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Forensic Engineering: A Case Study

Auto Service Manuals Reveal Clues

With each new case involving the investigation of a component or system of an automobile, we purchase the manufacturer's Service Manual. These books are very detailed and well illustrated. They are intended to guide a technician in the diagnosis of problems, and repair or replacement of component parts.

Review of these manuals gives an engineer the opportunity to become more familiar with the design details of a product, including the design strengths and weaknesses. Very often, the Service Manual contains cautions and warnings to the service technician which can serve as important clues to the forensic engineer.

As an example, we once investigated an accident in a front-drive car where the automatic transaxle jammed, causing an accident, shortly after one front axle and C-V joint were replaced. I removed the new front axle and C-V joint, finding no sign of a problem. On consulting the Service Manual, I read the following:

"CAUTION: Do not dislodge the rollers in the constant velocity (CV) joint during removal. If the rollers become dislodged, the roller needle bearings could fall into the transaxle."

I next removed the transaxle oil pan, in which I found roller needle bearings matching the ones in the C-V joint. Some were crushed, indicating that they had become caught in the transaxle gears.

Clearly, the needle bearings had been dropped accidentally into the transaxle during removal of the original axle and C-V joint, as warned against in the Service Manual. This evidence was pivotal in convincing a jury that the mechanic was at fault.

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