| Table 1: | Hydraulic Brake Booster Diagnosis--Noisy Brake Booster |
System Tests
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. Perform the following procedure before beginning any extensive testing.
- Inspect all of the power steering and the brake pipe connections for leaks and restrictions.
- Ensure that the master cylinder is properly filled.
- Ensure that the power steering pump reservoir is properly filled.
- Inspect the power steering pump belt for wear and improper tension. Refer to Symptoms - Drive Belt in Engine Mechanical 4.8L, 5.3L, 6.0L.
- Test the power steering pump pressure. Refer to Power Steering System Test in Power Steering System.
Notice: 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.
If the power steering fluid contains air, bleed the power steering system. Refer to Power Steering System Bleeding in Power Steering System.
Noise from the relief valve is normal when applying the brakes. Firmly applying the brake pedal while the vehicle is parked also causes noise. The noises are the result of air allowed into the fluid during these conditions. The air remains in the fluid only temporarily.
Power steering pump noise can be confused with transmission, rear axle, or generator noise.
The following noises come from the hydraulic booster. The noises may be cause for a customer concern. Noises may be normal and temporary. Noises may be a sign of wear, or evidence of air in the hydraulic booster or the power steering system
| • | You may notice a moan or low frequency hum (usually accompanied by a vibration in the brake pedal or steering column) 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 1 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. | Test the fluid level. Fill the fluid reservoir as needed. Refer to Master Cylinder Reservoir Filling . |
| 2. | Turn OFF the engine. |
| 3. | Allow the vehicle to sit for 1 hour. Any air in the system will escape. |
| 4. | Verify that the moan is no longer present. If the moan persists, refer to Power Steering System Test in Power Steering System. |
| • | You may hear a high-speed fluid noise when fully applying the brake pedal. The noise is normal. |
| • | You may notice a slight hiss while the accumulator pressure is in use. The hiss is hydraulic fluid escaping through the accumulator valve. The noise is normal. |
| • | If the accumulator is empty when the engine is started, you may hear a hissing noise during the first brake application or steering maneuver. The noise is fluid rushing through the accumulator charging orifice. The noise is normal. The noise should be heard only 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. Inspect the accumulator. Refer to Accumulator Leak-Down Test . |
Problem | Action | ||||
|---|---|---|---|---|---|
DEFINITION: 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 hissing, clunk, click and clatter noises. | |||||
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Important: Loud hissing noises at, or below, normal pedal effort 11 N (25 lb) warrant investigation. 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 condition:
| Clunk, clatter, or clicking noises will be heard when you quickly release the brake pedal from hard efforts 222-444 N (50-100 lb). | ||||
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Verify the concern. Define the type of noise, and when the noise occurs. Did you verify the concern? | -- | Go to Step 2 | -- | ||||||
2 | Does the noise occur during high brake pedal efforts, with quick pedal release? | -- | Go to Step 3 | Go to Step 4 | ||||||
3 | Compare the system to a known acceptable system. Does the acceptable system make the same noise? | -- | System OK | Go to Step 1 | ||||||
4 | Does the noise occur under any of the following conditions?
| -- | Go to Step 5 | -- | ||||||
5 |
Does the acceptable system make the same noise? | -- | System OK | Go to Step 1 |
