Hydro-boost brake assist and power steering share one hydraulic supply (four campaigns)

Why it happensSome vehicles were built with an out-of-specification hydro-boost housing relief valve bore.

SOME OF THESE VEHICLES WERE PRODUCED WITH AN OUT-OF-SPECIFICATION BRAKE HYDRO-BOOST HOUSING RELIEF VALVE BORE.— [1]

If leftThe valve O-ring seal may fracture, and steering effort may be slightly higher while braking or parking.

CONSEQUENTLY, THE VALVE O-RING SEAL MAY FRACTURE. STEERING EFFORTS MAY BE SLIGHTLY INCREASED WHILE BRAKING OR PARKING.— [1]

RepairDealers replace the hydro-boost relief valve.

DEALERS ARE TO REPLACE THE HYDRO-BOOST RELIEF VALVE.— [1]

Why it happensThe hydraulic brake booster pressure accumulator may crack and separate from the hydro-boost assembly during normal operating conditions.

THE HYDRAULIC BRAKE BOOSTER PRESSURE ACCUMULATOR MAY CRACK AND SEPARATE FROM THE HYDRO-BOOST ASSEMBLY DURING NORMAL OPERATING CONDITIONS.— [2]

If leftThe loss of fluid would increase steering and braking effort, and a crash may occur without prior warning.

THE LOSS OF FLUID WOULD CAUSE INCREASED STEERING AND BRAKING EFFORT AND A CRASH MAY OCCUR WITHOUT PRIOR WARNING.— [2]

RepairDealers test the hydro-boost assembly for functional operation and replace it if it fails.

DEALERS WILL TEST THE HYDRO-BOOST ASSEMBLY FOR FUNCTIONAL OPERATION OF THE TWO-FUNCTION VALVE. IF THE HYDRO-BOOST ASSEMBLY FAILS THE TEST, DEALERS WILL REPLACE THE ASSEMBLY FREE OF CHARGE.— [2]

If leftA hydraulic pump driveshaft can fracture, causing immediate loss of power steering assist.

EQUIPPED WITH A HYDRAULIC PUMP DRIVESHAFT THAT CAN FRACTURE, RESULTING IN IMMEDIATE LOSS OF HYDRAULIC POWER STEERING ASSIST.— [3]

If leftOn hydro-boost vehicles the same failure removes brake power assist once the reserve pressure is used up.

ON CERTAIN VEHICLES EQUIPPED WITH HYDRO-BOOST POWER BRAKES, THE SAME CONDITION CAN RESULT IN LOSS OF POWER ASSIST FOR BRAKING AFTER THE RESERVE PRESSURE IS DEPLETED.— [3]

CautionAn inoperative pump raises steering and braking effort but does not completely remove the ability to steer or slow the vehicle.

AN INOPERATIVE PUMP CAN CAUSE INCREASED STEERING EFFORT, AND IN HYDRO-BOOST EQUIPPED VEHICLES ALSO INCREASED BRAKING EFFORT, BUT DOES NOT COMPLETELY ELIMINATE THE ABILITY TO STEER OR SLOW THE VEHICLE.— [3]

RepairDealers replace the hydraulic pump.

DEALERS WILL REPLACE THE HYDRAULIC PUMP.— [3]

CautionThis action is deemed a safety improvement campaign and is not conducted under the Safety Act.

THIS ACTION IS DEEMED A SAFETY IMPROVEMENT CAMPAIGN AND IS NOT BEING CONDUCTED UNDER THE SAFETY ACT.— [3]

Why it happensUnder extreme steering manoeuvres, such as turning the wheel fully left or right while braking, the hose may fracture and leak fluid.

UNDER EXTREME STEERING MANEUVERS, SUCH AS TURNING THE STEERING WHEEL FULLY TO THE LEFT OR RIGHT WHILE BRAKING, THE HOSE MAY FRACTURE AND LEAK FLUID.— [4]

If leftPower steering assist would be lost and steering effort would rise.

IF THIS WERE TO OCCUR, POWER STEERING ASSIST WOULD BE LOST AND INCREASED STEERING EFFORT WOULD BE REQUIRED.— [4]

If leftOn hydro-boost vehicles the hose failure could also remove power brake assist.

ON VEHICLES EQUIPPED WITH HYDRO-BOOST POWER BRAKES, IT COULD ALSO RESULT IN LOSS OF POWER BRAKE ASSIST AND INCREASED BRAKING EFFORT WOULD BE REQUIRED.— [4]

If leftPower steering fluid sprayed onto hot engine parts could cause an engine compartment fire.

IF THE POWER STEERING FLUID SPRAY WERE TO SPRAY ONTO HOT ENGINE PARTS, AN ENGINE COMPARTMENT FIRE COULD OCCUR.— [4]

RepairDealers inspect the power steering hoses for two suspect date codes and replace them if required.

