Understand the engineering behind repair procedures to ensure the best repairs
Who would build a car this way?
That lament has a special place in the history of collision repairs. Up to the 1980s, repairers relied mainly on experience and training picked up on the job to perform body work. All that changed when several engineering trends began driving vehicle design over the course of the new wave decade.
First, unibody structures became the industry standard as a way to meet rising safety and fuel efficiency standards. Also, the number of electronic and power conveniences significantly increased and became mainstays on popular vehicles. Finally, Detroit automakers , fighting for their existence in brutal competition with Asian imports, began turning to ever more revolutionary (and sometimes poorly thought out) designs to appeal to new car buyers.
Repairers were left howling in frustration at engineering that grew increasingly sophisticated and rarely seemed to take reparability into account. In time, their voices and those from insurers and motorists began driving home the notion that the same cars sold today would need fixed tomorrow. Automakers responded with designs more palatable to repairers and more detailed repair instructions that made what happened at the factory more of an afterthought than an issue.
Still, shops can benefit by increasing their understanding of how manufacturing practices influence repair decisions. Here’s a look, using repair steps and information supplied by GM, at how factory engineering and processing creates some unexpected repair courses.
Door dilemmas
GM’s position on outer door panel replacement is a departure from most of the auto industry. The industry standard is a partial panel or belt-cut replacement. GM instead recommends complete door panel replacement for door damage, except on its full-size pickups and SUVs where this kind of sectioning is preferred.
Why?
It all goes back to factory production techniques. The outer door panels on these GM vehicles are hemmed around the door glass opening. Only factory hemming tools, which aren’t true options for shops, can be used for full replacements (there’s no room to sneak in a dolly behind the inner panel during a shop repair).
In these instances, you’ll need the following repair steps, along with other applicable GM repair guidelines and instructions:
- Disconnect the negative battery cable and disable the SIR system.
- Remove all related panels and components. Repair any damage.
- Remove the door assembly.
- Cut the door panel where sectioning is to be performed.
- Inspect the door guard beam for damage. If any damage is found, the door must instead be replaced. (Failure to do so can compromise the structural integrity of the vehicle and cause personal injury if the vehicle is involved in a collision.) If there is no damage, proceed with this repair.
- Grind the edges of the outer door panel to separate the outer panel from the door shell.
- Remove the outer door panel.
- Remove the anti-corrosion materials and sealers from the repair area, as necessary.
- Straighten the edges of the door shell.
- Cut the replacement panel in corresponding locations to overlap the remaining original panel by approximately 1 in. (25 mm) at each joint.
- Use a grinding disk to grind the surface of the door shell mating flanges to bare steel.
- Scuff the opposing mating surfaces of the door outer panel to remove the gloss of the E-Coat.
- Clean the mating surfaces. Note: Do not allow the door to completely cure off the vehicle since doing so will make proper alignment of the door outer panel to the door shell difficult.
- Apply a 1/8–1/4 in. (3–6 mm) bead of door panel bonding adhesive to both of the mating surfaces.
- Use a small acid brush to spread a coat of adhesive covering all the bare metal surfaces. This will supply corrosion protection.
- Apply a 3/8–1/2 in. (9–13 mm) bead of door panel bonding adhesive to the mating surfaces of the service panel. Note: Be careful not to pull the panels apart after being joined together. Instead, slide the panels against each other to realign the panels.
- Install the outer panel to the door shell, and clamp it into position.
- Use a hammer to re-hem the hem flanges around the door shell. Hammer in stages along the hem flanges.
- Remove any excess adhesive from the panel area with a lacquer thinner.
- Install the door by first examining the outer panel to ensure proper alignment. Adjust if necessary.
- Weld the outer panel into place.
- Clean and prepare all welded surfaces for application of foam.
- Apply a GM-approved flexible foam in a continuous bead between the inner safety beam and outer door panel. Fill any gaps by taking multiple passes behind the side impact sensor portion of the beam.
- Reinstall all related panels and components.
- Enable the SIR system and connect the negative battery cable.
