OT- Weird TX/RX circuit

[Ttelmah]

Agreed totally.

The way I understand these, is that they have two internal thermostats, and one external control input. The internal thermostats are used 'alone' if the unit is jumpered to run in the dumbest mode. You have a simple external thermostat, and if this goes on, and the temperature of the 'cold' side is above the required temperature, and the 'heat dump' fluid is in an acceptable range, the system comes on. However you have then two options. One is to speed control the blower (there is a daughter board for this), and the other is using an air thermostat. In each case, there are jumper changes, or extra parts shown in the wiring to use these options (hence the suggestion to look at the wiring diagrams).
There are then options on some modules to use an external display (could be the unit being talked about), or to add a 'smart' controller. The small version of this is the MBS controller. Now this is currently the MBS6, but there were older versions (have never seen an MBS5, but have seen MBS4's and MBS3's).

Honestly the easiest way to use this will be to ignore the current display , and instead jumper the unit for 'dumb' mode and get it working. Then to control it, all that would be needed would be a speed control input signal (again there are jumper details for how to set this to just use a DC voltage), and on/off control. Then ones own control unit can read temperature from where required, and operate these.

I'd suspect it is not coming on, because one or more of the inputs is not correctly set up (there are options for things like water being present on the cooler etc.). In 'dumb' mode, the control inputs are very standard on these units (you can use controllers from other manufacturers, just as with things like your central heating - standard thermostats, switches etc...).

A signal is not going to tell you anything, till you see it working.