OK an update here. I've been looking into this and it appears that the 3 key (315Mhz) and 4 key fobs (902Mhz) use different frequencies.

https://www.f150forum.com/f118/3-4-b...-start-475871/

It sounds like the Remote Transceiver Module above the rear passenger window is either one or the other. I wonder why ford made the fobs run on different frequencies? This means the two keys are incompatible.

 

Maybe because they're two different systems provided by two different vendors; not just the fobs, but also the transceiver in the truck. I suspected as much. Ford probably didn't specify the two systems run on different frequencies - nor did they specify they run on the same frequency. That doesn't matter to Ford, all they care about is that the fobs for a new truck match its transceiver. Compatibility is not a decision point for new truck buyers so it's not worth any time, effort or expense.

 

Standard fobs of the era which only transmit a signal one-way to the RTM were 315 mHz. Smart fobs which both transmitted and received signals (these have the little LED) used the 902 mHz frequency. I could not find any other reason for this other than the 902 mHz has a much longer range.

 

I suspect your problem may be what I also encountered with a key fob bought on ebay. These key fobs operate on a specific radio frequency. My locksmith cut the new flip out key and then programmed it. After doing so, it functioned to start/stop the engine, but that is all. His subsequently tested both fobs to determine on what radio frequency they operated. They were operating on different frequencies. Consequently, the second fob did not communicate with the system in operation on the truck.

In my case the factory fob runs on the 902.37 Mhz frequency, while the ebay fob was running on a 315.00 Mhz frequency.

Unfortunately, there is no way to change the frequency setting -

 

I don't mean to highjack you thread but I ran into a similar situation when I purchased and installed a new Lariat gauge cluster in my XLT. For whatever reason, one key when used would only allow the truck to go over 80 MPH. No more under any circumstances. Not really an issue unless trying to pass in traffic, but. Since I figured that someone may have programed one of the keys to do so and I never realized it, I tried reprogramming it. I started the process using the good key. Shut the engine down to switch keys and for whatever reason, the mykey programming cannot be found or accessed at all. It disappeared from the menu completely. Since the only thing changed was the cluster, I have to think it has something to do with keying also. For now, I just live with it. I have two good keys and one not so much. If anybody may know how to reset this in Forscan it would be appreciated.