Dana Super 70 Pinion Seal Leaking: When to Worry About RV Differential Seepage

The Quick Answer

Yes, a slight weeping or "sweating" pinion seal on your Dana Super 70 rear differential is fairly common, especially on RVs that have been in service for several years. Given that you've owned your RV for 5 years and noticed this condition from day one with 24,000 miles on the odometer, what you're observing is likely normal seepage rather than a problematic leak. The fact that you've never seen actual drips on pavement after five months of winter storage is a good indicator that this is minor seepage, not a significant leak requiring immediate attention. The sweating at the weld spots on the axle tube to differential housing is also relatively normal due to the expansion and contraction cycles these components experience, along with the pressurization that occurs during operation.

Your approach of monitoring fluid levels annually or bi-annually is absolutely appropriate for this level of seepage. Since you just performed a differential fluid change to Amsoil, you have a perfect baseline to monitor any actual fluid loss. Most RV technicians consider light seepage acceptable as long as the differential maintains proper fluid levels and no significant drops are observed over time. The key distinction here is between seepage (which leaves a slight film or dampness) and actual leaking (which would result in drops or puddles).

What's Causing This

The sweating you're observing on your Dana Super 70 pinion seal occurs due to several factors inherent to RV operation and the design of these heavy-duty differentials. The pinion seal, which prevents gear oil from escaping where the drive pinion shaft exits the differential housing, experiences significant stress due to the rotational forces and the substantial weight loads that RV rear ends must handle. Over time, the seal lip can develop microscopic imperfections or slight hardening that allows minimal amounts of gear oil to migrate past the sealing surface.

Temperature cycling plays a major role in this phenomenon. When your RV sits in storage, the differential cools and contracts, creating a slight vacuum effect. When you begin driving, the differential heats up from friction and operation, causing expansion and increased internal pressure. This pressure differential can force small amounts of gear oil past seals that might otherwise contain the fluid perfectly. The Dana Super 70, being a robust commercial-grade differential, actually runs at higher temperatures than lighter automotive units, which accelerates this process.

The sweating at the weld joints between the axle tubes and differential housing is caused by a different but related mechanism. These welds, while structurally sound, can develop microscopic porosity over time due to thermal cycling and the stresses imposed by RV loads. The gear oil, being relatively thin and under pressure, can slowly migrate through these tiny passages. This is particularly common in differentials that operate under the demanding conditions typical of RV service, including frequent stop-and-go driving, grade climbing, and heavy loading.

Another contributing factor is the age and mileage of your unit. At 24,000 miles over 5 years, your differential has experienced numerous heat cycles and varying load conditions. The original factory seals and gaskets have had time to take a "set" and may have lost some of their initial flexibility. This doesn't mean they're failing catastrophically, but rather that they've reached a state where minimal seepage is normal and expected.

Tools You'll Need

For monitoring and addressing minor pinion seal seepage on your Dana Super 70, you'll need a specific set of tools and supplies. The primary diagnostic tool is a quality flashlight or headlamp to clearly observe the seal area and distinguish between fresh seepage and old, accumulated residue. A mechanics mirror or inspection mirror will help you examine hard-to-see areas around the differential housing and weld joints.

For cleaning and assessment purposes, gather some degreaser (avoid brake cleaner near rubber seals), clean rags or shop towels, and disposable gloves. A wire brush or small scraper will help remove old oil residue so you can accurately assess current seepage rates. You'll also want masking tape and a permanent marker to mark areas you've cleaned, allowing you to monitor new seepage over time.

To check fluid levels properly, you'll need the appropriate tools to remove the differential fill plug. For the Dana Super 70, this typically requires a 3/8-inch square drive plug, though some variants use an Allen wrench. Having a torque wrench capable of 15-25 foot-pounds will ensure proper plug reinstallation (consult manufacturer specifications for exact torque). A small funnel and pump or squeeze bottle designed for gear oil will facilitate adding fluid if needed.

For documentation purposes, consider having a digital camera or smartphone to photograph the affected areas. This creates a baseline record that you can compare to future inspections. A small ruler or measuring tape placed in photos provides scale reference for assessing any changes in seepage patterns.

