556s in cold weather

What we used to do years ago (before the better bypasses were around) was remove the ring around the igniton and zip tie it around the key and then hide it really well under the dash.  This completely bypasses the OEM security so you would want to get the customers permission to this.

I would guess it has something to do with the frequency that the coil is tuned to.  Weather has an effect on resistance and resistance plays a role in tuning coils.  I have no actual first hand knowlege, but that is my assumption.

It is obviously something wrong in the module itself as the OEM key never quits working!

I'm guessing DEI would use the same quality of components for their entire line of modules (at least automotive range).  I would also doubt the coil wire insulation would only be breeched on every 556S made and not any of the other transponder modules.

From memory the modules quit working around 32 degrees or so.  They've had this problem since 2001 or so, back when I was installing.  It's interesting to hear they are still having the same issues this long after.

And I do beg to differ about weather not effecting electrical properties - if you get a chance measure the current that it takes to energize a standard automotive relay.  A relay I had sitting here initially pulled 127mA.  However, the longer it stayed energized the less current it pulled.  Why is that?  After 20 seconds or so it was less then 125mA (this was on a regulated labratory 12vdc power supply).  Voltage remained constant and current is a direct function of voltage and resistance, meaning the resistance HAS to be changing (94.48 ohms vs 96 ohms).

I then threw the relay in the freezer for 15 mintues.  It was around 17 degrees when I pulled it out.  I energized the relay again and whoa, look at the current!  145mA!  That's a coil resistance of 82.8 ohms.  So, from going from ~65 to ~17 degrees the resistance of my admittingly cheap coil dropped >10%.  This shows, undoubtedely, that electrical properties do change with weather changes.

This little test can also be done with an ohm meter, but it's much more interesting to actually energize the relay to see the current change.  This is because the coil is warming up, thus changing the resistance, thus changing the amount of current needed.

In transponder systems there is a signal that is transmitted from the car to the passive key.  When the key is close enough to the signal the signal energizes the chip in the key and the chip in the key transmits it's own signal back to the car.  The key doesn't reflect the signal as that wouldn't really provide security.  Each chip is given a statistically unique ESN and that is why keys have to be programmed to cars.  

With a transponder bypass installed the factory transponder ring isn't used as a transmitter anymore, just a reciever so I would assume the problem would have to be in the key, not in the ring.

What you are witnessing is the mechanical properties of the conductive material(copper) as it changes through temperature. Copper has a positive co-effcient. It will expand and contract, through rising and falling temperatures, respectively. This causes the coil inductive properties to increase and decrease, as the coil shrinks and expands, respectively. What you saw with your meter was the inductive properties being influenced by the capactive properties of your meter, as well as your power supply. This manifests itself as a reactive load and not purely resistive.

Note: My statement "However, extreme(-300 F/+2M F) temperatures do." should be taken purely as an example. I have no idea at what tempreature copper become a super-conductor, nor when it ceases to acts as a conductor. They're certainly extreme, in that they don't naturally happen here on earth.