Whether you drive a front-wheel-drive (FWD) or rear-wheel-drive (RWD) vehicle, the differential plays a crucial role in ensuring smooth and efficient operation. In RWD cars, the differential is a standalone unit responsible for distributing power to the wheels, while in FWD vehicles, it’s integrated into the transmission. This distinction is important because it affects how repairs and maintenance are approached.
While symptoms for FWD and RWD are similar, the repair procedure differs.
In this guide, we’ll focus on RWD-mounted differentials, detailing common symptoms that indicate trouble, and methods for diagnosis.
Understanding these signs and procedures can help you maintain your car’s performance and avoid spendy repairs.
What is a Differential and Its Function?
A differential is a key component found in the drivetrain of vehicles, including both rear-wheel-drive (RWD) and front-wheel-drive (FWD) configurations a four-wheel drive will have two differentials, one for each axle. Its primary function is to allow the wheels to rotate at different speeds while providing independent torque to each wheel.
This is important because when turning a corner, the outer wheels on an axle need to rotate faster than the inner wheels. Without a differential to accommodate this difference, the vehicle would skid uncontrollably on every turn.
In RWD vehicles, the differential is a standalone unit mounted on the rear axle. It consists of gears and bearings that distribute power from the driveshaft to the wheels, ensuring optimal traction.
In front-wheel-drive (FWD) vehicles, the differential is integrated into the transmission housing and connects directly to the front axle shafts.
This design simplifies packaging and reduces drivetrain complexity. However, it typically requires more specialized repair procedures not commonly undertaken by local garages. Specialized repair shops with the necessary expertise and tools normally handle these repairs.
Understanding the type and function of your car’s differential is important for diagnosing and addressing potential issues.
While the symptoms of both types of differentials are similar, the repair differs. In the following sections, we’ll explore common signs of differential problems, how to diagnose them, and an outline of the repair techniques; however, we will cover RWD-mounted differentials only.
Signs Of Diff Trouble & Possible Causes
Here are common signs that indicate your differential might be experiencing problems:
Unusual Noises: Whining, grinding, howling or clunking sounds, especially during acceleration or deceleration, may indicate worn gears, backlash issue, pinion preload issue, or bearing issue within the differential.
Leaking Fluid: Visible leaks or puddles of differential fluid underneath the vehicle, often around seals (such as the pinion seal, axle seals or cover gasket), indicate seal failure. Other possibles include loose oil filler bung or diff cover damage caused by grenaded pinion tooth.
Vibrations: Excessive vibrations felt through the vehicle, particularly at higher speeds, may suggest issues with the differential gears or bearings.
Overheating: A hot or burning smell coming from the differential area may indicate overheating due to lack of lubrication or excessive friction.
Difficulty Turning: Resistance or hesitation when turning, especially at low speeds, could indicate binding gears or insufficient lubrication within the differential.
Uneven Tire Wear: Differential problems can cause uneven tire wear, typically on the inner or outer edges of the tires, due to improper power distribution to the wheels.
Visible Damage or Play: Damage to the casing or back cover is common, either from an external impact or from an internal component disintegrating and striking the housing.
Dashboard Warning Lights: In some more sophisticated vehicles, differential issues may trigger warning lights on the dashboard related to the drivetrain or limited slip control system part of the stability control systems.
Next, we’ll look at what may be causing each of these symptoms.
Diagnosing Differential Issues
A differential can fail in various ways. Diagnosis depends on the specific symptoms you’re experiencing. First, I’ll explain how I inspect a differential and look for signs of trouble. Then, we’ll explore each symptom and its corresponding repair for RWD-mounted differential.
- Visual Inspection: Begin by visually inspecting the differential housing for signs of leaks, such as wet spots or dripping fluid. Check the area around seals and gaskets, including the pinion and axle seals. Check also for excessive play in the driveshaft or half shafts.
- Road Test: Take your vehicle for a test drive to observe any unusual noises or vibrations. Pay attention to how the car handles during turns and accelerations, noting any resistance or grinding sensations.
- Listen for Noises: Listen carefully for abnormal sounds, such as whining, howling or clunking. These noises often indicate worn gears, pinion preload issues, bearings, or insufficient lubrication.
- Check Differential Fluid: Inspect the differential fluid level and condition. Milky or foamy fluid indicates water contamination, while low fluid levels can suggest leaks or internal issues. Glittering particles suggest a bearing or gear wear issue and a tear-down is needed.
- Perform a Driveline Inspection: Inspect the driveshaft and half shafts for signs of damage or excessive play. Worn universal joints (U-joints), damaged or out-of-balance propshaft, or CV joints can contribute to differential-related symptoms.
- Use Diagnostic Tools: Utilize diagnostic tools, such as a stethoscope or chassis ears, to pinpoint the source of noises more accurately. These tools help isolate differential-related sounds from other drivetrain components.
