10.2 - Transfer Case¶

Specifications¶

Gear Ratios¶

Crawl Ratio¶

Behind an automatic the torque converter multiplies torque further at low speed, so effective crawl is deeper still than the geared figure suggests. That is the accepted tradeoff for the 4:1 — see Selection Notes.

Selection Notes¶

The 4:1 Rock-Trac was chosen over the 2.72:1 JK Sport Command-Trac for the deeper low range (owner decision, 2026-09-18).

• Ratio (primary reason): 4:1 gives ≈96.7:1 geared versus ≈65.8:1 for the 2.72:1 case. The deeper range is the point — it trades the ability to hold momentum on fast terrain for genuine low-speed control on technical terrain.
• Converter multiplication — accepted: paired with the 8HP70 automatic, the converter multiplies torque below stall, pushing effective crawl deeper than 96.7:1. An automatic needs less geared reduction than a manual because the converter handles creep, so this setup will be very slow in 4L-1st. Accepted deliberately.
• Native to the chassis: the NP241OR is the factory TJ/LJ Rubicon case, so crossmember mounting and the floor shifter/linkage arrangement are the factory ones for this vehicle rather than adapted from a JK case. Note this does not extend to the driveshafts — both are custom Tom Woods units measured at the stretched 110.5" wheelbase regardless of which case is fitted (see Driveshafts).
• Front output side matches the axle: the NP241OR's front output is on the driver's side,³ and the Super Duty Dana 60 front axle is a driver's-side differential drop.⁸ The front driveshaft therefore runs down the driver's side without crossing the vehicle — no clearance conflict with the R2.8's oil pan to design around.
• Strength: the Rock-Trac's reinforced 4:1 planetary is the heavier-duty variant of the NP241 family. The R2.8's torque is well within either case.

Adapter (8HP70 → NP241OR)¶

An adapter is required — there is no native bolt-up. The 8HP70 is not the case's factory transmission, so it mates through a transmission-to-transfer-case adapter.¹¹

DomiWorks publishes both sides of this exact pairing: the adapter's fitment list is "NP231, NP241, NP242, NP243, NP246, NP249" from an 8HP70/75 Dodge RAM EcoDiesel — and the donor here is an 8HP70 EcoDiesel out of a RAM 1500.⁶ The NP241OR is an NP241; its 4:1 planetary is internal and does not change the input face the adapter bolts to, so fitment is expected. Confirm in one line with the vendor anyway, since the listing does not spell out the OR variant (TBD #313).

Specifications: 115 mm overall, anodized aluminum, case tilt adjustable in 5° increments through a full 360° rotation, with an integrated support bracket for the crossmember and cushions.⁶

• DomiWorks 24004001 — $1,005.90 from DriftHQ (US stock), versus ≈8,495 SEK direct from DomiWorks plus import.⁶
• Alternative — Advance Adapters: no off-the-shelf 8HP70 → stock NP241OR kit. Their 8HP70 tooling (output assembly 50-9905, housing 51-9905) is built to mate to an Atlas. Their previously noted NP241 input gears are JK-specific and no longer relevant to this case. Call to spec a 23-spline build, or go Atlas.

Parts list (DomiWorks path)¶

On the 8HP70 (transmission) side, the only DomiWorks part is the conditional spacer plate. The 8HP70's factory output shaft is used as-is — it is already a 4WD/transfer-case output (this is a 4x4 Ram donor); no coupler or output adapter is needed from DomiWorks.

Critical measurement: DomiWorks' standard fit is 96 mm from the rear plane to the axle tip. The longer outputs that need the spacer plate are called out as some Jeep Grand Cherokee EcoDiesel units, so a RAM 1500 donor is expected to be standard — measure before ordering rather than assume.⁶

Fluid Specifications¶

4WD Position Sensing¶

The dash 4WD and 4LO indicators are driven from the case's OEM position switch, translated by the PMU. The NP241OR is cable/lever-shifted and has no discrete position contact, so nothing on the case can ground the HDX 4x4 input directly.