DEALERS WILL INSPECT THE POWER STEERING HOSE(S) FOR TWO SUSPECT DATE CODES AND REPLACE THEM IF REQUIRED.— [4]

First signsA lack of steering assist could occur when the brakes are applied and the steering wheel is turned with the vehicle stopped or during parking-lot manoeuvres.

A lack of steering assist could occur if the brakes are applied and turning the steering wheel with the vehicle stopped or during parking lot type maneuvers.— [5] Condition

CautionThat concern is aggravated by worn tires, low tire pressure, low pump pressure and oversize tires.

This concern can be aggravated by worn tires, low tire air pressure, low power steering pump pressures, and tires larger than stock size.— [5] Condition

Applies to2003-2006 GMC Yukon, Yukon Denali and Yukon XL.

2003-2006 GMC Yukon, Yukon Denali, Yukon XL— [5] Models

First signsThe condition goes away or greatly improves once the vehicle begins to move or is above parking lot speeds.

This condition will go away or greatly improve once the vehicle begins to move or is above parking lot speeds.— [6]

How to confirmPerform the "Power Steering System Test" found in SI and make any necessary repairs based on system diagnosis.

Perform the "Power Steering System Test" found in SI. Make any necessary repairs based on system diagnosis.— [6]

How to confirmTo tell whether a 670 gear box is fitted, inspect the mounting bolts that attach the stub shaft housing; a 670 gear box has only 3 mounting bolts.

To determine if the vehicle is equipped with a 670 gear box, inspect the mounting bolts that attach the stub shaft housing. A 670 gear box will only have 3 mounting bolts.— [6]

RepairIf a C/K 1500 Series vehicle is equipped with a 670 gear box, replacing it with a 680 gear box may increase the customer satisfaction.

If a C/K 1500 Series vehicle is equipped with a 670 gear box, replacing it with a 680 gear box may increase the customer satisfaction.— [6]

RepairWhen replacing a 670 gear box with a 680 gear box, replace the power steering gear, union fitting and both inlet O-ring seals.

If replacing a 670 gear box with a 680 gear box, it will be necessary to replace the power steering gear, union fitting and both inlet O-ring seals.— [6]

RepairParts (C/K 1500 series only): 88965542 Power Steering Gear, 26025709 Union Fitting, 26001594 Inlet O-ring Seal.

Parts Information (C/K 1500 Series Only) Part Number Description 88965542 Power Steering Gear 26025709 Union Fitting 26001594 Inlet O-ring Seal— [6]

CautionFor C/K 1500 series vehicles, if diagnostics did not lead to any repair, the concern is a normal operating characteristic.

FOR C/K 1500 SERIES VEHICLES ONLY - If diagnostics did not lead to any repair, this concern is a normal operating characteristic.— [6]

CautionIf the stub shaft housing has no mounting bolts or has 4 bolts, do not proceed with this bulletin; the vehicle does not have a 670 gear box.

If the stub shaft housing does not have any mounting bolts or has 4 bolts - DO NOT PROCEED WITH THIS BULLETIN. The vehicle is not equipped with a 670 gear box.— [6]

CautionIt is recommended in all cases that the proper tire size is fitted and tire pressure is set to the manufacturer's specification on the door placard, especially for the front tires.

In all cases, it is recommended that the proper tire size and the maximum tire pressure is set according to the manufacturer’s specification as noted on the door placard; especially for the front tires.— [6]

Applies toThe engineering analysis closing report EA04-012, on hydraulic pump shaft failures, covers 2000 - 2004 Chevrolet and GMC C/K/G 2500 and 3500 series vehicles equipped with a Hydro-Boost system.

ENGINEERING ANALYSIS CLOSING REPORT EA04-012 SUBJECT: Hydraulic Pump Shaft Failures VEHICLES: 2000 - 2004 Chevrolet and GMC C/K/G 2500 and 3500 series vehicles equipped with a Hydro-Boost system— [7]

Applies toThe population table of the closing report has a row for the GMC Yukon XL, with a total of 16,115 vehicles.

Yukon XL 2,604 5,518 2,590 3,013 2,390 16,115— [7]

First signsThe hydraulic pump shaft can shear, resulting in a deterioration of brake effectiveness, extended stopping distances, and complete loss of power steering assist.

ALLEGED DEFECT: The hydraulic pump shaft can shear, resulting in a deterioration of brake effectiveness, extended stopping distances, and complete loss of power steering assist.— [7]

Why it happensThe Hydro-Boost system links the power brake and power steering systems together by a common hydraulic pump.