- Internal Inspection: Finally, if I believe we are looking at a diff issue, I’ll remove the rear cover and inspect the pinion, ring gear and spyder gears for signs of damage and wear.
Mechanic’s tip: Tires can often cause road noise resembling differential issues. Before assuming a problem with the differential, carefully inspect the tires for cupping, feathering, or uneven wear patterns.
A faulty wheel bearing can also create a similar noise. To check for this, drive slowly and swerve from side to side. If the noise tone changes significantly, it’s a failed wheel bearing.
Now, let’s look at each symptom and its likely cause.
- Unusual Noises (Whining/Howling or Grinding):
- Possible Causes: Backlash incorrect, gear wear, bearing wear, incorrect pinion preload, low differential fluid.
- Check & Fix: Lift the vehicle on jack stands. Run the vehicle and listen closely around the differential with a mechanic’s stethoscope to pinpoint the noise’s source. Check the differential fluid level and condition; replacing the fluid can sometimes help. Persistent noises need a bearing and gear inspection.
- Clunking or Banging on Shifting:
- Possible Causes: Backlash incorrect, loose axle, bad prop-shaft universal joint, damaged/worn ring gear, or pinion gear or incorrect pinion preload.
- Check & Fix: Check the prop shaft, half shafts and axle. Check pinion preload and backlash.
- Vibrations During Driving:
- Possible Causes: Imbalanced wheels, worn-out CV joints, misaligned drive shaft.
- Check & Fix: Inspect tires for abnormalities and ensure they’re balanced. Look at the CV joints for wear, do a visual once-over and check drive shaft’s alignment.
- Fluid Leaks:
- Possible Causes: Worn axle seals, pinion seal, back cover gasket, oil filler/drain bung seal, or cracked differential casing.
- Check & Fix: Spot leaks by tracing fluid paths back to potential seal issues. Replace leaking seals and check for cracks or damage to the casing. Always refill with fresh differential fluid after repairs.
- Uneven Tire Wear:
- Possible Causes: Misaligned wheels, uneven suspension wear.
- Check & Fix: Check wheel alignment and inspect the condition of suspension components. Adjust alignment and replace worn suspension parts for even tire contact.
How to Repair Differential Problems
Fixing differential issues in your vehicle requires specialized techniques. While some tasks are manageable for a DIY mechanic, many need professional tools, a controlled environment, and advanced skills. Here’s an outline for tackling common differential problems.
- Adjusting Pinion Preload: To adjust pinion preload, use a rotational torque wrench and a new crush sleeve to achieve the correct resistance. Alternatively, some setups may use shims instead of a crush sleeve for preload adjustment. You will likely need a puller to remove the Yoke; this is DIY doable but at the upper end.
- Replacing Pinion Seals: If you’ve identified leaks around the pinion seal, replace it promptly to prevent fluid loss and potential damage to internal components. You will need a new crush sleeve, and the pinion preload will need adjustment, so a rotational torque wrench and large breaker bar are required. DIY doable but at the upper end.
- Replacing Bearings: Worn or damaged bearings cause noise and vibration issues and, if not fitted correctly, could seize locking the rear diff. This is typically a job for a shop with the correct tools and knowledge.
- Adjusting backlash: The contact between the ring gear and pinion gear, known as backlash, often requires checking and adjustment. This process involves a precise, step-by-step approach using a dial gauge to measure the clearance. Adjustments can typically be made using side adjusters, though some axles rely on shims. While possible for experienced DIY mechanics, this task is on the more advanced end of home repairs.
- Routine Maintenance: Change differential fluid regularly as per manufacturer recommendations to maintain proper lubrication and cooling. Use the recommended fluid type and ensure proper filling procedures to prevent overfilling or underfilling. This is an easy DIY chore.
- Replacing axle seals (independent rear suspension): A leak at the half-shaft axle seal is common for cars with independent suspension. Drain the diff. Remove the shaft, pry out the seal, and fit a new one. The diff oil will need to be replaced. This is a medium DIY difficulty.
- Replace axle seal (Live rear axle): For live rear axles such as those fitted to pick-up trucks, remove the brake drum/rotor, drain the diff, remove the pin and C-clip, and slide the axle out. Remove the seal and fit a new one, replace the axle, and fill with fresh oil. This is a medium DIY difficulty.
Consider seeking professional assistance for complex repairs involving full differential rebuilds or transmission-integrated differentials in FWD vehicles. These tasks require specialized tools and expertise for safe and effective completion.
When working on a differential, it’s important to reinstall all components exactly as they were removed. While parts like bearing caps may appear identical, they are specific to their original positions. Be thorough in your approach—label everything carefully and take plenty of photos.
The prop shaft is a balanced component. Its alignment with the yoke is precise, so it must be reinstalled correctly to avoid vibration issues.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.