Design Article
Teardown and repair reveals design choices in Dodge key fob
Maurice Wright
9/9/2008 12:52 PM EDT
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.
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Haldor
9/10/2008 9:31 AM EDT
Nice analysis. I agree stuff like automatic alarms should be something the owner could disable.
Service tech tip. Clearing solder from a PCB through hole typically requires suction from a solder bulb or a vacuum desoldering station. A reliable field expediant is to thoroughly heat the solder joint then slam the PCB flat down on a slightly resiliant surface (like an antistatic mat on a work bench). The PCB stops when it hits the bench, but inertia pulls the molten solder out of the hole. Clean up any solder splatter from the PCB and you are good to go.
If this doesn't work the first time try adding a bit of solder to the hole. This works best if the hole is completely full of solder. Also make sure the solder is completely melted. You need to do the slam quickly before the solder gets a chance to resolidify.
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Hube
9/11/2008 4:02 AM EDT
I'm not sure the flaws in design rely dolely on poor design. I have examined too this problem and found marketing reasons. Keys become unstable when it is easier to push customer towards a new car........
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The MicroMan
9/11/2008 3:36 PM EDT
I'd like to see a little more details of the design described. No real info on the electronics. I don't like designs with extra, unused buttons - looks sloppy, confuses users. Yes, many systems fail because of something other than electronics - like battery mounts or connectors. But look at Dodge's point of view: you had 3 good years before the quality problem showed up and then they can recoup some costs by charging an arm and leg for the replacement - which has no bearing on the BOM cost. When I do a repair such as you did I mumble to myself "it's not right, but I should get 5 more good years out of it" (I can usually improve on the original).
But PLEASE, someone do some editing. Just because spell-checking doesn't catch it doesn't mean the following words are correct: "safety, feature", When [With], batter, On half [One half], were was, indemnified [identified], devices [device], handless [handles (what is hand-less??)], work. [work?], and clean hole. There are also various comma problems. Humans are still the best. Plus Figures 3 & 4 look identical to me.
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rhusain
9/16/2008 5:29 PM EDT
The solution is to simply bypass the cars alarm and start mechanism and put a toggle key underneath the steering wheel as a start switch.
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musiklab
9/17/2008 3:17 AM EDT
Hi. Making such cheezy stuff decide if you´re stuck is not very nice.
However, the repair will probably last as long as a new fob, if made in the same sloppy way. Regarding the cost , manufacturing such an item in the thousands should not cost more than 3-5$US a piece.
Caution: I would not recommend the solder slam method- especially with SMD pcb´s a microscopic solder splat may be too small to see and foil any further attempts to repair by destroying some circuit part when applying the battery. Better play it safe and use suction wire.
In a through hole, I usually wet the suction wire with a tiny bit of solder, the cut it to a point that will go into the Through hole and lean it up with heat. you can also clean it using a pencil graphite core (0.5mm) or stainless steel wire and gentle heat to push out the solder. In the key fob, I would have applied a little fresh solder on the top part of the solder tag, and simply pushed it down using the soldering iron.
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Linearbits
11/11/2008 1:06 PM EST
My 2002 Wrangler has a simpler but similar key. It only has the RFID anti-theft part. The dealer warned me when I bought it that a new key would cost $75 in 2001 money. I tend to keep things forever so I started wondering if I could even get a new key in ten years and what it would cost.
First I bought a $3 key without the chip at Wal-Mart; it gets me in if I lock my keys in the car. Like yours, it starts the engine but it dies in a few seconds. I discovered that using the bogus key in the ignition, but holding the real key near the ignition lock for a few seconds made the computer happy. Then I could take it away and it kept running until it turned it off.
Next I found a locksmith in California on the internet who sold the chip keys for $12 or $15. His page explained how to "introduce" them to the cars' computer. It's a modestly complex sequence of turning the ignition on and off a few times at the right rate taking your cures from various lights on the instrument panel. First you demonstrate you have a real key, and then you tell it to remember your new key number. I now have four valid keys for the Jeep. I don't know how many numbers the computer can hold, but it's at least four.
The dealer probably has a fancier way to add a key and you are probably paying to use that special equipment and for labor. On the other hand it only took me 5 minutes per key to do the job manually once I found out how.
Obvious flaw in the system. If there is a receiver chip in the car that says everything is OK, its output can be clugged up to always say things are cool.
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