Rear Sectioning Procedures for the General Motors 200 Series Van

Introduction

General Motors has recently released a completely new mini-van for 1990. Three models are available: The Chevrolet Lumina APV, Pontiac Trans Sport, and the Oldsmobile Silhouette.

Although this vehicle has a Sheet Molded Compound (SMC) skin with unique repair challenges, this bulletin is concerned only with the repair of the rear unibody structure. SMC repairs are discussed in Tech-Cor Research Bulletin 1990-1. The majority of the fixed exterior panels of this vehicle are attached to the steel subframe with adhesives instead of mechanical fasteners. If panel removal is required to expose the damaged structure a new method of panel removal is required. A panel can be removed by breaking the adhesive bond using a heat gun while applying pressure.

This rear sectioning procedure has been developed because full replacement of the unirails at factory seams is not practical because of seam location. The section joint is located at the crossmember which is positioned directly above the axle, in Photograph 1.

This location offers the following advantages:

• Located forward of the primary crush zones.
• The crossmember forms a cradle to support the rail section joint.
• Crossmember flange supports the floor.
• Welding can be performed from the top with cargo floor removed.
• No additional seam is visible from beneath the vehicle.

Lifting Vehicle

Caution should be taken when lifting this vehicle. The rocker panel flange which consists of one layer of steel covered by the SMC panel has little strength. If any upward or side force is applied it will crush. General Motors recommends lifting the vehicle by its jacking points located at each wheel or using suspension components of sufficient strength.

Vehicle Holding and Fixturing

Because of the lack of strength in the down standing rocker flanges a special holding fixture has been developed by the equipment manufacturers for this particular vehicle. Although ordinary pinch weld clamps can be used to secure the front of this van, Photograph 2, no such area is available for fixturing the rear. Some repair equipment manufacturers are developing clamps that utilize the rear axle trailing arm mounting brackets, Photograph 3. These types of special brackets or hardware are necessary when performing moderate to heavy corrective pulls.

Repair Information for the GM 200 Series Van

Normal safety precautions must always be observed when working with collision damaged vehicles and related repair equipment. Prior to replacement of any rear unibody rails, the following procedures should be completed to allow for proper sectioning techniques:

• Remove necessary exterior SMC panels (Refer to Tech-Cor Research Bulletin 1990-1, SMC Repairs For The GM 200 Van, Rear Quarter Panel Splice steps #1-8) and trim to gain proper access to the damaged area.

• Pull and align rear unibody structure so that proper dimensions are attained. This must be done prior to cutting and removal of any primary structural member.

Tech-Cor does not recommend a butt joint or a butt joint with insert for this section repair. Because of several advantages for welding and the application of corrosion protection, we recommend a lap joint with a final overlap of 1/16" minimum to 1/4" maximum with a continuous edge weld around the rail interior at the section joint. This overlap provides necessary measurement tolerances during the cutting and fit-up of both the replacement and original structure. To achieve this overlap, carefully flair the end of the original rail so that the replacement rail fits inside with proper overlap, Photograph 4 and 5.

When the rail section procedure is completed, the remainder of the assembly and reinstallation of other unibody parts can be completed. All adjoining seams and welded areas should be prepared as described in the WELDING section of this bulletin. Corrosion protection including refinishing of replacement components should be performed as outlined in Tech-Cor Research Bulletin 1986-7, RESTORING CORROSION PROTECTION DURING VEHICLE REPAIR.

Rear Sectioning Procedures for the GM Series Van

Read this bulletin completely before making any section cuts on the damaged or replacement rail.

This repair sequence applies to the partial replacement of either unirail. This procedure should not vary greatly from standard Tech-Cor unibody repairs discussed in earlier bulletins except for the method of panel removal. Once the unibody has been dimensionally restored the following sequences can begin:

1. Remove rear corner pillar supports. Grind top edge weld and remove spots welds with a hole cutter.

2. Remove all spot welds holding the cargo floor from the section joint rearward. This includes the rear flange of the section crossmember.

3. Using a cutoff wheel, cut the cargo floor just behind the section crossmember's forward flange, Photograph 4.
   

4. Rough cut the damaged unirail just rearward of the crossmember using a reciprocating saw.

5. Cut the rear body crossmembers close to the undamaged unirail, Photograph 6. These crossmembers are replaced by sectioning after the replacement rail is installed.
   

6. A spare tire mounting crossmember extends between the two rails, four hidden spot welds hold it in place, Photograph 7. The spare tire crossmember can be broken away from the damaged rail using a panel cutter from underneath.

7. Remove the damaged floor/rail assembly by prying. Additional cutting may be required.

8. Before the partial rail can be installed, a section of the remaining rail needs to be removed. Using a small cutoff wheel cut the rail in the middle of the rear crossmember and remove, Photograph 4. Caution should be taken not to damage the crossmember.

9. Flare the end of the existing rail to allow the replacement rail to fit inside and overlap from 1/16" - 1/4", Photograph 4.
   

10. Cut the replacement rail to required length.

11. Check fit and length of replacement rail. The replacement should lie flat on top of the original rail and have an overlap of 1/16" minimum to 1/4" maximum, Photograph 5.
    

12. Before welding the replacement components all bare metal should be covered with a weld-through coating to ensure corrosion protection. Read the WELDING section before proceeding.

13. Clamp rail into place, M.I.G. weld the rail from the top using the techniques discussed in this bulletin.

14. Install spare tire crossmember between the rails.

15. Dress down any new or original welds that will inhibit proper fit of the replacement floor section.

16. Cut the replacement cargo floor to allow for a 3/4" to 1" overlap. Install cargo floor. Place MIG plug welds every inch to fasten the front of the replacement floor to the original cargo floor. The remainder of the replacement floor is attached with MIG plug welds placed in the same locations as the originals that were removed.

17. The two outer rear body crossmembers can now be replaced. Stagger the section joints on each side of the energy absorber mounting area at the undamaged rail. The location of the two section joints are shown in Photograph 6 and Photograph 7. Photograph 7 shows the section locations utilized for a left rail replacement. The same procedures apply for the partial replacement of a damaged right rail.
    
    

18. Install rear corner pillar supports.

19. Prime, paint, and restore corrosion protection as described in Tech-Cor Research Bulletin 1986-7, RESTORING CORROSION PROTECTION DURING VEHICLE REPAIR.

Welding

Prior to any welding operations, the weld site must be cleaned without removing factory corrosion protective coatings or thinning the base metal. Do not remove any preexisting galvanized coatings. Do not use an oxyfuel (oxyacetylene) gas torch. After cleaning, a weld-through coating must be applied to all bare metal in the weld sites. All welding should be performed with a M.I.G. welder (Metal Inert Gas, now called G.M.A.W. or Gas Metal Arc Welding). Welding wire should be either 0.023" (0.584 mm) or 0.025" (0.635 mm) diameter wire which meets or exceeds the American Welding Society Standard AWS-E-70S-6) with a mixture of 75% argon and 25% carbon dioxide shielding gas. Do not concentrate all welding efforts on one area, but alternate sites so that each welded area has time to cool down before the welding is continued. All continuous welds must be made in 1/2" - 3/4" (13-19 mm) increments which helps to minimize heat build up and panel distortion. All plug welds are made by welding around the edge of a 5/16" (8 mm) diameter hole. The welding equipment must also be properly adjusted by making and testing practice welds on similar materials.

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.