Owner's Problem Description
The owner reports a vibration-based humming noise from the rear brakes of their 1995 Volvo 850. This issue began during wet weather and has been occurring more frequently. Notably, the noise only manifests when the brake pedal is fully released, ceasing immediately upon pedal depression. Despite a recent oil change and brake check by a trusted mechanic, the brakes were deemed in good condition. The owner seeks diagnostic assistance.
Professional Technical Analysis
The 1995 Volvo 850, equipped with rear disc brakes and a standard ABS system, has reported a unique vibration noise emanating from the rear brakes, particularly when the brake pedal is fully released. This symptom typically arises under wet conditions and increases in frequency over time. The vehicle's mileage is approximately 150,000 miles, which can correlate with typical wear patterns in brake components. The absence of noise upon pedal application suggests that the issue may not stem from traditional brake wear but rather from resonance or vibration issues linked to the brake calipers or pads. Notably, Volvo issued a Technical Service Bulletin (TSB 803) regarding the rear brake shims for the 1993-1997 Volvo 850 models, which addresses similar resonance vibrations. This TSB indicates that a specific shim kit (P/N 272272-6) can minimize these vibrations. Testing should confirm that the brake pads are properly seated and that the shims are in place. Furthermore, inspecting the brake rotor for warping or uneven wear may also be necessary, as these factors could contribute to vibration under specific conditions. Overall, the vehicle's current condition requires a thorough inspection of the brake system components and adherence to the manufacturer's recommendations to ensure safety and reliability.
Possible Causes
Most common causes (ordered by frequency):
- Rear Brake Shim Resonance: The 1995 Volvo 850 may experience resonance vibrations due to inadequate shimming. According to TSB 803, new shims (P/N 272272-6) are required to minimize these vibrations, particularly when brakes are hot after repeated use. The absence of proper shims can lead to noise during pedal release, indicating a resonance condition rather than a mechanical failure. Insufficient or worn shims contribute to increased vibration frequencies, especially in wet conditions. - Noise occurs only when the brake pedal is released; previous mechanic inspection confirmed brake condition without visible wear.
- Potential Brake Pad Issue: Although the mechanic reported the brakes as being in fine condition, the brake pads may still be exhibiting uneven wear or not seated properly. This can lead to vibration noises when the brakes are applied lightly or released completely, especially under wet conditions. Inspecting for proper pad installation and wear patterns can provide additional insights. - Mechanic's inspection report noted pads were in good condition, but did not assess for proper seating or uneven wear.
- Warped Brake Rotors: Warping of the brake rotors can result in vibration and noise during braking or when the brakes are released. This issue often goes unnoticed until symptoms become pronounced. Measuring the rotor runout with a dial gauge can help determine if warping is present. The acceptable runout specification for Volvo 850 rotors is typically less than 0.003 inches (0.08 mm). - Vibration noise increases with brake usage; symptoms may appear after prolonged driving. Inspecting rotor condition is recommended.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Visual Inspection: Begin with a thorough visual inspection of the rear brake components, including calipers, pads, and rotors. Check for any signs of uneven wear, damage, or missing shims. Ensure that the pads are properly seated within the caliper brackets. Use a flashlight to inspect for any debris or contaminants that could affect brake performance.
- STEP 2 - Rotor Runout Measurement: Use a dial indicator to measure the runout of the brake rotors. Acceptable limits should be below 0.003 inches (0.08 mm). If excessive runout is detected, the rotors may need resurfacing or replacement to ensure proper braking performance.
- STEP 3 - Brake Pad Inspection: Remove the rear brake pads and inspect for signs of uneven wear or improper installation. Measure the thickness of the pads; they should be above 3mm for optimal performance. If pads appear uneven, consider replacing them with OEM parts (P/N 272271-8) for better fit and function.
- STEP 4 - Shim Replacement: Based on TSB 803, replace the rear brake shims with the updated shim kit (P/N 272272-6) designed to minimize resonance vibrations. Ensure all components are properly torqued to manufacturer specifications. Reassemble the brakes and perform a test drive to verify noise elimination.
