... for about 5 seconds. I was moving my boat the other day and started the truck up after hooking the boat, truck started up then died. I was like "WTF!?!?!?". Truck is an '06 with only 30K miles we bought new. Never had any problems with it at all. Tried to restart it and it would turn over fine, just not start. I got out to open the hood, took keys with me (habit as I usually have my four year old in the car/truck with me so never leave keys inside in case he locks the doors or tries to start the car/truck- he is always in a car seat but he is old enough now to undo it himself...). On pulling out the key I noticed I had accidently used the XJ key on the Dodge. It fits and starts it up but the Dodge key has the transponder in it- without it it disables the ignition so no spark.
Dodge key fits into the XJ too, although the big plastic housing for the remote/transponder is too big so it doesn't seat in the ignition far enough to turn. I was hopefull I could get rid of one key on my keychain, but I guess not.

Just thought I would share as I found it funny.

~K

 

yeah, it's weird. i found out a few jeep keys from before the transponder will start my 91 dakota.
my 92 xj, wifies 97 tj, buddies 95 zj, friends 98 tj, and probably many more.
but my key won't start their vehicles.... they will even unlock my doors too.

but my 99 tj or my buddies 00 tj won't. never tried my buddies 00 xj yet, forgot all about it.

 

Not terribly surprising.

Figure a five-cut key (five tumblers) with four or five tumbler depths gives you 5^4 possible combinations, or 625 possible "cuts" (at a scratch figure. It's usually closer to 5,000-10,000 possibles, typically working with 6^5 - 7,776 - or so.)

Figure that these keys are made and used by the millions. Figure also that there are some "master" combinations cut in as well - that's how locksmith's "try keys" work (it's a chain of about three dozen keys with various "master cuts" on them, and one set is usually good for about twenty year's worth of vehicles of a particular lock make.)

Why do you think transponders came into use? Since you have a limited number of key "cuts" available, you have to do something to make them all different, and increasing the number of "cuts" means increasing the length of the key, or we have to go back to having a "right way" and a "wrong way" to insert the key in the keyway (something that most people probably aren't willing to put up with, which is why "double-cut" keys are reversible.) Thus, you either end up with keys a foot long, or you end up with keys that have transponders in them (use a 16-bit keycode, and you get 2^16 transponder combinations, or 65,536. Couple that with variations in key cuts, and you get 2^16 * 6^5 = 65,536 * 7,776 ~= fifty million possible combinations. Now we're getting useful variation for security purposes!)

And, the hardware to make a 16-bit transponder is dirt cheap, so that made it easy. As semiconductors get cheaper, they can increase the bit depth of the memory array used to house the transponder code - 2^32 ~=430 million combinations, and you multiply that by the just-short-of eight thousand we have from physical key cuts, and you're getting some serious "security by obscurity."

Give it time - I figure before too long, when you buy a car, you won't get keys with it. You'll just get a set of bracelets or something with transponders in them set to 128- or 256-bit codes. Increment by one for each vehicle that rolls off the line, and possibly have a four-bit "manufacturer prefix" so each manufacturer can use the full WORD length in setting their key codes (by each manufacturer having a different NIBBLE to set them apart. It would be easier than having to programme finger- or thumbprint access anyhow.

(NIBBLE - four-bit combination
(BYTE - eight-bit combination
(WORD - any combination of whole BYTES. 16-bit and 32-bit are typical WORD lengths, with 16 bits being called a WORD and 32 bits being called a LONG or LONGWORD.)

Oh - see if the Dodge key will work in the XJ. The XJ doesn't need the transponder signal, and will therefore ignore it. The physical key will probably work in the lock, and you'll still be able to be quit of a key...

 

im glad we got 5-90 on this forum everytime i see a post from him its just filled with good information keep it up man you know your stuff is all i can say

 

A fellow does what he can. There are some advantages to a heavily chequered past, you know...

 

Since my lovely daughter *lost* the key and remote for her GC, we only have 1 key left (fortunately it IS a transponder key, so we can drive the vehicle).

Question: Is it possible to buy a couple of *new* keys and remotes off eBay and program them myself, or will I have to bite de bullet and go to the Stealership...?

 

For our old Durango, (2001) you needed two keys to program a third. So we had to get the first spare programmed at the dealer. Then we could do our own.