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Brake Assist System Diagnosis

WARNING: This page is about a different car, the 2007 Hummer H2. However, it is still accessible from the selected car via links, so may be relevant.
Brake Assist System Diagnosis

Step Action Yes No
DEFINITION: This diagnostic table is designed to diagnose ONLY the components of the brake ASSIST system in order to determine if the brake ASSIST system is operating properly. You will be directed by the appropriate Symptom table to go to other brake system diagnostic tables as appropriate.
1 Were you sent here from a Brake Symptom table? Go to Step 2  Go to Diagnostic Starting Point - Hydraulic Brakes 
2 Is the symptom related to brake system noise? Go to Step 4  Go to Step 3 
3 Is the symptom related to the brake system self-applying when the steering wheel is being turned? Go to Step 11  Go to Step 9 
4 Check the hydraulic assist system operation for the presence of abnormal noise. Refer to Hydraulic Brake Assist System Noise Inspection .
Did the system operate normally from a noise standpoint?		 Go to Step 20  Go to Step 5 
5
  1. Check the power steering system fluid level and adjust the level if necessary. Refer to Checking and Adding Power Steering Fluid .
  2. Inspect the power steering pump drive belt for proper tension and excessive wear.
  3. Inspect the power steering pressure and return hoses for proper routing, possible ground-outs, possible restrictions, and/or damage.
Did you find and correct a condition?		 Go to Step 20  Go to Step 6 
6
  1. Position the vehicle on the ground with wheels straight ahead.
  2. Start the engine and allow it to idle.
  3. Apply the brake pedal with moderate force 130-200 N (30-45 lb). Observe for noises.
  4. Release the brakes and turn the steering wheel slightly from side to side. Observe for noises.
  5. Compare the noise levels between brake apply and steering input.
Was the abnormal noise heard ONLY during brake apply?		 Go to Step 12  Go to Step 7 
7 Perform the Power Steering System Test procedure to determine if there are any restrictions in the system. Refer to Power Steering System Test .
Did you find and correct a condition?		 Go to Step 20  Go to Step 8 
8 During the noise inspection, did the hydraulic brake booster produce abnormal noises? Go to Step 12  Go to Step 20 
9 Inspect for proper brake pedal travel. Refer to Brake Pedal Travel Measurement and Inspection .
Is the brake pedal travel distance within the acceptable limits?		 Go to Step 11  Go to Step 10 
10
  1. Inspect for worn, missing, misaligned, bent or damaged brake pedal system components.
    • For the brake pedal pushrod component inspection, refer to Brake Pedal Pushrod Inspection  .
    • Inspect the brake pedal bushings for excessive wear and/or damage and inspect the brake pedal for a misaligned, bent and/or damaged condition.
  2. Replace the brake pedal system components that are worn, missing, misaligned, bent or damaged.
Did you find and replace any worn, missing, misaligned, bent or damaged brake pedal system components?		 Go to Step 12  Inspect for internal and external fluid leaks and air in the hydraulic brake system
Go to Hydraulic Brake System Diagnosis 
11
  1. Check the power steering system fluid level and adjust the level if necessary. Refer to Checking and Adding Power Steering Fluid .
  2. Inspect the power steering pump drive belt for proper tension and excessive wear.
  3. Inspect the power steering pressure and return hoses for proper routing, possible ground-outs, possible restrictions, and/or damage.
Did you complete the inspections and any necessary adjustments or replacements?		 Go to Step 12  -
12 Perform the hydraulic booster functional test:
  1. With the ignition OFF, depress the brake pedal several times to deplete the power reserve.
  2. Depress and maintain 178 N (40 lb) of force on the brake pedal.
  3. Start the engine and observe pedal operation.

Note: The brake pedal should fall slightly, then hold.
Did the brake pedal fall slightly after the engine was started, then hold?		 Go to Step 16  Go to Step 13 
13 Perform the Power Steering System Test procedure to determine if there are any restrictions in the system. Refer to Power Steering System Test .
Did you find and correct a condition?		 Go to Step 14  Go to Step 19 
14 Perform the hydraulic booster functional test:
  1. With the ignition OFF, depress the brake pedal several times to deplete the power reserve.
  2. Depress and maintain 178 N (40 lb) of force on the brake pedal.
  3. Start the engine and observe pedal operation.

Note: The brake pedal should fall slightly, then hold.
Did the brake pedal fall slightly after the engine was started, then hold?		 Go to Step 16  Go to Step 19 
15 Perform the Power Steering System Test procedure to determine if there are any restrictions in the system. Refer to Power Steering System Test .
Did you find and correct a condition?		 Go to Step 14  Go to Step 19 
16 Perform the short-term or accumulator reserve leak-down test:
  1. Start the engine and allow it to idle.
  2. Charge the accumulator:

    Turn the steering wheel to full-lock position for 2 seconds ONLY and return to rest.

  3. Turn the ignition OFF.
  4. Wait 90 seconds.
  5. Depress the brake pedal 2 times to check power reserve. Observe pedal effort.

Note:
  • Both pedal applies should be assisted
  • The amount of assist could diminish slightly with the second apply
  • If 2 power assisted pedal applications could not be performed, the accumulator is losing its gas charge

Were both pedal applies power assisted?		 Go to Step 17  Go to Step 18 
17 Perform the long-term reserve leak-down test:
  1. Start the engine and allow it to idle.
  2. Charge the accumulator:

    Turn the steering wheel to full-lock position for 2 seconds ONLY and return to rest.

  3. Turn the ignition OFF.
  4. Wait 1 hour.
  5. Depress the brake pedal 2 times to check power reserve. Observe pedal effort.

Note:
  • Both pedal applies should be assisted
  • The amount of assist could diminish slightly with the second apply
  • If 2 power assisted pedal applications could not be performed, the hydraulic booster assembly is not retaining power reserve

Were both pedal applies power assisted?		 Go to Step 20  Go to Step 19 
18 Replace the hydraulic brake booster accumulator assembly, if applicable.
Did you complete the replacement?		 Go to Step 20  -
19 Replace the hydraulic brake booster assembly. Refer to Power Brake Booster Replacement .
Did you complete the replacement?		 Go to Step 20  -
20 Install or connect any components that were removed or disconnected during diagnosis.
Did you complete the operation?		 Brake Assist System OK
Return to Symptom Table		 -
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