General Motors Two-Stage Energy Absorbers

Purpose

In 1985 General Motors introduced a two-stage bumper energy absorber on a substantial volume of model vehicles. During an impact above 5 mph, these two-stage absorbers may sustain internal operational changes which will dramatically limit their ability to control damage in future low-speed (below 5 mph) impacts. This bulletin provides a brief description of the two-stage absorber and presents information which will help identify which units must be replaced.

Introduction - GM Single-Stage Energy Absorber

For many years, GM used a single-stage hydraulic energy absorber as the primary method for controlling low speed damage. It consisted of (Figure 1) a gas-filled piston tube assembly and a cylinder tube. The piston assembly is filled with an inert gas under pressure. The cylinder tube assembly is filled with hydraulic fluid.

Operation

In a non-operational situation the gas pressure in the piston tube maintains the unit in the extended position.

During a low-speed impact, the energy absorber is collapsed forcing the hydraulic fluid from the cylinder tube into the piston tube through the brass orifice. The rate at which the fluid enters the piston tube is regulated by the metering pin. This action of regulating the fluid flow provides the bumper system with energy absorbing protection by converting motion energy into heat during oil pressurization and fluid flow.

The hydraulic fluid that is forced from the cylinder into the piston tube displaces the floating piston by compressing the gas behind it. After the impact, the compressed gas behind the floating piston forces the hydraulic fluid back into the cylinder tube assembly, extending the unit to its original position. During a high-speed impact, any energy not absorbed by the energy absorber will be transferred inward, to the vehicle structure.

Two-Stage Energy Absorber

In an effort to better control collision forces and resulting damage, General Motors added a second stage to the single stage unit. The purpose of the second stage is to help manage the energy created in accidents above 5 mph.

The two stage energy absorber is nearly identical in structure to the single-stage absorber with the exception of the metering pin orifice (Figure 2).

The first stage functions the same as in the single-stage absorber; however, the second stage is designed to provide additional protection at speeds above 5 mph. During higher speed impacts, the orifice is forced out of the piston cap allowing the hydraulic fluid to pass at a faster pace from the cylinder tube into the piston tube. The faster flow of the hydraulic fluid allows additional energy absorption during higher speed collisions.

After a high speed impact the orifice is no longer in its piston cap seat. This means that although the absorber will still function adequately above 5 mph, the first stage of the absorber, which was designed to operate in the 1-5 mph range, is no longer effective and the unit MUST be replaced. Dispose of malfunctioning energy absorbers using operating precautions and procedures prescribed by the manufacturer.

Diagnosis

Depending on the angle of vehicle impact and the rigidity of the object being struck, the second stage floating orifice may be driven out of its seat rendering the low speed stage of the two-stage absorber inoperative. Several methods can be used to determine when a two-stage absorber must be replaced.

• Leakage:
  Check the crimped or mounting end of both the right and left units for hydraulic fluid (oil) leakage. If oil is dripping from the crimped or mounting areas of the absorber the unit must be replaced.
• Damage:
  Inspect the energy absorber assembly (piston tube, frame bracket and cylinder tube) for evidence of visible distortion. If any distortion is present, the unit must be replaced. Be careful not to mistake normal scuffing of the piston tube as impact distortion. Piston scuffing will normally occur when the unit is stroked.
• Manual Inspection:
  With the bumper structure removed, grasp the absorber unit firmly, shake, and listen for any loose internal components. If a rattling noise can be heard from inside the unit, the brass-orifice has been dislodged and the unit must be discarded.
• Bench Test:
  Repair facilities that have an arbor or similar type press can, at their discretion, perform the following test. This test was designed for pre-testing service units prior to assembly on a vehicle and to check units that have been removed during vehicle repair. With the use of the press, compress the energy absorber approximately 3/8". Upon release the unit should return to its normal position. If it does not return to its original position, internal damage is likely and the unit must be discarded. Normal safety procedures should be observed when performing this test.
• Rotation Test:
  With the energy absorber removed from the vehicle, attempt to rotate the piston tube assembly while holding the cylinder steady. If the piston tube rotates freely, the stop ring is damaged and the unit MUST be replaced.

Conclusion

Exhibit 1 details the current make and model General Motors vehicles using this two-stage energy absorbing unit. This list was current as of this writing but is subject to change by General Motors.

Exhibit 1
General Motors 1985 - 92
Two-Stage Energy Absorber

C/H Body	Buick LeSabre and Park Avenue Cadillac DeVille and Fleetwood Oldsmobile 88 and 98 Pontiac Bonneville	Front and Rear 1985 - 1992
E/K Body	Buick Reatta and Riviera Cadillac Seville Oldsmobile Eldorado and Toronado	Front 1986 - 1992
J Body	Cadillac Cimmaron Chevrolet Cavalier	Front 1985 - 1988 Front 1985 - 1987
L Body	Chevrolet Beretta and Corsica	Front and Rear 1988 - 1992
U Body Van	Lumina APV Oldsmobile Silhouette Pontiac Transport	Front and Rear 1989 - 1992
V Body	Cadillac Allante Saturn SC Coupe	Front 1987 - 1992 Rear 1991 - 1992
B-Body	Chevrolet Caprice Sedan Chevrolet Caprice Sedan and all Wagons Chevrolet Caprice Sedan Buick RoadMaster and All Wagons	Front 1990 and 1/2 Front 1991 Front 1991 and 1/2

Damage appraisers and estimators need to carefully inspect General Motors vehicles which use the two-stage energy absorber. After an impact it is possible that the vehicle may sustain little cosmetic or bumper damage which could lead the estimator to wrongly conclude that the absorber is undamaged. For this reason some or all of the previous tests should be performed to determine the full functional capability of the absorber. An absorber that has been damaged or altered by an impact above 5 mph and is not replaced, could cause unnecessary vehicle damage in subsequent impacts at low speeds (1-5 mph).

For additional information on the installation, removal and inspection of these two-stage energy absorbing units, refer to the appropriate make and model General Motors Service Manual.

The information provided in this bulletin is for educational purposes only. Although every effort has been made to ensure the accuracy of the information contained in this bulletin, Tech-Cor assumes no responsibility or liability for any repairs performed using information from any publication issued by Tech-Cor.

Any person performing repairs must determine whether any suggested or recommended procedures or repairs are suitable or appropriate for the particular vehicle being repaired. The repairer remains solely responsible for such determination, as well as for the proper completion of the repairs.

Reproduction of this bulletin is not permitted without the written approval of Tech-Cor.