The Hydro-Boost system links the power brake and power steering systems together by a common hydraulic pump.— [7]

If leftThe Hydro-Boost unit includes a pneumatic accumulator that serves as a reserve and provides a transition between power assisted and manual braking when pressure supply is lost. The number of accumulator-assisted stops varies with previous steering and braking inputs.

The Hydro-Boost unit includes a pneumatic accumulator, which provides a transition between power assisted braking and manual braking in the event of pressure supply loss and serves as a reserve system. The number of accumulator-assisted stops varies depending on previous inputs from the steering and braking maneuvers.— [7]

If leftLoss of boost with a depleted reserve sharply increases brake pedal effort: a driver used to applying 20 lbs for a moderate 0.3 g deceleration may have to apply 130 lbs or more.

The results show that the loss of boost, when coupled with a depleted reserve, sharply increases brake pedal effort. Drivers who become accustomed to applying 20 lbs for a moderate 0.3 g deceleration, may not realize that they have to apply 130 lbs or more if they lose power-assist.— [7]

If leftIn the steering test, the test vehicle required 45 in-lb (= 5.08 N·m) with the system intact and 300 in-lb (= 33.9 N·m) with the system failed.

The test vehicle required 45 in-lb for the intact system and 300 in-lb for the failed system.— [7]

Why it happensGM's response identifies several factors that may contribute to hydraulic pump shaft failures: pressure plate deflection/pump seizure, pressure plate flatness, improper ring/rotor clearance, and use of an oversized/tapered rotor.

GM's response identifies several factors that may contribute to hydraulic pump shaft failures, including: pressure plate deflection/pump seizure; pressure plate flatness; improper ring/rotor clearance; and use of an oversized/tapered rotor.— [7]

Applies toGM is recalling all subject vehicles built between May 1999 and October 14, 1999. Significant changes went into production in early October 1999, and vehicles built after that date have lower warranty failure rates.

GM is conducting a recall on all subject vehicles build between May 1999 and October 14, 1999. Significant changes were incorporated into production in early October 1999, and vehicles built after that date have lower warranty failure rates.— [7]

RepairUnder GM's field action, 98,221 MY 2000 vehicles built from May 1, 1999 to October 14, 1999 are recalled (NHTSA Recall No. 05V-005) and have the latest version of the pump installed.

Under this program, 98,221 MY 2000 vehicles built from May 1, 1999 to October 14, 1999 will be recalled (NHTSA Recall No. 05V-005) and have the latest version of the pump installed.— [7]

Why it happensThe recalled vehicles have exhibited high warranty rates and consist of all vehicles equipped with pumps built with reground salvaged rotors.

These recalled vehicles have exhibited high warranty rates and consist of all vehicles equipped with pumps built with reground salvaged rotors.— [7]

Applies toThe last product change, requested by GM, went into vehicle production on February 2, 2004: the pressure plate within the hydraulic pump was significantly redesigned for better pump performance during high demand maneuvers such as parking.

The last product change, requested by GM, was incorporated into vehicle production on February 2, 2004. The pressure plate within the hydraulic pump was significantly redesigned to provide better pump performance during high demand maneuvers, such as parking.— [7]

RepairThe "post pressure plate design" pumps are available as service replacements, are compatible with previous applications and have superseded the production of previous pumps.

The "post pressure plate design," was tested and found to be capable to endure the full test protocol, where the original design seized on the first of 34 cycles. These pumps are now available as service replacements, are compatible with previous applications and have superseded the production of previous pumps.— [7]

How to confirmThe PI document instructed technicians to diagnose the vehicle using the Power Steering Test Procedures if an owner complained about difficulty steering during "parking lot type maneuvers"; if the vehicle passed this test, no work was required, as GM viewed this as a normal phenomenon.

PI01863³ instructed technicians to diagnose the vehicle using the Power Steering Test Procedures if an owner complained about difficulty steering during "parking lot type maneuvers." If the vehicle passed this test, no work was required, as GM viewed this as a normal phenomenon.— [7]

RepairPI01897, published March 23, 2004, added that if the customer's situation warranted, a replacement pressure control valve (PN: 26095470) was available to provide additional steering assist.

On March 23, 2004, GM published PI01897.⁴ PI01897 contained instructions similar to the prior PI, but added that if the customer's situation warranted, a replacement pressure control valve (PN: 26095470) was available to provide additional steering assist.— [7]

CautionThe closing of this investigation does not constitute a finding by NHTSA that no safety-related defect exists on the remaining population; ODI will continue to monitor the remaining population and reserves the right to take further action if warranted.

The closing of this investigation does not constitute a finding by NHTSA that no safety-related defect exists on the remaining population. ODI will continue to monitor the remaining population and reserves the right to take further action if warranted by the circumstances.— [7]