Hydraulic Booster Noise Diagnosis Process
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
DEFINITION: Brake booster noises that generate a customer complaint may be normal and temporary. However, they may be a sign of wear, or evidence of air, in the hydraulic booster or power steering systems. | ||||||||||
1 | Verify the complaint. Define the type of noise, and when the noise occurs. Did you verify the complaint? | -- | -- | |||||||
2 | Does the noise occur during high brake pedal efforts with quick pedal release? | -- | -- | |||||||
3 | Refer to the Normal Operating Noises section that follows. Compare the system to a known acceptable system. Does the acceptable system make the same noise? | -- | System OK | |||||||
4 | Does the noise occur under the following conditions?
| -- | -- | |||||||
5 |
Does the acceptable system make the same noise? | -- | System OK | |||||||
Noise Diagnosis
| • | A moan or low frequency hum (usually accompanied by a vibration in the brake pedal or steering column) may be noticed during parking or other low speed maneuvers. The moan may be caused by insufficient power steering fluid, or air in the fluid. Holding the steering wheel turned as far as possible in one direction holds the power steering pump at relief pressure. Holding the pump at relief pressure for more than 5 seconds causes air to enter the system. Verify the cause of the noise using the following procedure: |
| 1. | Check the fluid. Fill as needed. |
| 2. | Turn the engine OFF. |
| 3. | Allow the vehicle to sit for 1 hour to let the air in the system escape. |
| 4. | Verify that the moan is no longer present. If the moan persists, refer to Power Steering System Test Procedure . |
| • | A high-speed fluid noise may be heard when fully applying the brake pedal. The noise is normal. |
| • | A slight hiss while the accumulator pressure is in use may be noticed. This is hydraulic fluid escaping through the accumulator valve. This noise is normal. |
| • | A hissing noise may be heard during the first brake application or steering maneuver if the engine is started and the accumulator is empty. The noise is fluid rushing through the accumulator charging orifice. This noise is normal but should only be heard once after the accumulator is emptied. If the noise continues, even though no apparent accumulator pressure assist occurred, the noise may indicate that the accumulator is not holding pressure. Check the accumulator. Refer to Hydraulic Booster Functional Test . |
Power steering pump noise can be confused with noise from the transmission, rear axle, or generator. If excessive noise is present, remove the power steering pump drive belt to check if the noise is coming from the power steering pump. Refer to Drive Belt Replacement and Power Steering Pump Replacement .
Air in the system causes noise. Fill and bleed the system when a hose has been disconnected. Refer to Hydraulic Brake System Bleeding .
The hydraulic booster uses fluid pressure from the power steering system. A malfunctioning power steering system can affect the hydraulic booster. A malfunctioning booster can affect the steering system. Before beginning extensive testing, perform the following procedure:
- Check all power steering and brake pipe connections for leaks and restrictions.
- Ensure that the master cylinder and the power steering reservoir are properly filled. If the power steering fluid or the brake fluid contain air, refer to Locating Fluid Leaks in Power Steering System, and .
- Check the power steering pump belt for wear and improper tension. Refer to Drive Belt Replacement in Engine Mechanical.
- Check the power steering pump pressure. Refer to Power Steering System Test Procedure .
Important: Power steering fluid and brake fluid cannot be mixed. If brake seals contact power steering fluid or steering seals contact brake fluid, seal damage will result.
Normal Operating Noises
Normal hydro-boost units produce characteristic noises. These noises usually occur when the brake pedal is manipulated in an unusual manner. The general types of normal operating noises are as follows:
Hissing | Clunk/Click/Clatter | ||||
|---|---|---|---|---|---|
Hissing noises that occur with a brake pedal effort of 266 N (60 lb) or more are normal. The noise is particularly noticeable with the vehicle motionless. The noise will increase in intensity with the following conditions:
Important: : Loud hissing noises at, or below normal pedal effort 111 N (25 lb) warrant investigation. | Clunk, clatter, or clicking noises will be heard when you quickly release the brake pedal from hard efforts 222-444 N (50-100 lb). |
