So did you get back to them with a block diagram or schematic of what you tried, to see if they could tell you why it doesn't work?

They are the experts on how these units communicate. They are the best people to tell you why it isn't working.

When you contact them, tell them that your modems transmit all the time, and you're only passing TXD and RXD between them. I think that may be the problem.

 

Got the specifications of the modem IC used by Axel Counter. Its OKi Semiconductors MSM6947.
This IC has got ability to shut off transmitter when RS1 is made high.

 

Right. Have you spoken to the manufacturers of the axle counter? They are your best bet.