Maury Wright
TechOnline

Keyless entry systems have become common in today's cars and are certainly a convenience, and a safety, feature for drivers. But keyless entry can turn terribly inconvenient when a key fob breaks and replacements can be expensive. Just such an occurrence prompted this story on the teardown and repair of a key fob for a 2005 model year Dodge. And while we realize that his isn't exactly a digital home topic, the technologies of the digital home are pervading the auto space, and we figured that there are lessons aplenty in the fob design.

Chrysler and Dodge vehicles from the 2005 timeframe use a keyless entry system developed originally for the Mercedes brand. The system actually implements the driver convenience and safety functions, and an immobilization function designed to prevent vehicle theft. As you can see in Figure 1, the key fob includes a pair of buttons to lock and unlock the vehicle, and a panic button that can be pushed to sound an alarm. There is a fourth button located above the lock and unlock buttons that is not used in this specific fob but that in others can be used to operate features such as a power lift gate.

Figure 1: The keyless entry fob for 2005 model year Dodge vehicles includes lock, unlock, and panic buttons.
(Click on image to enlarge)

The keyless entry functions are straightforward when the fob is operating properly. When the fob stops working -- whether from a dead battery or an actual product failure -- the system becomes quite inconvenient. Anytime the vehicle is locked the alarm system is active. Indeed, whether the driver pushes the door lock button before exiting the vehicle or via the keyless system, the alarm engages. There is no provision to disable the system. When a dead fob, you can use the key to open the door, but the alarm sounds immediately. You must insert the key into the ignition and turn the switch a notch clockwise to enable auxiliary power and end the alarm.

Figure 2: A 3V, 20-mm CR2032 Lithium batter powers the fob.
(Click on image to enlarge)

The fob relies on a 3V, 20-mm, CR2032 Lithium battery (Figure 2). A single screw on the back of the fob holds the halves of the plastic enclosure together. Replacing the battery is relatively easy if that's the only problem with a non operational fob. The mechanical design could be cleaner. The small pcb in the fob is not anchored to either half of the enclosure. On half does have some small guide pins that mate with small holes in the pcb.