This maintenance manual provides technical instructions for the service and repair of Meritor Double-Reduction Drive Units featuring a Spiral Bevel-Spur design and an Inter-Axle Differential (IAD). These heavy-duty units utilize a two-stage reduction process—a primary spiral bevel gear set followed by a secondary spur gear set—to provide high torque for specialized applications.

1. Technical Adjustments and Specifications

Proper assembly is critical for operational longevity and requires adhering to precise parameters:

Pinion Bearing Preload: Controlled by the thickness of the spacer between the bearings. For new bearings, the preload should be 15–25 lb-in (1.7–2.8 N·m); for used bearings in good condition, it is 5–25 lb-in (1.7–2.8 N·m).
Main Differential Bearing Preload: The standard specification is 15–35 lb-in (1.7–3.9 N·m). This can also be measured by expansion between bearing caps: 0.002–0.009 inch (0.05–0.229 mm) for standard models and 0.006–0.013 inch (0.15–0.33 mm) for 180 series models.
Ring Gear Backlash: For new gear sets, backlash should be set to a nominal 0.015 inch (0.38 mm), within a range of 0.010–0.020 inch (0.25–0.51 mm).
Runout and Resistance: Maximum allowable ring gear runout is 0.008 inch (0.20 mm). The maximum rotating resistance for differential gears is 50 lb-ft (68 N·m) applied to one side gear.

2. Core Assembly and Service Highlights

Matched Gear Sets: Drive pinions and ring gears are matched sets and must always be replaced together.
Pinion Depth Adjustment: Controlled by a shim pack between the bearing cage and the carrier. Technicians must calculate new shim thickness based on the old pack measurement and the Pinion Cone (PC) variation numbers die-cut on the gears.
Ring Gear Installation: Ring gears must be heated in water to 160°F–180°F (71°C–82°C) for 10 to 15 minutes before installation to permit a press-free fit over the case half pilot.
Inter-Axle Differential (IAD): The IAD allows for speed differences between the forward and rear tandem axles. If the unit includes a Driver-Controlled Main Differential Lock (DCDL), it must be shifted and held in the engaged (locked) position during carrier removal or installation to provide necessary clearance.

3. Safety and Lubrication

Towing: To prevent internal damage, axle shafts must be removed before towing a vehicle with its drive wheels on the ground.
Lubrication: Use only API GL-5 hypoid gear oil. Sump temperatures must never exceed 250°F (121°C), as excessive heat will damage internal components.