Two cars on one key fob

The answer is no.

 

To add to the above correct answer, the underlying problem comes from the rolling keycodes used in electronic locks. The same code is never used twice, so both the car and fob need to know the next code in the sequence or everyone gets confused. With two cars, there is no way for a singe fob to keep synchronized with both.

(Note that this is a simplified explanation, but it conveys the general idea.)

Tom

 

You could certainly have all of the keyfobs work plugged into the dash of both cars. Just do the 5 step programming instructions in the first post of the thread http://priuschat.com/forums/knowled...lly-how-program-non-sks-key-fob-detailed.html to add each keyfob to the other car.

It is also probably possible to make a mechanical key that works in both cars. The lock uses 4 cuts from each side of the key, ignoring the other 4 cuts. A "blended" key would have the pattern for one car in one orientation, and the pattern for the other car when reversed. The theory seems ok, but I've never heard of anyone actually trying it.

The proximity/remote buttons functions are doubtful. The dealers all say it can't be done, and they have the equipment to try it. I'd like to see the results of doing a "Smart Code Reset" to add a keyfob to a second car. That would allow adding the keyfob to the 2nd car. What would happen to functionality on the first car is anybody's guess.

 

There is no technical reason why you can't sync two receivers to one transmitter (rolling code). The forward code sync is sufficient that if you at least hit the other car once every ~200 times or so it will still stay in sync. (the code will sync up to 255 "button presses" ahead)

Here's how it works: A new code is sent each button press. This is used to prevent replay attacks, where an eavesdropper records the transmission and replays it at a later time to cause the car to unlock. The codes are pseudo-random numbers that are mathematically generated such that the receiver can calculate the next code going to be sent. It will accept up to 255 codes "in the future" in case you accidentally press buttons while out of range of the receiver. It will NOT accept any codes that have already been received, which prevents the replay attack. A pointer is incremented in the receiver each time a new code is received that prevents old codes for being valid.

When you get a new transmitter, you have the car in "learn" moide to update it's pointer to wherever the new fob happens be transmitting at. (pseudo-random number determines the start) There are typically 16,777,215 different codes (24 bit), though some systems use 32 bit. (4,294,967,295 codes)

This applies only to the fob transmit on button press. Not sure about Smartkey, someday I'm going to have to do an analysis. I suspect it simply returns the current pseudo-random number in the sequence each time the car polls it, and then it updates the pointer just as if a button were pressed. If this is the case, then that's the rub; the car is polling the fob every few seconds, so after about 10 minutes in the car, the fob will pass 255 codes and then be out of sync with the other car.

So bottom line is:

You CAN program multiple cars to recognize the RFID transponder. (proven)
You can program multiple cars to recognize the fobs lock/unlock buttons. (theoretical)
You cannot use a fob with smartkey functions on multiple cars. (also theoretical)

 

We've have previously discussed all of this. In theory it should work fine. In practice no one has had any luck. The problem is compounded by the lack of any solid information. Obviously, manufacturers are reluctant to publish details about security systems. As you point out, the Smart Key System is where the problem is insurmountable at this point.

As for maintaining sync, even when out of the allowable range (the 255 figure above), two button presses should restore synchronization. This is the method used in Microchip's Keeloq system, which is the one most commonly used. The two-button-push method works well, as that is the normal response when something doesn't work: try it again.

Tom

 

I can't speak for the newer cars, but as far as using one key cut for one car on one side and a different car on the other side, here is what I did. A few years back I had a Lincoln Town Car and a Ford Pickup. I had a key made that had a cut for the Town Car on one side and The pickup on the other side. It worked great with either car. I ground a notch on one side of the "handle" part of the key so that I would know which side was which.

 

Keeloq is one of the most common, but many OEM's do not use it. I'm pretty sure Denso does not.

In Keeloq, there is a 16 bit sync window, (32,767 button presses!) and they require a second transmission if outside of only 16 presses. The way to test if you have a Keeloq is to press the button over 16 times out of range of the car. If it still works on the first press when back in range, is is NOT a Keeloq system.

Most of the OEM's use the 8 bit sync window without the 2nd press requirement. I have seen some running at 10 bits though (1023 presses). I suspect having a lower sync window is a way to generate more dealer revenue.

 

Still on topic (the parenthetical question by the OP), Prius is not a Latin noun, but an adjective (forward, as in forward thinking). The appropriate plural is Priora, which seems like a superb term to me, but it will probably not catch on. "Prii" is incorrect Latin.

If one takes Prius to be a singular proper English noun, then the standard plural would be to add "es" to a noun ending in "s". As in Jones and Joneses. Use of an apostrophe would be wrong, since that is for possessive, as if a Prius owned something. So the inevitable plural of Prius is Priuses.

People in France, Germany, etc., may completely hijack this thread with their own language definitions.

 

Or you can follow Toyota's lead, and use "Prius" as the plural of "Prius".

Tom