Smart Key Case Replacement
 

Over time, my supply of keys for each of my cars inevitably dwindles. For our S430, one of our Smart Key's buttons cracked, and its operation became intermittent, even with new batteries. The emergency key's plastic hole is broken off, so it wouldn't stay on my key ring (I hated the thing anyway, since unlocking the door with the metal key always set off the alarm). So I'm down to one good Smart Key. I will probably order another key from the dealer ($198), but I decided to salvage the important parts of my damaged Smart Key, which should still work.

My choices were to get an old Smart Key from somebody, or a new Smart Key case, and then transfer the innards of my old key to a new one. I found a new case on ebay, and bought it for less than $40.

The new case is just the outer plastic shell, with no metal blade, spring loaded release mechanism, battery holder or circuit board. After removing the routine parts as described in the owner's manual for battery replacement, I could look inside the old case and see the edge of the circuit board, with a plastic piece on top of it. This spacer piece is held in place by two posts that fit into holes in the board. The challenge was to pull those parts out.

I decided to insert a pick into the rectangular hole in the plastic piece and pry the thing out. I held the pick parallel to the edge of the circuit board to do this. I partially succeeded , but broke off a piece of the plastic in the process. The plastic part was partially out, leaving room for me to carefully use needle nose pliers to pull the board out. Next time I may try to hold the pick perpendicularly, or maybe I'll just put a cutting wheel on my Dremel and take the case apart.

I then simply put the plastic piece back on the circuit board, and slid it into its new home. It is possible to put it in crooked, so it took a couple tries. I then put the battery holder and blade back in, and it seems to work perfectly.

I'm guessing that the case I got may not have been made by the same company that supplies them to MB, as the word "PANIC" isn't printed straight, but I'm optimistic that this thing will last a long time. Getting an old one (in good shape, at the right price of course) may have been a better way to go because then I would have gotten to practice removing the guts of the donor key before working on my original key, and maybe had an extra plastic spacer piece as well.

Update
 

Recently the aforementioned key has started to act up. It locks and unlocks the doors just fine, but does crazy things when used in the ignition. Sometimes a CEL comes on. Yesterday I started the car and backed out of the garage, and put it in Park. But then I couldn't get it out of Park.

The other 2 keys I have work fine. I'm surprised because I would expect a malfunctioning key to fail to start the car-I don't know why the car would change its mind about the key after it is running.

Oh well.

Had a similar problem. Bought a replacement case via eBay. Seemed to work at first but then started refusing to release the ignition lock. Sometimes wiggling and reinserting multiple times it would work. Put circuit board back in original case and problems disappeared.

Did you clean the infrared port on the key and the ignition tumbler?

No, I didn't know that anything could be cleaned. Any particular cleaner I should use?

I also bought a new case off of ebay a few years ago for my 01 E320. Made in China. If you compare the OE and Chinese cases you will see that the red plastic window at the end of the key on the Chinese unit is smaller and a different shade of red from the OE case. IMO these differences make the Chinese case unreliable. My spare key with the Chinese case works intermittently in the ignition cylinder sometimes unlocking the steering wheel but not authorizing starting, other times doing nothing. Frequently if I remove the key and flip it 180 degrees and reinsert the key it will function. Finally gave up on the Chinese cased key as the main key and keep it solely as a spare. Shame MB won't supply a replacement case in this "green" age. Mark

I tried cleaning the case and the ignition switch, and also tried rotating the key 180 degrees, but the car still does tricks with that key. I guess I have to cough up another $200 if I want another backup key, or perhaps if I take it apart I will notice something fixable (not likely).