NP231
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Overview
A medium Duty part-time transfer case, the np231 features shift on-the-fly 4hi, a good lo range, and massive aftermarket support. Cast in aluminum, the case should have a tag on the back saying New Process Gear (or New Venture Gear), Model 231. New versions of the NP231 are referred to as NVG231 or NV231 which stands for New Venture Gear. The tag on the back of the transfer case will reflect the name change. Only the name is different. A NV231 HD model is used in some ZJs. Presumably this is a "heavy duty" version for Jeeps with the big V8s, which features a wider chain and corresponding sprockets etc.
There are versions of this chain driven transfer case that take 21 and 23 spline input shafts. The 23 spline version is used behind the AX-15 and NV3550. The 21 spline is used behind the AX-5 and the BA-10/5. The output for the front driveshaft is on the drivers side. Low range is 2.72 and high is 1.00.
Tech tips
If you own a 21 spline NP231 and you want to swap in a 23 spline transmission, you can either look for a cheap 23 spline NP231 at the junk yard or go to your local Jeep Parts Dealer and order up a new input shaft part #5300-6085. This is the part you need to convert your NP231 to 23 spline input.
Model number explanation
Sankar K. Mohan of New Venture Gear defined what the numbers on their T-cases mean:
The first number is the number of speeds:
- "1" = one speed (high range). Example – the NV 136.
- "2" = two speeds (high range and low range). Example – the NV 231.
The second number is the strength:
- The NV 231 is designated "3" in strength, not as heavy duty as their largest T-cases, which go up to a "7" (eg, NV 273, for vehicles with a GVW of 17,500 lb).
The third number designates the type of T-case:
- "1" = part-time 4WD
- "2" = full-time 4WD with an open center differential plus lockable part-time option – like the Selec-Trac NV 242 of the Jeep Cherokee and Dodge Durango, or the Hummer’s NV 242HD AMG.
- "3" = electrically shifted.
- "4" = not currently used.
- "5" = Torsen-type differential.
- "6" = computer controlled multi-plate wet clutch, like GM’s AutoTrac NV 246.
- "7" = GeroDisc – like the Grand Cherokee’s Quadra-Trac II NV 247.
- "8" = not currently used.
- "9" = viscous coupling.
Usage
Since 1988, the NP231 has been the "Command Trac" transfer case used in the XJ Cherokee, MJ Comanche, and YJ Wrangler. It has been the "Command Trac" transfer case in the ZJ Grand Cherokee since its introduction. The new TJ Wrangler continues to use the NP231, but the NP231 used in the TJ has a different tail housing that allows the removal of the drive shaft with out losing transfer case fluid.
Both versions of the NP231 used in the Wranglers have an over all length that is long for a short wheel base vehicle. This can cause driveline vibration and premature u-joint failure especially for a lifted vehicle. There are kits to convert the tail housing to a standard yoke to allow the use of a longer rear drive shaft. The kit is called a SYE (slip yoke eliminator), it is recommended to get one when going with more than 4" of lift on the wrangler models (although in some cases modifications are needed in as little as 2" of lift, but in that small of a lift you can get by with a transfercase drop). The SYE kit is available as a standard fixed yoke or a super short shaft kit. Short shaft kits cost a bit more but allow you to run a longer driveshaft to decrease driveline vibes, and therefore allowing you to have more lift. There is also a 4:1 low range reduction and a "2 lo" (allows you to drive in 2wd but in your low range setting) kit for the NP231. Wranglers are only available with "Command Trac", so the NP231 or NP207 (the NP241 in rubicons)are the only transfer cases available from the factory in any Wrangler model.
Operation
Using the 231 is straight forward. During normal operation on road the shifter will be in the 2-hi possition. The operator may shift into 4-hi while driving if the situation requires more traction. Shifting into 4-lo may only be done once the vehicle is stopped or moving slowly and the clutch is engaged. While in 4-lo, vehicle speed must not exceed 40 MPH. When shifting back to 4-hi or 2-hi from 4-lo the vehicle must be stopped or moving slowly and the clutch is engaged.
links
Source: http://jeep.off-road.com/jeep/article/articleDetail.jsp?id=260351
Technical Info
| Hi-Range Ratio | 1:1 |
| Lo-Range Ratio | 2.72:1 |
| Drive Type | Chain |
| Case Material | Cast Aluminum |
| Shift pattern | 2-hi | 4-hi | N | 4-lo |
| Shift on the fly | Yes (4-hi up to ~45mph) |
| Dry weight | ~75 lbs |
| Oil capacity | 2 Quarts |
| Rated GVW | Please contribute here |
| Input shaft spline count | 21 & 23 |