Step-by-Step Fix

Begin by establishing a proper assessment routine for your Dana Super 70's pinion seal seepage. Park your RV on level ground and allow the differential to cool completely, preferably after sitting overnight. WARNING: If accessing the differential requires getting under the RV, ensure proper jack stands or ramps are used - never rely on the RV's leveling jacks for safety support. Clean the entire pinion seal area, the surrounding differential housing, and the weld joints using degreaser (avoid brake cleaner near rubber seals) and shop rags. Remove all old oil residue, dirt, and grime to create a clean baseline for monitoring.

Once cleaned, mark the areas you've addressed with masking tape and note the date with a permanent marker. This creates reference points for future inspections. Take detailed photographs of the clean pinion seal area and weld joints from multiple angles, ensuring good lighting to capture the true condition of the surfaces.

Establish a regular inspection schedule, ideally every 30-60 days initially, then extending to quarterly once you understand your differential's seepage pattern. During each inspection, look for new oil film or dampness in the previously cleaned areas. Use your flashlight to examine the pinion seal closely, checking for any change in the amount or extent of seepage. Document your findings with photos and notes, creating a log of the differential's condition over time.

Check the differential fluid level every six months or before long trips. Remove the fill plug using the appropriate Allen wrench, being careful not to round the edges. The proper fluid level should reach the bottom of the fill hole when the RV is positioned level both front-to-back and side-to-side - this is critical for accurate fluid level assessment. If fluid level is low, add the same Amsoil gear oil you used during your recent service, but add only small amounts at a time to avoid overfilling.

Monitor the seepage pattern for any significant changes. If the amount of seepage increases noticeably, if you begin finding actual drops on the ground, or if fluid levels drop more than a quarter-inch over six months, then more aggressive action may be warranted. However, for the level of seepage you've described, this monitoring approach should be sufficient to ensure your differential remains properly lubricated while avoiding unnecessary repairs.

Consider implementing a seasonal inspection routine where you thoroughly clean and assess the differential at the beginning and end of camping season. This timing coincides with your storage preparation and pre-season checkout, making it easy to incorporate into existing maintenance schedules and routines. Document any changes in seepage patterns that might correlate with seasonal temperature changes or extended storage periods.

When to Call a Pro

While the level of seepage you've described is manageable through monitoring and maintenance, certain conditions warrant professional attention. If you observe drops forming on the ground, fluid levels dropping significantly between checks, or seepage increasing to the point where the pinion seal area remains consistently wet rather than just showing light film, professional service is recommended. Differential losing more than half a quart of fluid over a six-month period, this indicates seepage has progressed to actual leaking that requires seal replacement. A qualified RV technician with experience in Dana Super 70 differentials should perform this work, as proper pinion seal replacement requires setting precise pinion bearing preload and gear mesh patterns.

Contact a professional immediately if you notice metal particles in the differential fluid during level checks, hear unusual noises from the rear end during operation, or observe any fluid that appears milky or contaminated. These symptoms can indicate more serious internal problems that require immediate diagnosis and repair. Additionally, if the seepage suddenly increases dramatically or you begin finding puddles rather than just dampness, professional intervention becomes necessary.

The weld joint seepage you mentioned should be evaluated by a professional if it progresses beyond light sweating. If you notice actual beads of oil forming at weld joints or if the sweating area expands significantly, have the differential housing inspected for structural integrity. While minor porosity is common, extensive weeping could indicate fatigue cracks that might compromise the housing strength.

Consider professional service if you're uncomfortable performing the monitoring and maintenance procedures yourself, or if you lack the tools and experience to properly assess differential fluid levels. Many RV service centers offer differential inspection services that can provide professional documentation of your differential's condition and establish service intervals appropriate for your specific use patterns.

Finally, if your RV is still under warranty, document the seepage with photos and service records, then consult with your dealer about whether this condition is covered. While minor seepage is often considered normal, having professional documentation can be valuable for warranty purposes and helps establish a maintenance history that may be important for future service needs.