Sensor: 5-position transfer case switch, OE 5083138AA (Omix-Ada 18676.52) — a two-wire resistive plunger switch mounted on top of the case, riding the internal shift rail; each detent presents a different resistance.⁹ This switch is a listed application for the 03-06 TJ NV241OR, so the sensor should already be on the case — confirm on the actual unit and record its per-detent resistances (TBD #145).

Signal chain¶

The HDX 4x4 terminal is ground-activated and EXTRA(+) is 12V-activated, so the 4LO leg drives the HDX directly from a PMU high-side output while the 4x4 leg needs a relay to sink it — the PMU switches high-side only.¹⁰

If the switch is missing from the donor case, it is an available service part (≈$50), or fall back to two discrete ground-closing switches on the shift linkage — 4H+4L → HDX 4x4 terminal, 4L only → HDX EXTRA(−) — which removes the PMU, relay, and both firewall pins from the design.

Outstanding Items¶

Build Tasks¶

• Acquire the NP241OR — in hand; the JK NV241 is also on hand and is kept as a spare/fallback⁷
• Verify the unit is an OR (4:1) and not a Command-Trac — red and silver tag on the rear identifies it¹
• Re-confirm the adapter path for the OR case with DomiWorks before ordering (TBD #313)
• Measure 8HP70 output-shaft protrusion (≤~96 mm, else order DomiWorks spacer plate or shorten)
• Confirm the donor case's input spline and stickout against the DomiWorks requirement
• Confirm the 5-position switch is fitted; record its connector/pinout (TBD #145)
• Measure switch resistance at each detent (2H / 4H / N / 4L) and record the table (TBD #145) — the switch is external, on top of the case, so this is done at its connector while moving the lever through the detents. Not blocked: the case is on hand, no teardown and no parts needed⁷
• Size the An 1 divider from the measured resistances; set PMU thresholds; verify 2H reads "no 4x4"
• Cap and label the unused tailhousing VSS connector

Vendor Inquiry — DomiWorks (draft)¶

Draft email to confirm fitment and parts before ordering:

Your listing already answers fitment, price, tilt, the crossmember bracket, and the 96 mm protrusion standard. These are the four questions it does not:

Subject: NP241OR (Rubicon 4:1) fitment — Ram 1500 EcoDiesel 8HP70 adapter

Hi DomiWorks,

I'm mating a Ram 1500 EcoDiesel 8HP70 (4x4) to a Jeep TJ Rubicon
NP241OR "Rock-Trac" transfer case (4.0:1 low range, 2003-2006). Your
adapter lists NP241 fitment from the 8HP70/75 EcoDiesel, which is
exactly my pairing, but I want to confirm four things before ordering.

1. The listing says NP241 — does that cover the Rubicon NP241OR
   specifically? I understand the 4:1 planetary is internal and the
   input face is common to the family, but I'd like that confirmed.

2. Input gear: you specify a 23-spline gear, ZTNP22390 or equivalent.
   The NP241OR's factory input is already 23-spline short (1-1/8"
   stickout). Does the factory gear work as-is, or does it have to be
   replaced with the ZTNP22390? This decides whether I need to open the
   case at all.

3. Bearing / seal / hardware: are the input bearing, front seal, and
   adapter mounting bolts included, or separate?

4. Clearance: does the adapter clear the OR case's 5-position position
   switch on top of the case, and the short tailhousing VSS boss?

Donor details:
- Transmission: ZF 8HP70 (Chrysler 845RE), Ram 1500 EcoDiesel, 4x4
- Transfer case: Jeep TJ Rubicon NP241OR Rock-Trac (4:1 low range)

Thanks,
[name]

Related Documentation¶

• Transmission
• Driveshafts
• HDX Control Module - 4x4 and EXTRA(+) indicator inputs
• PMU Inputs - An 1 position sensing
• PMU Outputs - OUT19 (4x4 relay) and OUT24 (4LO)