A car can usually be in three settings: on, off, and accessory mode. When the driver turns the key, the ignition switch rotates and the detent plunger fits into the matching slot on the settings plate.
GM’s ignition switch defect related to the detent plunger. The part used from 2002 to 2006 was 10.6 millimeters long and did not provide enough torque to keep the plunger in the correct setting on the settings plate. (An engineer’s rendering of the ignition switch is available on the New York Times website.)
The detent plunger could become dislodged, usually if driving over a rough area and/or if the driver jostled the keychain. As the Valukas (2014, p. 29) report explains, “the amount of effort required to rotate the Ignition Switch was too low, permitting it to move under certain circumstances from the Run position to the Accessory or Off position, when it was not the drivers attempt to do so.”
Shorter drivers were more likely to nudge keychains, and heavy keychains were more apt to pull the switch in such a way that the detent plunger shifted position.
I am short, 5’2″, and while I don’t drive a General Motors vehicle, the steering column is similar across cars. In the picture to the left, you can see that my knee is very close to the keyring, and if I went over a pothole or bumpy service, my knee would inadvertently hit the keyring. In fact, I have experienced this. (Read more about keyrings in my post Key to the Issue.)
When the detent switch fails in a moving car, it can turn the car off or place it in accessory mode. As a result, the car’s airbag does not deploy and it loses power steering and the anti-lock brake system.
The 10.2 mm detent switch did not meet GM’s specifications, but it was used anyway. Engineers noticed that Chevy Cobalts could unexpectedly power off as early as 2002. Knowledge of the defect was widespread by 2005 when a Cobalt in accessory mode hit a tree with no airbag deployment and killed the driver. GM teams investigated solutions, one being a new detent plunger that would cost $0.90 plus a one-time plate charge of $400,000. Due to the expense, the team decided not to authorize the replacements.
In 2006, engineer Ray DeGiorgio authorized Delphi, manufacturer of the ignition switch, to use a longer detent plunger with a tighter coil. Contrary to GM policy, he used the same part number as the defective switch and had no one else sign off on the change. As a result, there was no way to distinguish between the old and new switches, so some newer GM models were built with the defective ignition.
Most of the fatal crashed related to the defective ignition switch have been linked to failed airbags.
CBS News created an animation to explain the defect.
Jenner & Block. (2014). Report to board of directors of General Motors Company regarding ignition switch recalls. Chicago, IL: Valukas, A.