Sensor sensitivities
The previous door sectioning steps included instructions to disable\enable the Supplemental Inflatable Restraint (SIR) system, which need to be performed with detailed GM instructions. That fact and the relationship between the SIR and vehicle design makes this area another where factory knowledge can be significant. Perhaps the most important bit of this knowledge is that simply disabling\enabling the SIR system requires detailed steps because this sensitive system is engineered to work under catastrophic circumstances—essentially as the vehicle is being destroyed in a collision. Ignoring the factory repair procedures can prove disastrous as well since doing so can result in violent air bag deployment while the vehicle is being serviced or a system failure during a future collision.
When disabling\enabling the SIR system in GM vehicles, always follow these steps:
Disabling Procedure – Air Bag Fuse
- Turn the steering wheel so the vehicle wheels point straight ahead.
- Turn the ignition to the OFF position.
Caution: The SDM may have more than one fused power input. To prevent an accidental SIR deployment, remove all fuses supplying power to the sensing diagnostic module (SDM). Note that with the SDM fuses removed and the ignition switch in the ON position, the air bag warning indicator still illuminates. This is normal operation and doesn’t indicate a SIR system malfunction.
- Locate and remove the fuse(s) supplying power to the Inflatable Restraint Sensing and Diagnostic Module. (Refer to SIR Schematics or Electrical Center Identification Views in the GM Service Information.)
- Wait two minutes before working on the system. This is necessary to drain the reserve power supply and prevent the air bags from deploying.
Enabling Procedure – Air Bag Fuse
- Turn the ignition to the OFF position. 2.
- Install the fuse(s) supplying power to the Inflatable Restraint Sensing and Diagnostic Module. (Again, refer to SIR Schematics or Electrical Center Identification Views in the GM Service Information.)
- Turn the ignition switch to the ON position. The air bag indicator will flash or illuminate.
- Turn the ignition to the OFF position.
- If the air bag warning indicator doesn’t respond as described, perform the Diagnostic System Check – Vehicle. Refer to Diagnostic System Check - Vehicle in the GM Service Information.
Disabling Procedure – Negative Battery Cable
- Turn the steering wheel so the vehicle wheels point straight ahead.
- Turn the ignition to the OFF position.
- Disconnect the negative battery cable from the battery.
- Wait two minutes before working on system.
Enabling Procedure – Negative Battery Cable
- Turn the ignition to the OFF position.
- Reconnect the negative battery cable to the battery.
- Turn the ignition to the ON position. The air bag indicator will flash or illuminate.
- Turn the ignition to the OFF position.
- If the air bag warning indicator doesn’t respond as described, perform the Diagnostic System Check — Vehicle. (Refer to Diagnostic System Check - Vehicle in the GM Service Information.)
Repairers need to be just as careful when dealing with potentially damaged impact sensors.
Shops can locate malfunctioning impact sensors by performing a diagnostic trouble code (DTC) procedure. But not all sensors will show immediate problems, and damage such as cuts in the wire insulation and slight bending of the mounting brackets might not be visible. These sensors can still fail following a repair, placing motorists in dire situations in the event of another collision.
For this reason, GM mandates the replacement of all impact sensors in the “accident damage area.” This area is defined as the portion of a vehicle that has been crushed, bent or otherwise damaged in an accident. An “otherwise damaged” portion can be, in the case of a front-end collision for example, the area forward of the radiator where an impact sensor is mounted.
GM also sets the following stipulations for impact sensor replacement:
- Replacement is necessary whether or not the sensors appear damaged or the air bags have deployed.
- Always verify the correct replacement part number.
- Never substitute sensors with components from a different vehicle.
- Never use salvaged parts for SIR system repairs.
Building understanding
If repairers wanted to, they could spend the better part of their time researching design and repairs. Of course, they’d soon go out of business. That isn’t the point here.
Shops already have their hands full keeping up with training demands. Taking a look, when time permits, at the engineering processes behind repairs is too beneficial to ignore. Building this kind of understanding of collision repair work only adds to the level of expertise and excellence shops can bring to their services. This ongoing quest is one journey all shops can take.