ok one thing we didn't do is determine what transmission is in this unit, it's either a 2 speed power shift or just a normal powershift.
If there is a switch on the dash with transmission symbols or other symbols indicating hi or low or 1st or 2nd speeds this would be a 2 speed control switch.
in either case it probably has a transmission control module in the dash area located on the right hand side probably mounted underneath the firewall kick panel area.
There is also a mention in a schematic showing a 10 way dual micro relay base supporting forward and reverse relays.
(this is probably later serial numbers.

After i found a parts breakdown diagram it does show this small module mounted underneath the dash on the right hand side. There is a high possibility your loss of or weak signal may be due to that being either bad or the connector is corroded. The connector on it will be a rectangle shaped 12 wire conductor molex type plug, it will more than likely have only 10 wires hooked to it, the diagram shows it has 2 open conductor slots not being used.
terminals 1, 2, 3, 4, 5, 7, 8, 11, and 12 are being used, terminals

Now it could have a different shaped deutz type plug that is more rounded utilizing all 12 wires

So can you give me a serial number off the truck?
This would be very helpful in determining what might actually be on this truck.