The TJ's NP231 transfer case takes about 2 quarts of ATF+4 and the fluid change takes 30 minutes with hand tools — the transfer case is unfussy about ATF brand, but it does need clean fluid every 30,000 miles to keep the planetary gears and chain happy.
The NP231 (later relabeled NV231 after New Venture Gear took over) is the standard TJ transfer case for non-Rubicon builds. It's a chain-driven, part-time 4WD unit with a planetary low range. The lubrication system is straightforward: ATF splashes from a pump inside the case onto the chain, the planetary, and the bearings. Clean fluid keeps the chain from wearing the case and keeps the bearings from pitting.
Mopar's spec is ATF+4. The forums spend a lot of energy debating whether Dexron/Mercon or other ATFs work — the short answer is yes, they do, the transfer case isn't picky like a modern automatic transmission. ATF+4 is the safe call because it's the manufacturer spec and Walmart sells it for $9 a quart. If you have leftover Valvoline MaxLife from a transmission service, that's fine in the transfer case too.
Capacity is the more useful question. The shop manual says 2.2 pints (about 1.1 quarts), but the real fill range from owners who pump until fluid comes out the fill hole is 1.5 to 2 quarts. The "correct" answer is: pump fluid in until it runs out the fill hole, then stop. Don't trust a fixed number — fluid level varies with how level the rig is parked and where the fill hole is positioned on the case. Park as level as you can manage.
The plugs are both 1/2" drive square plugs — no socket, stick a 1/2" ratchet directly into the plug. Drain plug is at the bottom of the case; fill plug is on the rear of the case, roughly halfway up. Crack the FILL plug first. If it won't budge, do not drain. A stuck fill plug means you can drain a transfer case you can't refill, and you'll be towing the Jeep instead of driving it.
A common upgrade is a magnetic drain plug — replaces the factory plug, catches metal fuzz, and gives you a diagnostic check at the next service. About $15 from B&M, AFE, or any magnetic-plug vendor.
| Part | Vendor | Est. price |
|---|---|---|
| Mopar ATF+4 (1 quart) | Mopar | ~$9 |
| Valvoline MaxLife ATF (1 quart) | Valvoline | ~$7 |
| Mopar transfer case fill/drain plug crush washer | Mopar | ~$2 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.