Maury Wright
TechOnline

Our failed fob led to a quick trip to Radio Shack and the purchase of a new battery. But after opening the plastic enclosure, we could see that we might have more than a battery problem. There were was clearly an issue with the metal clip designed to hold the battery. The battery that was in the fob tumbled out immediately after we removed the pcb from the enclosure. When we tried to insert the new battery the clip wouldn't hold it securely.

As you can see in Figure 3, the clip is designed to connect to the pcb in multiple places. Along each side of the clip, the design includes a pin that's supposed to go through a hole in the pcb. There is also a solder connection in the center on the back side of the clip although it is not a through-hole connection. Solder both holds the clip in place and provides the electrical connection to the positive battery terminal. In the case of our fob, the pin along the right side of the clip shown in Figure 3 wasn't soldered properly. At first we guessed that the pin had broken, but upon further review realized that it hadn't been seated properly in the pcb hole when originally manufactured. It's likely that the pin was once at least connected to the solder pool on top of the pcb because the fob had operated properly for several years.

Figure 3: A close examination shows that the battery clip wasn't properly soldered to the pcb.
(Click on image to enlarge)

We had previously checked into buying a third key for the vehicle. But extra keys range in price from $150 to $200. You can find cheaper keys on the Internet, but a look at the vehicle owners' manual had left us wary of the offers. And faced with the failed fob, we dug deeper into the operation of the fob and the keyless system.

The owners' manual states that one must have a Sentry key to operate the vehicle. Even if a locksmith could copy the key, it turns out that such a key would not operate the vehicle and of course wouldn't solve the alarm problem. The DaimlerChrysler engineers used what Philips (now NXP) called immobilizer technology to add a theft prevention feature. If you start the card with a key that the car doesn't identify as a Sentry key, the engine runs 2 seconds and then the system turns the engine off.

Arguably we should have read the manual long before the fob failure, but it was both a surprise to find out about the theft deterrent and a puzzle as to how the system worked. The key itself didn't appear to have any mechanical feature that would allow the system to recognize a Sentry key. And clearly the system could recognize a Sentry key even if a battery was not in the fob, although the keyless entry requires a battery.

Figure 4: A Philips (NXP) PCF7941 keyless entry IC also provides immobilizer technology.
(Click on image to enlarge)

Figure 4 offers a close up view of the pcb. There's a single chip on the pcb indemnified as a Philips PCF7941. There's not much on the web about the IC in question, although a Remote Keyless Entry web page mentions the devices. It appears to integrate the immobilizer technology developed for a prior IC along with a RISC microcontroller that handless keyless entry and convenience functions. It even appears that the fob stores convenience features such as memorized seat positions and sound system preferences.