on a flat surface, with the parking brake engaged and wheels chocked.
- Use jack stands if you lift the vehicle—not just a jack.
- Transmission/gear oil can be hot; allow time to cool.
- Wear gloves and eye protection.
- Dispose of used fluid per local regulations.

Components — what each part is and what it does
- Transmission/Gearbox (NSG370): the mechanical gearbox housing where gears, shafts, bearings live. Has two metal ports where cooler lines attach.
- Transmission fluid/gear oil: lubricates and carries heat away. Some manuals specify a particular fluid—always check the factory service manual for the exact type and capacity.
- Cooler lines (metal hard lines and rubber hoses): carry hot fluid from the gearbox to the cooler and return cooled fluid back. Lines include fittings and clips/retainers.
- Transmission cooler core: two possible types:
- External air cooler: a small plate/tube cooler mounted in front of the radiator/condenser, cooled by air flow.
- Integrated cooler (most common): passages inside the engine radiator that let transmission fluid pass through while engine coolant flows around it, exchanging heat.
- Fittings and O‑rings/seals: connect lines to the gearbox and cooler. O‑rings seal the connection.
- Mounting brackets/clips: hold the cooler and lines in place.
- Drain plug: on the transmission case to drain fluid.
- Fill/level plug: used to refill and check fluid level (some transmissions are filled through the filler on the case or via a fill tube).
- Quick‑connect couplers (on some models): used where lines meet the radiator or cooler for easy disconnect.

How the cooling system works (theory)
- Friction in the gearbox produces heat. Transmission fluid carries that heat.
- Fluid is pumped (or moves by pressure/thermal circulation) through lines to the cooler.
- If the cooler is integrated in the radiator, the transmission fluid runs through a small separate loop inside the radiator where heat transfers to engine coolant and out through the radiator.
- If external, air flowing over the cooler removes heat directly.
- Cooled fluid returns to the gearbox, keeping gear temperatures in a safe range.
- If fluid gets too hot, it loses lubricating properties and the gearbox can suffer accelerated wear, slipping, or damaged synchros and bearings.

Why this repair is needed (common reasons)
- Leaks from cracked cooler cores, corroded fittings, torn hoses, failed O‑rings, or damaged lines.
- Blocked cooler passages (internal corrosion or debris) reducing cooling efficiency.
- Physical damage from road debris or impact.
- Radiator replacement/rebuild that includes replacing the integrated cooler.
- Preventive replacement when hoses are old or O‑rings brittle.

Signs you need to repair/replace the cooler or lines
- Visible fluid leak under vehicle near front or under the gearbox.
- Low transmission fluid level / frequent top‑ups required.
- Burning smell, dark/burnt fluid color.
- Elevated gearbox temperatures (if you have a temperature gauge).
- Gear noise, grinding, hard shifting (fluid degradation or low level).
- External damage to cooler, corrosion, or cracked hoses.

Tools and supplies you’ll need
- Basic hand tools: sockets, ratchet, wrenches, screwdrivers, pliers.
- Line wrenches for fittings; quick-disconnect tool if required.
- Drain pan, funnels.
- New O‑rings/seals (use OEM or high‑quality replacements), hose clamps.
- Replacement cooler or radiator (or replacement rubber hoses/metal lines) as required.
- Replacement transmission fluid (manufacturer specified). Do not guess fluid type—use the factory spec.
- Torque wrench (for bolts/fasteners).
- Shop rags, gloves, safety glasses.
- Jack stands and ramps or a lift.

Step‑by‑step repair procedure (replace cooler or cooler lines)
1. Preparation
- Park on level ground, chock wheels. Disconnect the negative battery terminal if you’ll be working around the radiator (prevents fan/accident).
- Allow engine and transmission to cool.

2. Drain transmission fluid
- Place a drain pan under the transmission drain plug or under the cooler line quick‑disconnect low point.
- Remove drain plug (if the unit has one) or disconnect lowest cooler hose/line to drain fluid. Allow sufficient time to drain.
- Collect and dispose of fluid properly.

3. Access and inspect
- Locate the cooler: external (in front of the radiator) or integrated (check radiator for cooler lines).
- Inspect lines, connections, clamps, and the cooler core for corrosion, dents, damage, or leaks.
- Note orientation and routing of lines so you can reinstall correctly.

