ARB AAM 11.5" Rear Air Locker Differential - 30-Spline, RD139
RD139
The ARB RD139 Air Locker Differential is a selectable locking differential designed for AAM 11.5" rear axles using 30-spline axle shafts. Built for heavy-duty Dodge Ram and Ram trucks, this air-operated locker provides maximum traction on demand for towing rigs, trail trucks, overland builds, and demanding off-road applications. With the flip of a dash-mounted switch, compressed air activates the internal locking mechanism, forcing both rear axle shafts to rotate together when additional traction is needed.
When activated, its patented locking mechanism provides positive mechanical engagement for improved traction through rocks, mud, snow, and steep terrain. The durable two-piece steel carrier, minimal moving parts, and reinforced annular air seal are engineered for reliable operation under heavy drivetrain loads. Compatible with all gear ratios in supported AAM 11.5" rear axle configurations, the RD139 combines selectable traction with predictable on-road handling.
Includes:
- (1) ARB RD139 AAM 11.5" Air Locker Differential Assembly
- (1) Dash-Mounted Activation Switch
- (1) Air Solenoid Valve
- Air Line
- Bulkhead Fitting
Important: A compatible ARB Air Compressor is required for operation and is sold separately. Verify the installed AAM 11.5" rear axle, 30-spline configuration, and ring gear mounting requirements before ordering.
Specifications
| Manufacturer | ARB |
|---|---|
| Part Number | RD139 |
| Product Type | Selectable Air Locking Differential |
| Axle Platform | AAM 11.5" |
| Ring Gear Diameter | 11.50" / 292.1mm |
| Axle Position | Rear |
| Axle Spline Count | 30-Spline |
| Axle Shaft Diameter | 1.54" / 39.1mm Nominal |
| Gear Ratio Compatibility | All Ratios |
| Ring Gear Bolt Count | 12 |
| Ring Gear Bolt Circle | 220mm / 8.66" |
| Ring Gear Mounting Bore | 187.2mm / 7.37" |
| Carrier Compatibility | Compatible AAM 11.5" Rear Differential Applications |
| Differential Operation | Selectable Open / Fully Locked |
| Engagement Type | Pneumatic / Compressed Air |
| Carrier Construction | Heavy-Duty Two-Piece Steel Design |
| Locking Mechanism | Patented Timed Locking Mechanism |
| Air Seal | Steel-Reinforced Elastomer-Bonded Annular Seal |
| Material | Steel |
| Finish | Silver |
| Installation Type | Differential Carrier Replacement |
| Primary Application | AAM 11.5" Rear Axle Traction Upgrades for Dodge Ram & Ram Heavy-Duty Trucks |
| Required Components | Compatible Air Compressor, Air Supply, Differential Bearings & Installation Components |
| Known Incompatibilities | Incorrect Axle Platform, Spline Count or Ring Gear Mounting Configuration |
| Quantity | One Air Locker Differential Assembly |
Fitment
Designed for compatible AAM 11.5" rear differential applications using 30-spline axle shafts. Vehicle fitment depends on the installed axle configuration, engine, and drivetrain package.
| Make | Model | Year Range | Axle Application |
|---|---|---|---|
| Dodge | Ram 2500 | 2003–2010 | AAM 11.5" Rear - Equipped Models Only |
| Dodge | Ram 3500 | 2003–2010 | AAM 11.5" Rear - Equipped Models Only |
| Ram | 2500 | 2011–2019 | AAM 11.5" Rear - Equipped Models Only |
| Ram | 3500 | 2011–2019 | AAM 11.5" Rear - Equipped Models Only |
Fitment Notes:
- Fits compatible AAM 11.5" rear axle applications with 30-spline shafts.
- Compatible with all gear ratios according to ARB's RD139 installation specifications.
- Vehicle year and model alone do not guarantee compatibility.
- Not every Ram 2500 or 3500 uses the same rear differential configuration.
- Verify the installed axle model, spline count, ring gear mounting dimensions, and carrier bearings before ordering.
- Requires a compatible compressed air supply for locker engagement.
- ARB air compressor sold separately.
- Professional differential installation and air line routing are recommended.
Builder Note: The AAM 11.5" is a popular heavy-duty rear axle platform used in diesel-powered trucks, towing applications, and off-road builds. However, Ram trucks were equipped with different rear axle configurations depending on model year and drivetrain. Before installing the RD139, confirm the axle identification and differential dimensions rather than selecting the locker based on vehicle year alone.
FAQ
What axles does the ARB RD139 Air Locker fit?
The RD139 is designed for compatible AAM 11.5" rear differential applications using 30-spline axle shafts. It is commonly used in select Dodge Ram and Ram heavy-duty trucks equipped with this axle platform.
Will the ARB RD139 fit my Ram 2500 or Ram 3500?
It fits compatible Ram 2500 and 3500 rear axle configurations equipped with the AAM 11.5" differential. Verify the installed axle before ordering because different axle options were available across model years and drivetrain packages.
What gear ratios are compatible with the RD139?
ARB's RD139 installation specifications identify compatibility with all gear ratios in supported AAM 11.5" rear axle applications. Correct ring gear mounting and differential setup are still required.
Does the ARB RD139 require an air compressor?
Yes. A compatible air compressor and air supply system are required to activate the locker. The ARB air compressor is sold separately.
How does the ARB RD139 Air Locker work?
Compressed air activates an internal mechanical locking mechanism that connects both axle shafts, forcing the rear wheels to rotate together. When disengaged, the differential operates as a conventional open unit.
Can I use the RD139 in a daily-driven truck?
Yes. When disengaged, the Air Locker allows normal wheel-speed differentiation during cornering and street driving. The driver can engage the locker when additional traction is needed off-road.
What is the difference between an ARB Air Locker and an automatic locker?
An ARB Air Locker provides driver-controlled engagement using compressed air. An automatic locker engages and disengages mechanically based on its design and driving conditions. The ARB allows the driver to choose when the differential is locked.
Does the RD139 stay locked all the time?
No. The RD139 operates as an open differential when disengaged. It only locks when the driver activates the air-operated locking system.
Does the RD139 include the activation switch and air line?
Yes. The supplied kit contents include the Air Locker differential assembly, activation switch, solenoid valve, air line, and bulkhead fitting. The compressor is sold separately.
Can the RD139 be installed during a ring and pinion gear change?
Yes. Installing an Air Locker during a re-gear is practical because the differential carrier is already being removed. Proper bearing preload, ring gear backlash, and tooth contact pattern must be established during installation.
Does this Air Locker fit every AAM 11.5" axle?
No. Compatibility must be confirmed against the specific axle configuration, spline count, carrier dimensions, and ring gear mounting requirements. The RD139 is designed for supported 30-spline AAM 11.5" rear applications.
Is the RD139 suitable for towing and off-road use?
Yes. The selectable design is well suited for heavy-duty trucks that need normal differential operation on the road and additional traction in demanding off-road conditions. The locker does not increase the vehicle's factory towing or axle load rating.
