Design Article
Teardown and repair reveals design choices in Dodge key fob
Maurice Wright
9/9/2008 12:52 PM EDT
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.
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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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