4. Disconnect cooler lines
- Use correct line wrenches or quick‑disconnect tools. Have rags handy (some residual fluid will leak).
- Remove line-to-radiator/cooler fittings and line-to-gearbox fittings. Remove any retaining clips.
- Cap or plug open transmission ports quickly to prevent contamination and excessive fluid loss.

5. Remove old cooler/core or lines
- If replacing an external cooler, unbolt mounting brackets and remove cooler.
- If replacing the radiator with integrated cooler, you’ll be removing the radiator—follow service manual for radiator removal because it involves draining coolant and removing electrical connectors, shrouds, fan, and condenser lines (if equipped).
- Remove and replace damaged hoses or hard lines. Replace any rusted brackets.

6. Replace seals and install new parts
- Replace O‑rings/seals on fittings with new ones lubricated with a little clean transmission fluid. Old O‑rings often cause leaks.
- Route new lines the same as old. Secure with original clips/brackets.
- Reinstall cooler or radiator per manufacturer instructions; torque mounting bolts to spec.

7. Reconnect fittings and torque
- Reconnect cooler lines to the gearbox and cooler/radiator. Make sure fittings are clean and threads/seats are undamaged.
- Tighten to proper torque or follow a snug‑then‑quarter‑turn rule only if you don’t have spec (preferred: check service manual for torque). Avoid overtightening which can damage threads or crush O‑rings.

8. Refill transmission fluid
- Fill with the correct type and amount of fluid specified by the factory. Filling is usually done through the fill/level plug on the gearbox.
- Many manual transmissions require filling until fluid starts to run out of the level hole—operate vehicle briefly and recheck level per manual.

9. Bleed and settle
- Some air can be trapped in lines; cycle the clutch and move through gears (with vehicle stationary or as instructed by manual) to circulate fluid.
- Recheck fluid level after cycling and after a short test drive.

10. Leak check and road test
- Start vehicle and with parking brake on, check for leaks at all fittings.
- Take a short drive, then recheck fluid level and inspect for leaks again.
- Monitor gearbox behavior; check for noise or overheating.

What can go wrong and troubleshooting
- Continued leaks: likely damaged O‑ring, misaligned line, or cracked fitting. Replace O‑rings and check seating surfaces.
- Overfilling: causes churning, foaming, overheating. If overfilled, drain to correct level.
- Underfilling: poor lubrication, overheating, abnormal gear wear. Refill to spec.
- Air in the system: causes foamy fluid and poor lubrication; bleed/recirculate and recheck level.
- Cross‑contamination: coolant in transmission fluid or vice versa (if radiator core leaks). If mixing occurs, radiator core must be replaced and transmission fluid flushed; contaminated fluid damages gearing.
- Wrong fluid type: can cause poor lubrication, premature wear, noisy shifting. If incorrect fluid used, drain and refill with correct fluid.
- Damaged radiator core (integrated cooler): requires radiator replacement; when replacing radiator, flush coolant and ensure new radiator is compatible with transmission cooler lines.

Common pitfalls and tips
- Always replace O‑rings when re‑connecting lines. They’re cheap and often the source of leaks.
- Keep everything clean—dirt entering the gearbox is harmful.
- Mark line routing with tape or take photos before removal so routing is correct on reassembly.
- If replacing an integrated cooler (radiator), remember you’ll also need to deal with engine coolant: drain/replace, bleed cooling system, and reconnect AC condenser carefully if it blocks access.
- Use the correct fluid and quantity. Consult the factory service manual or dealer for the exact fluid spec.
- If you smell burning after repair, stop and recheck fluid level and leaks.

Post‑repair checklist
- No visible leaks at gearbox or cooler lines.
- Fluid level at correct mark when hot/cold per manual instructions.
- Smooth shifts, normal gearbox noise.
- No burnt smell; fluid color reasonable for its age.
- Temperature readings normal (if you have a gauge).

If the cooler is integrated and leaking internally (coolant and transmission fluid mixing), this is a major repair: radiator replacement plus full transmission fluid flush is required.

Final note
- This is a general step‑by‑step for replacing or repairing the gearbox cooler/lines on an NSG370. Always refer to the Jeep factory service manual for model‑year‑specific procedures, exact torque figures, and the exact fluid specification. If you’re unsure about any step—especially radiator removal or fluid specifications—get the manual or professional help.

Straightforward and practical: replace cracked/damaged cooler or lines, always fit new seals, refill with the correct fluid, and confirm no leaks. That’s the core of keeping the NSG370 cool and reliable.
rteeqp73

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